Patchwork [PR43513,1/3] Replace vla with array - Implementation.

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Submitter Tom de Vries
Date July 27, 2011, 10:53 a.m.
Message ID <4E2FEE12.9040808@codesourcery.com>
Download mbox | patch
Permalink /patch/107023/
State New
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Tom de Vries - July 27, 2011, 10:53 a.m.
On 07/27/2011 01:50 PM, Tom de Vries wrote:
> Hi Richard,
> 
> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
> 
> 01_pr43513.3.patch
> 02_pr43513.3.test.patch
> 03_pr43513.3.mudflap.patch
> 
> The patch set has been bootstrapped and reg-tested on x86_64.
> 
> I will sent out the patches individually.
> 

The patch replaces a vla __builtin_alloca that has a constant argument with an
array declaration.

OK for trunk?

Thanks,
- Tom

2011-07-27  Tom de Vries  <tom@codesourcery.com>

	PR middle-end/43513
	* builtins.c (fold_builtin_alloca): New function.
	* tree.h (fold_builtin_alloca): Declare.
	* gimple-fold.c (gimple_fold_builtin): Use fold_builtin_alloca.
Richard Guenther - July 27, 2011, 11:12 a.m.
On Wed, 27 Jul 2011, Tom de Vries wrote:

> On 07/27/2011 01:50 PM, Tom de Vries wrote:
> > Hi Richard,
> > 
> > I have a patch set for bug 43513 - The stack pointer is adjusted twice.
> > 
> > 01_pr43513.3.patch
> > 02_pr43513.3.test.patch
> > 03_pr43513.3.mudflap.patch
> > 
> > The patch set has been bootstrapped and reg-tested on x86_64.
> > 
> > I will sent out the patches individually.
> > 
> 
> The patch replaces a vla __builtin_alloca that has a constant argument with an
> array declaration.
> 
> OK for trunk?

I don't think it is safe to try to get at the VLA type the way you do.
In fact I would simply do sth like

  elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
  n_elem = size * 8 / BITS_PER_UNIT;
  array_type = build_array_type_nelts (elem_type, n_elem);
  var = create_tmp_var (array_type, NULL);
  return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));

And obviously you lose the optimization we arrange with inserting
__builtin_stack_save/restore pairs that way - stack space will no
longer be shared for subsequent VLAs.  Which means that you'd
better limit the size you allow this promotion.

Alternatively this promotion could happen alongsize 
optimize_stack_restore using more global knowledge of the effects
on the maximum stack size this folding produces.

Thanks,
Richard.
Tom de Vries - July 27, 2011, 2:10 p.m.
On 07/27/2011 02:12 PM, Richard Guenther wrote:
> On Wed, 27 Jul 2011, Tom de Vries wrote:
> 
>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
>>> Hi Richard,
>>>
>>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
>>>
>>> 01_pr43513.3.patch
>>> 02_pr43513.3.test.patch
>>> 03_pr43513.3.mudflap.patch
>>>
>>> The patch set has been bootstrapped and reg-tested on x86_64.
>>>
>>> I will sent out the patches individually.
>>>
>>
>> The patch replaces a vla __builtin_alloca that has a constant argument with an
>> array declaration.
>>
>> OK for trunk?
> 
> I don't think it is safe to try to get at the VLA type the way you do.

I don't understand in what way it's not safe. Do you mean I don't manage to find
the type always, or that I find the wrong type, or something else?

> In fact I would simply do sth like
> 
>   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>   n_elem = size * 8 / BITS_PER_UNIT;
>   array_type = build_array_type_nelts (elem_type, n_elem);
>   var = create_tmp_var (array_type, NULL);
>   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
> 

I tried this code on the example, and it works, but the newly declared type has
an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
the memory access in the example potentially unaligned, which prohibits an
ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
with my current patch.

> And obviously you lose the optimization we arrange with inserting
> __builtin_stack_save/restore pairs that way - stack space will no
> longer be shared for subsequent VLAs.  Which means that you'd
> better limit the size you allow this promotion.
> 

Right, I could introduce a parameter for this.

> Alternatively this promotion could happen alongsize 
> optimize_stack_restore using more global knowledge of the effects
> on the maximum stack size this folding produces.
> 

OK, I'll look into this.

Thanks,
- Tom
Richard Guenther - July 27, 2011, 2:27 p.m.
On Wed, 27 Jul 2011, Tom de Vries wrote:

> On 07/27/2011 02:12 PM, Richard Guenther wrote:
> > On Wed, 27 Jul 2011, Tom de Vries wrote:
> > 
> >> On 07/27/2011 01:50 PM, Tom de Vries wrote:
> >>> Hi Richard,
> >>>
> >>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
> >>>
> >>> 01_pr43513.3.patch
> >>> 02_pr43513.3.test.patch
> >>> 03_pr43513.3.mudflap.patch
> >>>
> >>> The patch set has been bootstrapped and reg-tested on x86_64.
> >>>
> >>> I will sent out the patches individually.
> >>>
> >>
> >> The patch replaces a vla __builtin_alloca that has a constant argument with an
> >> array declaration.
> >>
> >> OK for trunk?
> > 
> > I don't think it is safe to try to get at the VLA type the way you do.
> 
> I don't understand in what way it's not safe. Do you mean I don't manage to find
> the type always, or that I find the wrong type, or something else?

I think you might get the wrong type, you also do not transform code
like

  int *p = alloca(4);
  *p = 3;

as there is no array type involved here.

> > In fact I would simply do sth like
> > 
> >   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
> >   n_elem = size * 8 / BITS_PER_UNIT;
> >   array_type = build_array_type_nelts (elem_type, n_elem);
> >   var = create_tmp_var (array_type, NULL);
> >   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
> > 
> 
> I tried this code on the example, and it works, but the newly declared type has
> an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
> the memory access in the example potentially unaligned, which prohibits an
> ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
> with my current patch.

Ok, so then set DECL_ALIGN of the variable to something reasonable
like MIN (size * 8, GET_MODE_PRECISION (word_mode)).  Basically the
alignment that the targets alloca function would guarantee.

> > And obviously you lose the optimization we arrange with inserting
> > __builtin_stack_save/restore pairs that way - stack space will no
> > longer be shared for subsequent VLAs.  Which means that you'd
> > better limit the size you allow this promotion.
> > 
> 
> Right, I could introduce a parameter for this.

I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
allow a size of PARAM_LARGE_STACK_FRAME / 10?

> > Alternatively this promotion could happen alongsize 
> > optimize_stack_restore using more global knowledge of the effects
> > on the maximum stack size this folding produces.
> > 
> 
> OK, I'll look into this.

