Patchwork [C] Don't reject valid code with _Alignas (PR c/61053)

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Submitter Marek Polacek
Date May 5, 2014, 8:27 p.m.
Message ID <20140505202703.GR11802@redhat.com>
Download mbox | patch
Permalink /patch/345880/
State New
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Comments

Marek Polacek - May 5, 2014, 8:27 p.m.
In this PR the issue is that we reject (valid) code such as
_Alignas (long long) long long foo;
with -m32, because we trip this condition:

   alignas_align = 1U << declspecs->align_log;
   if (alignas_align < TYPE_ALIGN_UNIT (type))
     {
       if (name)
         error_at (loc, "%<_Alignas%> specifiers cannot reduce "
                   "alignment of %qE", name);

and error later on, since alignas_align is 4 (correct, see PR52023 for
why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
is wrong here as that won't give us minimal alignment required.
In c_sizeof_or_alignof_type we already have the code to compute such
minimal alignment so I just moved the code to a separate function
and used that instead of TYPE_ALIGN_UNIT.

Note that the test is run only on i?86 and x86_64, because we can't (?)
easily determine which target requires what alignment.

Regtested/bootstrapped on x86_64-unknown-linux-gnu and
powerpc64-unknown-linux-gnu, ok for trunk?

2014-05-05  Marek Polacek  <polacek@redhat.com>

	PR c/61053
c-family/
	* c-common.c (min_align_of_type): New function factored out from...
	(c_sizeof_or_alignof_type): ...here.
	* c-common.h (min_align_of_type): Declare.
c/
	* c-decl.c (grokdeclarator): Use min_align_of_type instead of
	TYPE_ALIGN_UNIT.
testsuite/
	* gcc.dg/pr61053.c: New test.


	Marek
Marek Polacek - May 5, 2014, 8:31 p.m.
On Mon, May 05, 2014 at 10:27:03PM +0200, Marek Polacek wrote:
> In