Thanks,
Richard.
Michael Matz - July 27, 2011, 3:57 p.m.
Hi,

On Wed, 27 Jul 2011, Richard Guenther wrote:

> > > I don't think it is safe to try to get at the VLA type the way you do.
> > 
> > I don't understand in what way it's not safe. Do you mean I don't manage to find
> > the type always, or that I find the wrong type, or something else?
> 
> I think you might get the wrong type, you also do not transform code
> like
> 
>   int *p = alloca(4);
>   *p = 3;
> 
> as there is no array type involved here.

That's good, because you _can't_ transform that code into an array decl.  
See:

   for (int i = 0; i < 100; i++)
     p[i] = alloca(4);
   assert (p[0] != p[1]);

vs.
   char vla_cst[4];
   for (int i = 0; i < 100; i++)
     p[i] = &vla_cst;
   assert (p[0] != p[1]);

Tom: you can reliably detect if an alloca call is for a VLA by checking 
CALL_ALLOCA_FOR_VAR_P (on a tree call expression, but only if it's a 
builtin call) or gimple_call_alloca_for_var_p (on a gimple call stmt).


Ciao,
Michael.
Tom de Vries - July 27, 2011, 5:17 p.m.
On 07/27/2011 05:27 PM, Richard Guenther wrote:
> On Wed, 27 Jul 2011, Tom de Vries wrote:
> 
>> On 07/27/2011 02:12 PM, Richard Guenther wrote:
>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>
>>>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
>>>>> Hi Richard,
>>>>>
>>>>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
>>>>>
>>>>> 01_pr43513.3.patch
>>>>> 02_pr43513.3.test.patch
>>>>> 03_pr43513.3.mudflap.patch
>>>>>
>>>>> The patch set has been bootstrapped and reg-tested on x86_64.
>>>>>
>>>>> I will sent out the patches individually.
>>>>>
>>>>
>>>> The patch replaces a vla __builtin_alloca that has a constant argument with an
>>>> array declaration.
>>>>
>>>> OK for trunk?
>>>
>>> I don't think it is safe to try to get at the VLA type the way you do.
>>
>> I don't understand in what way it's not safe. Do you mean I don't manage to find
>> the type always, or that I find the wrong type, or something else?
> 
> I think you might get the wrong type,

Ok, I'll review that code one more time.

> you also do not transform code
> like
> 
>   int *p = alloca(4);
>   *p = 3;
> 
> as there is no array type involved here.
> 

I was trying to stay away from non-vla allocas.  A source declared alloca has
function livetime, so we could have a single alloca in a loop, called 10 times,
with all 10 instances live at the same time. This patch does not detect such
cases, and thus stays away from non-vla allocas. A vla decl does not have such
problems, the lifetime ends when it goes out of scope.

>>> In fact I would simply do sth like
>>>
>>>   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>>>   n_elem = size * 8 / BITS_PER_UNIT;
>>>   array_type = build_array_type_nelts (elem_type, n_elem);
>>>   var = create_tmp_var (array_type, NULL);
>>>   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
>>>
>>
>> I tried this code on the example, and it works, but the newly declared type has
>> an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
>> the memory access in the example potentially unaligned, which prohibits an
>> ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
>> with my current patch.
> 
> Ok, so then set DECL_ALIGN of the variable to something reasonable
> like MIN (size * 8, GET_MODE_PRECISION (word_mode)).  Basically the
> alignment that the targets alloca function would guarantee.
> 

I tried that, but that doesn't help. It's the alignment of the type that
matters, not of the decl.

So should we try to find the base type of the vla, and use that, or use the
nonstandard char type?

>>> And obviously you lose the optimization we arrange with inserting
>>> __builtin_stack_save/restore pairs that way - stack space will no
>>> longer be shared for subsequent VLAs.  Which means that you'd
>>> better limit the size you allow this promotion.
>>>
>>
>> Right, I could introduce a parameter for this.
> 
> I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
> allow a size of PARAM_LARGE_STACK_FRAME / 10?
> 

That unfortunately is too small for the example from bug report. The default
value of the param is 250, so that would be a threshold of 25, and the alloca
size of the example is 40.  Perhaps we can try a threshold of
PARAM_LARGE_STACK_FRAME - estimated_stack_size or some such?

>>> Alternatively this promotion could happen alongsize 
>>> optimize_stack_restore using more global knowledge of the effects
>>> on the maximum stack size this folding produces.
>>>
>>
>> OK, I'll look into this.
> 

Thanks,
- Tom
Richard Guenther - July 28, 2011, 8:47 a.m.
On Wed, 27 Jul 2011, Michael Matz wrote:

> Hi,
> 
> On Wed, 27 Jul 2011, Richard Guenther wrote:
> 
> > > > I don't think it is safe to try to get at the VLA type the way you do.
> > > 
> > > I don't understand in what way it's not safe. Do you mean I don't manage to find
> > > the type always, or that I find the wrong type, or something else?
> > 
> > I think you might get the wrong type, you also do not transform code
> > like
> > 
> >   int *p = alloca(4);
> >   *p = 3;
> > 
> > as there is no array type involved here.
> 
> That's good, because you _can't_ transform that code into an array decl.  
> See:
> 
>    for (int i = 0; i < 100; i++)
>      p[i] = alloca(4);
>    assert (p[0] != p[1]);
> 
> vs.
>    char vla_cst[4];
>    for (int i = 0; i < 100; i++)
>      p[i] = &vla_cst;
>    assert (p[0] != p[1]);

Hm, indeed ;)  At least not without more flow-sensitive analysis.

> Tom: you can reliably detect if an alloca call is for a VLA by checking 
> CALL_ALLOCA_FOR_VAR_P (on a tree call expression, but only if it's a 
> builtin call) or gimple_call_alloca_for_var_p (on a gimple call stmt).

Which actually hints that you should inline the folder into
gimple_fold_call/builtin where you still have this flag properly 
preserved.

Richard.
Richard Guenther - July 28, 2011, 9:22 a.m.
On Wed, 27 Jul 2011, Tom de Vries wrote:

> On 07/27/2011 05:27 PM, Richard Guenther wrote:
> > On Wed, 27 Jul 2011, Tom de Vries wrote:
> > 
> >> On 07/27/2011 02:12 PM, Richard Guenther wrote:
> >>> On Wed, 27 Jul 2011, Tom de Vries wrote:
> >>>
> >>>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
> >>>>> Hi Richard,
> >>>>>
> >>>>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
> >>>>>
> >>>>> 01_pr43513.3.patch
> >>>>> 02_pr43513.3.test.patch
> >>>>> 03_pr43513.3.mudflap.patch
> >>>>>
> >>>>> The patch set has been bootstrapped and reg-tested on x86_64.
> >>>>>
> >>>>> I will sent out the patches individually.
> >>>>>
> >>>>
> >>>> The patch replaces a vla __builtin_alloca that has a constant argument with an
> >>>> array declaration.
> >>>>
> >>>> OK for trunk?
> >>>
> >>> I don't think it is safe to try to get at the VLA type the way you do.
> >>
> >> I don't understand in what way it's not safe. Do you mean I don't manage to find
> >> the type always, or that I find the wrong type, or something else?
> > 
> > I think you might get the wrong type,
> 
> Ok, I'll review that code one more time.
> 
> > you also do not transform code
> > like
> > 
> >   int *p = alloca(4);
> >   *p = 3;
> > 
> > as there is no array type involved here.
> > 
> 
> I was trying to stay away from non-vla allocas.  A source declared alloca has
> function livetime, so we could have a single alloca in a loop, called 10 times,
> with all 10 instances live at the same time. This patch does not detect such
> cases, and thus stays away from non-vla allocas. A vla decl does not have such
> problems, the lifetime ends when it goes out of scope.

Yes indeed - that probably would require more detailed analysis.

> >>> In fact I would simply do sth like
> >>>
> >>>   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
> >>>   n_elem = size * 8 / BITS_PER_UNIT;
> >>>   array_type = build_array_type_nelts (elem_type, n_elem);
> >>>   var = create_tmp_var (array_type, NULL);
> >>>   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
> >>>
> >>
> >> I tried this code on the example, and it works, but the newly declared type has
> >> an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
> >> the memory access in the example potentially unaligned, which prohibits an
> >> ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
> >> with my current patch.
> > 
> > Ok, so then set DECL_ALIGN of the variable to something reasonable
> > like MIN (size * 8, GET_MODE_PRECISION (word_mode)).  Basically the
> > alignment that the targets alloca function would guarantee.
> > 
> 
> I tried that, but that doesn't help. It's the alignment of the type that
> matters, not of the decl.

It shouldn't.  All accesses are performed with the original types and
alignment comes from that (plus the underlying decl).

> So should we try to find the base type of the vla, and use that, or use the
> nonstandard char type?

I don't think we can reliably find the base type of the vla - well,
in practice we may because we control how we lower VLAs during
gimplification, but nothing in the IL constraints say that the
resulting pointer type should be special.

Using a char[] decl shouldn't be a problem IMHO.

> >>> And obviously you lose the optimization we arrange with inserting
> >>> __builtin_stack_save/restore pairs that way - stack space will no
> >>> longer be shared for subsequent VLAs.  Which means that you'd
> >>> better limit the size you allow this promotion.
> >>>
> >>
> >> Right, I could introduce a parameter for this.
> > 
> > I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
> > allow a size of PARAM_LARGE_STACK_FRAME / 10?
> > 
> 
> That unfortunately is too small for the example from bug report. The default
> value of the param is 250, so that would be a threshold of 25, and the alloca
> size of the example is 40.  Perhaps we can try a threshold of
> PARAM_LARGE_STACK_FRAME - estimated_stack_size or some such?

Hm.  estimated_stack_size is not O(1), so no.  I think we need to
find a sensible way of allowing stack sharing.  Eventually Michas
patch for introducing points-of-death would help here, if we'd
go for folding this during stack-save/restore optimization.

Richard.
Richard Guenther - July 28, 2011, 3:25 p.m.
On Thu, 28 Jul 2011, Tom de Vries wrote:

> On 07/28/2011 12:22 PM, Richard Guenther wrote:
> > On Wed, 27 Jul 2011, Tom de Vries wrote:
> > 
> >> On 07/27/2011 05:27 PM, Richard Guenther wrote:
> >>> On Wed, 27 Jul 2011, Tom de Vries wrote:
> >>>
> >>>> On 07/27/2011 02:12 PM, Richard Guenther wrote:
> >>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
> >>>>>
> >>>>>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
> >>>>>>> Hi Richard,
> >>>>>>>
> >>>>>>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
> >>>>>>>
> >>>>>>> 01_pr43513.3.patch
> >>>>>>> 02_pr43513.3.test.patch
> >>>>>>> 03_pr43513.3.mudflap.patch
> >>>>>>>
> >>>>>>> The patch set has been bootstrapped and reg-tested on x86_64.
> >>>>>>>
> >>>>>>> I will sent out the patches individually.
> >>>>>>>
> >>>>>>
> >>>>>> The patch replaces a vla __builtin_alloca that has a constant argument with an
> >>>>>> array declaration.
> >>>>>>
> >>>>>> OK for trunk?
> >>>>>
> >>>>> I don't think it is safe to try to get at the VLA type the way you do.
> >>>>
> >>>> I don't understand in what way it's not safe. Do you mean I don't manage to find
> >>>> the type always, or that I find the wrong type, or something else?
> >>>
> >>> I think you might get the wrong type,
> >>
> >> Ok, I'll review that code one more time.
> >>
> >>> you also do not transform code
> >>> like
> >>>
> >>>   int *p = alloca(4);
> >>>   *p = 3;
> >>>
> >>> as there is no array type involved here.
> >>>
> >>
> >> I was trying to stay away from non-vla allocas.  A source declared alloca has
> >> function livetime, so we could have a single alloca in a loop, called 10 times,
> >> with all 10 instances live at the same time. This patch does not detect such
> >> cases, and thus stays away from non-vla allocas. A vla decl does not have such
> >> problems, the lifetime ends when it goes out of scope.
> > 
> > Yes indeed - that probably would require more detailed analysis.
> > 
> >>>>> In fact I would simply do sth like
> >>>>>
> >>>>>   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
> >>>>>   n_elem = size * 8 / BITS_PER_UNIT;
> >>>>>   array_type = build_array_type_nelts (elem_type, n_elem);
> >>>>>   var = create_tmp_var (array_type, NULL);
> >>>>>   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
> >>>>>
> >>>>
> >>>> I tried this code on the example, and it works, but the newly declared type has
> >>>> an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
> >>>> the memory access in the example potentially unaligned, which prohibits an
> >>>> ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
> >>>> with my current patch.
> >>>
> >>> Ok, so then set DECL_ALIGN of the variable to something reasonable
> >>> like MIN (size * 8, GET_MODE_PRECISION (word_mode)).  Basically the
> >>> alignment that the targets alloca function would guarantee.
> >>>
> >>
> >> I tried that, but that doesn't help. It's the alignment of the type that
> >> matters, not of the decl.
> > 
> > It shouldn't.  All accesses are performed with the original types and
> > alignment comes from that (plus the underlying decl).
> > 
> 
> I managed to get it all working by using build_aligned_type rather that DECL_ALIGN.

That's really odd, DECL_ALIGN should just work - nothing refers to the
type of the decl in the IL.  Can you try also setting DECL_USER_ALIGN to 
1 maybe?

> 
> >> So should we try to find the base type of the vla, and use that, or use the
> >> nonstandard char type?
> > 
> > I don't think we can reliably find the base type of the vla - well,
> > in practice we may because we control how we lower VLAs during
> > gimplification, but nothing in the IL constraints say that the
> > resulting pointer type should be special.
> > 
> > Using a char[] decl shouldn't be a problem IMHO.
> > 
> >>>>> And obviously you lose the optimization we arrange with inserting
> >>>>> __builtin_stack_save/restore pairs that way - stack space will no
> >>>>> longer be shared for subsequent VLAs.  Which means that you'd
> >>>>> better limit the size you allow this promotion.
> >>>>>
> >>>>
> >>>> Right, I could introduce a parameter for this.
> >>>
> >>> I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
> >>> allow a size of PARAM_LARGE_STACK_FRAME / 10?
> >>>
> >>
> >> That unfortunately is too small for the example from bug report. The default
> >> value of the param is 250, so that would be a threshold of 25, and the alloca
> >> size of the example is 40.  Perhaps we can try a threshold of
> >> PARAM_LARGE_STACK_FRAME - estimated_stack_size or some such?
> > 
> > Hm.  estimated_stack_size is not O(1), so no.  I think we need to
> > find a sensible way of allowing stack sharing.  Eventually Michas
> > patch for introducing points-of-death would help here, if we'd
> > go for folding this during stack-save/restore optimization.
> > 
> 
> I changed the heuristics to this:
> 
> +  /* Heuristic: don't fold large vlas.  */
> +  threshold = (unsigned HOST_WIDE_INT)PARAM_VALUE (PARAM_LARGE_STACK_FRAME);
> +  /* In case a vla is declared at function scope, it has the same lifetime as a
> +     declared array, so we allow a larger size.  */
> +  block = gimple_block (stmt);
> +  if (!(cfun->after_inlining
> +        && TREE_CODE (BLOCK_SUPERCONTEXT (block)) == FUNCTION_DECL))
> +    threshold /= 10;
> +  if (size > threshold)
> +    return NULL_TREE;
> 
> The heuristics distinguishes between before and after inlining.
> 
> After inlining, vla's declared at function scope have the same lifetimes as
> declared arrays, and don't share their space. There should be no negative
> effects from folding an alloca in this case, but for safety we set a threshold
> of PARAM_LARGE_STACK_FRAME.
> 
> Before inlining, such a vla might be inlined and share its space with another
> vla, so we stick with the normal threshold before inlining.

That sounds reasonable, though the block check should probably use the
original VLA decl block, not that of the basic-block of the allocation,
but unfortunately we don't have access to that.  So I suppose using
the allocation basic-block BLOCK is good enough (still we don't
really care about BLOCK boundaries when doing CFG manipulations, so
the allocation bbs block may be not the outermost scope in more cases
than necessary).

> However, using this heuristic we still don't generate optimal code.
> 
> During the first pass_ccp, the folding is not done, because the size (40) is
> larger than the threshold 25. The threshold is 25, because inlining is not yet done.
> 
> During pass_fold_builtins, the folding is done because it's after inlining, but
> it's later than pass_iv_optimize, so that still doesn't yield the optimal size
> of 64.
> 
> The folding is not done during any of the other invocations or pass_ccp, because
> the argument has already become constant in the earlier invocation.

Yeah, that's the issue with relying on folding to do this transformation.

> Using this change, I manage to trigger folding during the second invocation of
> pass_ccp, before iv_optimize so we generate optimal code.
> 
> Index: gcc/tree-ssa-ccp.c
> ===================================================================
> --- gcc/tree-ssa-ccp.c (revision 173734)
> +++ gcc/tree-ssa-ccp.c (working copy)
> @@ -1727,6 +1727,13 @@ ccp_fold_stmt (gimple_stmt_iterator *gsi
>  	if (gimple_call_internal_p (stmt))
>  	  return false;
> 
> +        /* The heuristic of fold_builtin_alloca differs before and after
> +           inlining, so we don't require the arg to be changed into a constant
> +           for folding, but just to be constant.  */
> +        if (gimple_call_alloca_for_var_p (stmt)
> +            && get_constant_value (gimple_call_arg (stmt, 0)))

Probably reverse the get_constant_value check and the transformation
(gimple_call_alloca_for_var_p isn't a predicate as it has side-effects,
so its name should be changed).

> +          return true;
> +
>  	/* Propagate into the call arguments.  Compared to replace_uses_in
>  	   this can use the argument slot types for type verification
>  	   instead of the current argument type.  We also can safely
> 
> But, to me it feels like a hack. Do you have any ideas how to do this better?

It's somewhat of a hack, but at least it is more of a defined place
for this transformation - which then suggests to remove it from
generic folding and only keep calling it from CCP this way.

Richard.

> Attaching untested patch for reference (will test overnight).
> 
> Thanks,
> - Tom
> 
> 2011-07-28  Tom de Vries  <tom@codesourcery.com>
> 
> 	PR middle-end/43513
> 	* gimple-fold.c (params.h): Include.
> 	(fold_builtin_alloca): New function.
> 	(gimple_fold_builtin): Use fold_builtin_alloca.
> 	* tree-ssa-ccp.c (ccp_fold_stmt): Force folding of vla-related alloca.
> 	* Makefile.in (gimple-fold.o): Add $(PARAMS_H) to rule.
Tom de Vries - July 28, 2011, 5:20 p.m.
On 07/28/2011 06:25 PM, Richard Guenther wrote:
> On Thu, 28 Jul 2011, Tom de Vries wrote:
> 
>> On 07/28/2011 12:22 PM, Richard Guenther wrote:
>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>
>>>> On 07/27/2011 05:27 PM, Richard Guenther wrote:
>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>
>>>>>> On 07/27/2011 02:12 PM, Richard Guenther wrote:
>>>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>>>
>>>>>>>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
>>>>>>>>> Hi Richard,
>>>>>>>>>
>>>>>>>>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
>>>>>>>>>
>>>>>>>>> 01_pr43513.3.patch
>>>>>>>>> 02_pr43513.3.test.patch
>>>>>>>>> 03_pr43513.3.mudflap.patch
>>>>>>>>>
>>>>>>>>> The patch set has been bootstrapped and reg-tested on x86_64.
>>>>>>>>>
>>>>>>>>> I will sent out the patches individually.
>>>>>>>>>
>>>>>>>>
>>>>>>>> The patch replaces a vla __builtin_alloca that has a constant argument with an
>>>>>>>> array declaration.
>>>>>>>>
>>>>>>>> OK for trunk?
>>>>>>>
>>>>>>> I don't think it is safe to try to get at the VLA type the way you do.
>>>>>>
>>>>>> I don't understand in what way it's not safe. Do you mean I don't manage to find
>>>>>> the type always, or that I find the wrong type, or something else?
>>>>>
>>>>> I think you might get the wrong type,
>>>>
>>>> Ok, I'll review that code one more time.
>>>>
>>>>> you also do not transform code
>>>>> like
>>>>>
>>>>>   int *p = alloca(4);
>>>>>   *p = 3;
>>>>>
>>>>> as there is no array type involved here.
>>>>>
>>>>
>>>> I was trying to stay away from non-vla allocas.  A source declared alloca has
>>>> function livetime, so we could have a single alloca in a loop, called 10 times,
>>>> with all 10 instances live at the same time. This patch does not detect such
>>>> cases, and thus stays away from non-vla allocas. A vla decl does not have such
>>>> problems, the lifetime ends when it goes out of scope.
>>>
>>> Yes indeed - that probably would require more detailed analysis.
>>>
>>>>>>> In fact I would simply do sth like
>>>>>>>
>>>>>>>   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>>>>>>>   n_elem = size * 8 / BITS_PER_UNIT;
>>>>>>>   array_type = build_array_type_nelts (elem_type, n_elem);
>>>>>>>   var = create_tmp_var (array_type, NULL);
>>>>>>>   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
>>>>>>>
>>>>>>
>>>>>> I tried this code on the example, and it works, but the newly declared type has
>>>>>> an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
>>>>>> the memory access in the example potentially unaligned, which prohibits an
>>>>>> ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
>>>>>> with my current patch.
>>>>>
>>>>> Ok, so then set DECL_ALIGN of the variable to something reasonable
>>>>> like MIN (size * 8, GET_MODE_PRECISION (word_mode)).  Basically the
>>>>> alignment that the targets alloca function would guarantee.
>>>>>
>>>>
>>>> I tried that, but that doesn't help. It's the alignment of the type that
>>>> matters, not of the decl.
>>>
>>> It shouldn't.  All accesses are performed with the original types and
>>> alignment comes from that (plus the underlying decl).
>>>
>>
>> I managed to get it all working by using build_aligned_type rather that DECL_ALIGN.
> 
> That's really odd, DECL_ALIGN should just work - nothing refers to the
> type of the decl in the IL.  Can you try also setting DECL_USER_ALIGN to 
> 1 maybe?
> 

This doesn't work either.

  /* Declare array.  */
  elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
  n_elem = size * 8 / BITS_PER_UNIT;
  align = MIN (size * 8, GET_MODE_PRECISION (word_mode));
  array_type = build_array_type_nelts (elem_type, n_elem);
  var = create_tmp_var (array_type, NULL);
  DECL_ALIGN (var) = align;
  DECL_USER_ALIGN (var) = 1;

Maybe this clarifies it:

Breakpoint 1, may_be_unaligned_p (ref=0xf7d9d410, step=0xf7d3d578) at
/home/vries/local/google/src/gcc-mainline/gcc/tree-ssa-loop-ivopts.c:1621
(gdb) call debug_generic_expr (ref)
MEM[(int[0:D.2579] *)&D.2595][0]
(gdb) call debug_generic_expr (step)
4

1627	  base = get_inner_reference (ref, &bitsize, &bitpos, &toffset, &mode,
(gdb) call debug_generic_expr (base)
D.2595

1629	  base_type = TREE_TYPE (base);
(gdb) call debug_generic_expr (base_type)
<unnamed-unsigned:8>[40]

1630	  base_align = TYPE_ALIGN (base_type);
(gdb) p base_align
$1 = 8

So the align is 8-bits, and we return true here:

(gdb) n
1632	  if (mode != BLKmode)
(gdb) n
1634	      unsigned mode_align = GET_MODE_ALIGNMENT (mode);
(gdb)
1636	      if (base_align < mode_align
(gdb)
1639		return true;


Here we can see that the base actually has the (user) align on it:

(gdb) call debug_tree (base)
 <var_decl 0xf7e1b420 D.2595
    type <array_type 0xf7e1b360
        type <integer_type 0xf7e1b2a0 public unsigned QI
            size <integer_cst 0xf7d3d604 constant 8>
            unit size <integer_cst 0xf7d3d620 constant 1>
            align 8 symtab 0 alias set -1 canonical type 0xf7e1b2a0 precision 8
            min <integer_cst 0xf7dffaf0 0> max <integer_cst 0xf7dffb0c 255>
            pointer_to_this <pointer_type 0xf7e1b3c0>>
        BLK
        size <integer_cst 0xf7d5d070 constant 320>
        unit size <integer_cst 0xf7dde2a0 constant 40>
        align 8 symtab 0 alias set -1 canonical type 0xf7e1b360
        domain <integer_type 0xf7de12a0
                type <integer_type 0xf7d51000 unsigned int>
            SI
            size <integer_cst 0xf7d3d78c constant 32>
            unit size <integer_cst 0xf7d3d578 constant 4>
            align 32 symtab 0 alias set -1 canonical type 0xf7de12a0
            precision 32 min <integer_cst 0xf7d3d594 0>
            max <integer_cst 0xf7dde284 39>>
        pointer_to_this <pointer_type 0xf7e1b480>>
    addressable used ignored BLK file pr43513.c line 4 col 6
    size <integer_cst 0xf7d5d070 320> unit size <integer_cst 0xf7dde2a0 40>
    user align 32 context <function_decl 0xf7dfd480 foo3>>


>>
>>>> So should we try to find the base type of the vla, and use that, or use the
>>>> nonstandard char type?
>>>
>>> I don't think we can reliably find the base type of the vla - well,
>>> in practice we may because we control how we lower VLAs during
>>> gimplification, but nothing in the IL constraints say that the
>>> resulting pointer type should be special.
>>>
>>> Using a char[] decl shouldn't be a problem IMHO.
>>>
>>>>>>> And obviously you lose the optimization we arrange with inserting
>>>>>>> __builtin_stack_save/restore pairs that way - stack space will no
>>>>>>> longer be shared for subsequent VLAs.  Which means that you'd
>>>>>>> better limit the size you allow this promotion.
>>>>>>>
>>>>>>
>>>>>> Right, I could introduce a parameter for this.
>>>>>
>>>>> I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
>>>>> allow a size of PARAM_LARGE_STACK_FRAME / 10?
>>>>>
>>>>
>>>> That unfortunately is too small for the example from bug report. The default
>>>> value of the param is 250, so that would be a threshold of 25, and the alloca
>>>> size of the example is 40.  Perhaps we can try a threshold of
>>>> PARAM_LARGE_STACK_FRAME - estimated_stack_size or some such?
>>>
>>> Hm.  estimated_stack_size is not O(1), so no.  I think we need to
>>> find a sensible way of allowing stack sharing.  Eventually Michas
>>> patch for introducing points-of-death would help here, if we'd
>>> go for folding this during stack-save/restore optimization.
>>>
>>
>> I changed the heuristics to this:
>>
>> +  /* Heuristic: don't fold large vlas.  */
>> +  threshold = (unsigned HOST_WIDE_INT)PARAM_VALUE (PARAM_LARGE_STACK_FRAME);
>> +  /* In case a vla is declared at function scope, it has the same lifetime as a
>> +     declared array, so we allow a larger size.  */
>> +  block = gimple_block (stmt);
>> +  if (!(cfun->after_inlining
>> +        && TREE_CODE (BLOCK_SUPERCONTEXT (block)) == FUNCTION_DECL))
>> +    threshold /= 10;
>> +  if (size > threshold)
>> +    return NULL_TREE;
>>
>> The heuristics distinguishes between before and after inlining.
>>
>> After inlining, vla's declared at function scope have the same lifetimes as
>> declared arrays, and don't share their space. There should be no negative
>> effects from folding an alloca in this case, but for safety we set a threshold
>> of PARAM_LARGE_STACK_FRAME.
>>
>> Before inlining, such a vla might be inlined and share its space with another
>> vla, so we stick with the normal threshold before inlining.
> 
> That sounds reasonable, though the block check should probably use the
> original VLA decl block, not that of the basic-block of the allocation,
> but unfortunately we don't have access to that.  So I suppose using
> the allocation basic-block BLOCK is good enough (still we don't
> really care about BLOCK boundaries when doing CFG manipulations, so
> the allocation bbs block may be not the outermost scope in more cases
> than necessary).
> 
>> However, using this heuristic we still don't generate optimal code.
>>
>> During the first pass_ccp, the folding is not done, because the size (40) is
>> larger than the threshold 25. The threshold is 25, because inlining is not yet done.
>>
>> During pass_fold_builtins, the folding is done because it's after inlining, but
>> it's later than pass_iv_optimize, so that still doesn't yield the optimal size
>> of 64.
>>
>> The folding is not done during any of the other invocations or pass_ccp, because
>> the argument has already become constant in the earlier invocation.
> 
> Yeah, that's the issue with relying on folding to do this transformation.
> 
>> Using this change, I manage to trigger folding during the second invocation of
>> pass_ccp, before iv_optimize so we generate optimal code.
>>
>> Index: gcc/tree-ssa-ccp.c
>> ===================================================================
>> --- gcc/tree-ssa-ccp.c (revision 173734)
>> +++ gcc/tree-ssa-ccp.c (working copy)
>> @@ -1727,6 +1727,13 @@ ccp_fold_stmt (gimple_stmt_iterator *gsi
>>  	if (gimple_call_internal_p (stmt))
>>  	  return false;
>>
>> +        /* The heuristic of fold_builtin_alloca differs before and after
>> +           inlining, so we don't require the arg to be changed into a constant
>> +           for folding, but just to be constant.  */
>> +        if (gimple_call_alloca_for_var_p (stmt)
>> +            && get_constant_value (gimple_call_arg (stmt, 0)))
> 
> Probably reverse the get_constant_value check and the transformation

Done.

> (gimple_call_alloca_for_var_p isn't a predicate as it has side-effects,
> so its name should be changed).
> 
>> +          return true;
>> +
>>  	/* Propagate into the call arguments.  Compared to replace_uses_in
>>  	   this can use the argument slot types for type verification
>>  	   instead of the current argument type.  We also can safely
>>
>> But, to me it feels like a hack. Do you have any ideas how to do this better?
> 
> It's somewhat of a hack, but at least it is more of a defined place
> for this transformation - which then suggests to remove it from
> generic folding and only keep calling it from CCP this way.
> 

Done.

> Richard.
> 
>> Attaching untested patch for reference (will test overnight).
>>
>> Thanks,
>> - Tom
>>
>> 2011-07-28  Tom de Vries  <tom@codesourcery.com>
>>
>> 	PR middle-end/43513
>> 	* gimple-fold.c (params.h): Include.
>> 	(fold_builtin_alloca): New function.
>> 	(gimple_fold_builtin): Use fold_builtin_alloca.
>> 	* tree-ssa-ccp.c (ccp_fold_stmt): Force folding of vla-related alloca.
>> 	* Makefile.in (gimple-fold.o): Add $(PARAMS_H) to rule.

Thanks,
- Tom
Richard Guenther - July 28, 2011, 7:31 p.m.
On Thu, Jul 28, 2011 at 7:20 PM, Tom de Vries <vries@codesourcery.com> wrote:
> On 07/28/2011 06:25 PM, Richard Guenther wrote:
>> On Thu, 28 Jul 2011, Tom de Vries wrote:
>>
>>> On 07/28/2011 12:22 PM, Richard Guenther wrote:
>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>
>>>>> On 07/27/2011 05:27 PM, Richard Guenther wrote:
>>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>>
>>>>>>> On 07/27/2011 02:12 PM, Richard Guenther wrote:
>>>>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>>>>
>>>>>>>>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
>>>>>>>>>> Hi Richard,
>>>>>>>>>>
>>>>>>>>>> I have a patch set for bug 43513 - The stack pointer is adjusted twice.
>>>>>>>>>>
>>>>>>>>>> 01_pr43513.3.patch
>>>>>>>>>> 02_pr43513.3.test.patch
>>>>>>>>>> 03_pr43513.3.mudflap.patch
>>>>>>>>>>
>>>>>>>>>> The patch set has been bootstrapped and reg-tested on x86_64.
>>>>>>>>>>
>>>>>>>>>> I will sent out the patches individually.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> The patch replaces a vla __builtin_alloca that has a constant argument with an
>>>>>>>>> array declaration.
>>>>>>>>>
>>>>>>>>> OK for trunk?
>>>>>>>>
>>>>>>>> I don't think it is safe to try to get at the VLA type the way you do.
>>>>>>>
>>>>>>> I don't understand in what way it's not safe. Do you mean I don't manage to find
>>>>>>> the type always, or that I find the wrong type, or something else?
>>>>>>
>>>>>> I think you might get the wrong type,
>>>>>
>>>>> Ok, I'll review that code one more time.
>>>>>
>>>>>> you also do not transform code
>>>>>> like
>>>>>>
>>>>>>   int *p = alloca(4);
>>>>>>   *p = 3;
>>>>>>
>>>>>> as there is no array type involved here.
>>>>>>
>>>>>
>>>>> I was trying to stay away from non-vla allocas.  A source declared alloca has
>>>>> function livetime, so we could have a single alloca in a loop, called 10 times,
>>>>> with all 10 instances live at the same time. This patch does not detect such
>>>>> cases, and thus stays away from non-vla allocas. A vla decl does not have such
>>>>> problems, the lifetime ends when it goes out of scope.
>>>>
>>>> Yes indeed - that probably would require more detailed analysis.
>>>>
>>>>>>>> In fact I would simply do sth like
>>>>>>>>
>>>>>>>>   elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>>>>>>>>   n_elem = size * 8 / BITS_PER_UNIT;
>>>>>>>>   array_type = build_array_type_nelts (elem_type, n_elem);
>>>>>>>>   var = create_tmp_var (array_type, NULL);
>>>>>>>>   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
>>>>>>>>
>>>>>>>
>>>>>>> I tried this code on the example, and it works, but the newly declared type has
>>>>>>> an 8-bit alignment, while the vla base type has a 32 bit alignment.  This make
>>>>>>> the memory access in the example potentially unaligned, which prohibits an
>>>>>>> ivopts optimization, so the resulting text size is 68 instead of the 64 achieved
>>>>>>> with my current patch.
>>>>>>
>>>>>> Ok, so then set DECL_ALIGN of the variable to something reasonable
>>>>>> like MIN (size * 8, GET_MODE_PRECISION (word_mode)).  Basically the
>>>>>> alignment that the targets alloca function would guarantee.
>>>>>>
>>>>>
>>>>> I tried that, but that doesn't help. It's the alignment of the type that
>>>>> matters, not of the decl.
>>>>
>>>> It shouldn't.  All accesses are performed with the original types and
>>>> alignment comes from that (plus the underlying decl).
>>>>
>>>
>>> I managed to get it all working by using build_aligned_type rather that DECL_ALIGN.
>>
>> That's really odd, DECL_ALIGN should just work - nothing refers to the
>> type of the decl in the IL.  Can you try also setting DECL_USER_ALIGN to
>> 1 maybe?
>>
>
> This doesn't work either.
>
>  /* Declare array.  */
>  elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>  n_elem = size * 8 / BITS_PER_UNIT;
>  align = MIN (size * 8, GET_MODE_PRECISION (word_mode));
>  array_type = build_array_type_nelts (elem_type, n_elem);
>  var = create_tmp_var (array_type, NULL);
>  DECL_ALIGN (var) = align;
>  DECL_USER_ALIGN (var) = 1;
>
> Maybe this clarifies it:
>
> Breakpoint 1, may_be_unaligned_p (ref=0xf7d9d410, step=0xf7d3d578) at
> /home/vries/local/google/src/gcc-mainline/gcc/tree-ssa-loop-ivopts.c:1621
> (gdb) call debug_generic_expr (ref)
> MEM[(int[0:D.2579] *)&D.2595][0]
> (gdb) call debug_generic_expr (step)
> 4
>
> 1627      base = get_inner_reference (ref, &bitsize, &bitpos, &toffset, &mode,
> (gdb) call debug_generic_expr (base)
> D.2595
>
> 1629      base_type = TREE_TYPE (base);
> (gdb) call debug_generic_expr (base_type)
> <unnamed-unsigned:8>[40]
>
> 1630      base_align = TYPE_ALIGN (base_type);
> (gdb) p base_align
> $1 = 8
>
> So the align is 8-bits, and we return true here:

Ah, but this code should use get_object_alignment, not solely look
at the type.

Richard.

> (gdb) n
> 1632      if (mode != BLKmode)
> (gdb) n
> 1634          unsigned mode_align = GET_MODE_ALIGNMENT (mode);
> (gdb)
> 1636          if (base_align < mode_align
> (gdb)
> 1639            return true;
>
>
> Here we can see that the base actually has the (user) align on it:
>
> (gdb) call debug_tree (base)
>  <var_decl 0xf7e1b420 D.2595
>    type <array_type 0xf7e1b360
>        type <integer_type 0xf7e1b2a0 public unsigned QI
>            size <integer_cst 0xf7d3d604 constant 8>
>            unit size <integer_cst 0xf7d3d620 constant 1>
>            align 8 symtab 0 alias set -1 canonical type 0xf7e1b2a0 precision 8
>            min <integer_cst 0xf7dffaf0 0> max <integer_cst 0xf7dffb0c 255>
>            pointer_to_this <pointer_type 0xf7e1b3c0>>
>        BLK
>        size <integer_cst 0xf7d5d070 constant 320>
>        unit size <integer_cst 0xf7dde2a0 constant 40>
>        align 8 symtab 0 alias set -1 canonical type 0xf7e1b360
>        domain <integer_type 0xf7de12a0
>                type <integer_type 0xf7d51000 unsigned int>
>            SI
>            size <integer_cst 0xf7d3d78c constant 32>
>            unit size <integer_cst 0xf7d3d578 constant 4>
>            align 32 symtab 0 alias set -1 canonical type 0xf7de12a0
>            precision 32 min <integer_cst 0xf7d3d594 0>
>            max <integer_cst 0xf7dde284 39>>
>        pointer_to_this <pointer_type 0xf7e1b480>>
>    addressable used ignored BLK file pr43513.c line 4 col 6
>    size <integer_cst 0xf7d5d070 320> unit size <integer_cst 0xf7dde2a0 40>
>    user align 32 context <function_decl 0xf7dfd480 foo3>>
>
>
>>>
>>>>> So should we try to find the base type of the vla, and use that, or use the
>>>>> nonstandard char type?
>>>>
>>>> I don't think we can reliably find the base type of the vla - well,
>>>> in practice we may because we control how we lower VLAs during
>>>> gimplification, but nothing in the IL constraints say that the
>>>> resulting pointer type should be special.
>>>>
>>>> Using a char[] decl shouldn't be a problem IMHO.
>>>>
>>>>>>>> And obviously you lose the optimization we arrange with inserting
>>>>>>>> __builtin_stack_save/restore pairs that way - stack space will no
>>>>>>>> longer be shared for subsequent VLAs.  Which means that you'd
>>>>>>>> better limit the size you allow this promotion.
>>>>>>>>
>>>>>>>
>>>>>>> Right, I could introduce a parameter for this.
>>>>>>
>>>>>> I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
>>>>>> allow a size of PARAM_LARGE_STACK_FRAME / 10?
>>>>>>
>>>>>
>>>>> That unfortunately is too small for the example from bug report. The default
>>>>> value of the param is 250, so that would be a threshold of 25, and the alloca
>>>>> size of the example is 40.  Perhaps we can try a threshold of
>>>>> PARAM_LARGE_STACK_FRAME - estimated_stack_size or some such?
>>>>
>>>> Hm.  estimated_stack_size is not O(1), so no.  I think we need to
>>>> find a sensible way of allowing stack sharing.  Eventually Michas
>>>> patch for introducing points-of-death would help here, if we'd
>>>> go for folding this during stack-save/restore optimization.
>>>>
>>>
>>> I changed the heuristics to this:
>>>
>>> +  /* Heuristic: don't fold large vlas.  */
>>> +  threshold = (unsigned HOST_WIDE_INT)PARAM_VALUE (PARAM_LARGE_STACK_FRAME);
>>> +  /* In case a vla is declared at function scope, it has the same lifetime as a
>>> +     declared array, so we allow a larger size.  */
>>> +  block = gimple_block (stmt);
>>> +  if (!(cfun->after_inlining
>>> +        && TREE_CODE (BLOCK_SUPERCONTEXT (block)) == FUNCTION_DECL))
>>> +    threshold /= 10;
>>> +  if (size > threshold)
>>> +    return NULL_TREE;
>>>
>>> The heuristics distinguishes between before and after inlining.
>>>
>>> After inlining, vla's declared at function scope have the same lifetimes as
>>> declared arrays, and don't share their space. There should be no negative
>>> effects from folding an alloca in this case, but for safety we set a threshold
>>> of PARAM_LARGE_STACK_FRAME.
>>>
>>> Before inlining, such a vla might be inlined and share its space with another
>>> vla, so we stick with the normal threshold before inlining.
>>
>> That sounds reasonable, though the block check should probably use the
>> original VLA decl block, not that of the basic-block of the allocation,
>> but unfortunately we don't have access to that.  So I suppose using
>> the allocation basic-block BLOCK is good enough (still we don't
>> really care about BLOCK boundaries when doing CFG manipulations, so
>> the allocation bbs block may be not the outermost scope in more cases
>> than necessary).
>>
>>> However, using this heuristic we still don't generate optimal code.
>>>
>>> During the first pass_ccp, the folding is not done, because the size (40) is
>>> larger than the threshold 25. The threshold is 25, because inlining is not yet done.
>>>
>>> During pass_fold_builtins, the folding is done because it's after inlining, but
>>> it's later than pass_iv_optimize, so that still doesn't yield the optimal size
>>> of 64.
>>>
>>> The folding is not done during any of the other invocations or pass_ccp, because
>>> the argument has already become constant in the earlier invocation.
>>
>> Yeah, that's the issue with relying on folding to do this transformation.
>>
>>> Using this change, I manage to trigger folding during the second invocation of
>>> pass_ccp, before iv_optimize so we generate optimal code.
>>>
>>> Index: gcc/tree-ssa-ccp.c
>>> ===================================================================
>>> --- gcc/tree-ssa-ccp.c (revision 173734)
>>> +++ gcc/tree-ssa-ccp.c (working copy)
>>> @@ -1727,6 +1727,13 @@ ccp_fold_stmt (gimple_stmt_iterator *gsi
>>>      if (gimple_call_internal_p (stmt))
>>>        return false;
>>>
>>> +        /* The heuristic of fold_builtin_alloca differs before and after
>>> +           inlining, so we don't require the arg to be changed into a constant
>>> +           for folding, but just to be constant.  */
>>> +        if (gimple_call_alloca_for_var_p (stmt)
>>> +            && get_constant_value (gimple_call_arg (stmt, 0)))
>>
>> Probably reverse the get_constant_value check and the transformation
>
> Done.
>
>> (gimple_call_alloca_for_var_p isn't a predicate as it has side-effects,
>> so its name should be changed).
>>
>>> +          return true;
>>> +
>>>      /* Propagate into the call arguments.  Compared to replace_uses_in
>>>         this can use the argument slot types for type verification
>>>         instead of the current argument type.  We also can safely
>>>
>>> But, to me it feels like a hack. Do you have any ideas how to do this better?
>>
>> It's somewhat of a hack, but at least it is more of a defined place
>> for this transformation - which then suggests to remove it from
>> generic folding and only keep calling it from CCP this way.
>>
>
> Done.
>
>> Richard.
>>
>>> Attaching untested patch for reference (will test overnight).
>>>
>>> Thanks,
>>> - Tom
>>>
>>> 2011-07-28  Tom de Vries  <tom@codesourcery.com>
>>>
>>>      PR middle-end/43513
>>>      * gimple-fold.c (params.h): Include.
>>>      (fold_builtin_alloca): New function.
>>>      (gimple_fold_builtin): Use fold_builtin_alloca.
>>>      * tree-ssa-ccp.c (ccp_fold_stmt): Force folding of vla-related alloca.
>>>      * Makefile.in (gimple-fold.o): Add $(PARAMS_H) to rule.
>
> Thanks,
> - Tom
>

Patch

Index: gcc/builtins.c
===================================================================
--- gcc/builtins.c	(revision 175801)
+++ gcc/builtins.c	(working copy)
@@ -6193,6 +6193,43 @@  builtin_mathfn_code (const_tree t)
   return DECL_FUNCTION_CODE (fndecl);
 }
 
+/* Detects a vla-related alloca with a constant argument.  Declares fixed-size
+   array and return the address, if found, otherwise returns NULL_TREE.  */
+
+tree
+fold_builtin_alloca (tree lhs, tree arg)
+{
+  unsigned HOST_WIDE_INT size, n_elem, elem_size;
+  tree var_type, vla_type, elem_type, array_type;
+
+  if (lhs == NULL_TREE)
+    return NULL_TREE;
+
+  /* Detect constant argument.  */
+  if (TREE_CODE (arg) != INTEGER_CST || !host_integerp (arg, 1))
+    return NULL_TREE;
+  size = TREE_INT_CST_LOW (arg);
+
+  /* Detect a vla.  */
+  var_type = TREE_TYPE (SSA_NAME_VAR (lhs));
+  if (TREE_CODE (var_type) != POINTER_TYPE)
+    return NULL_TREE;
+  vla_type = TREE_TYPE (var_type);
+  if (TREE_CODE (vla_type) != ARRAY_TYPE)
+    return NULL_TREE;
+  if (TREE_CODE (TYPE_MAXVAL (TYPE_DOMAIN (vla_type))) == INTEGER_CST)
+    return NULL_TREE;
+  elem_type = TREE_TYPE (vla_type);
+  if (TREE_CODE (TYPE_SIZE_UNIT (elem_type)) != INTEGER_CST)
+    return NULL_TREE;
+
+  /* Declare a fixed-size array and return the address instead.  */
+  elem_size = TREE_INT_CST_LOW (TYPE_SIZE_UNIT (elem_type));
+  n_elem = size / elem_size;
+  array_type = build_array_type_nelts (elem_type, n_elem);
+  return build_fold_addr_expr (create_tmp_var (array_type, "vla_cst"));
+}
+
 /* Fold a call to __builtin_constant_p, if we know its argument ARG will
    evaluate to a constant.  */
 
Index: gcc/tree.h
===================================================================
--- gcc/tree.h	(revision 175801)
+++ gcc/tree.h	(working copy)
@@ -5321,6 +5321,7 @@  truth_value_p (enum tree_code code)
 
 /* In builtins.c */
 extern tree fold_call_expr (location_t, tree, bool);
+extern tree fold_builtin_alloca (tree, tree);
 extern tree fold_builtin_fputs (location_t, tree, tree, bool, bool, tree);
 extern tree fold_builtin_strcpy (location_t, tree, tree, tree, tree);
 extern tree fold_builtin_strncpy (location_t, tree, tree, tree, tree, tree);
Index: gcc/gimple-fold.c
===================================================================
--- gcc/gimple-fold.c	(revision 175801)
+++ gcc/gimple-fold.c	(working copy)
@@ -1246,6 +1246,9 @@  gimple_fold_builtin (gimple stmt)
       arg_idx = 1;
       type = 2;
       break;
+    case BUILT_IN_ALLOCA:
+      return fold_builtin_alloca (gimple_call_lhs (stmt),
+				  gimple_call_arg (stmt, 0));
     default:
       return NULL_TREE;
     }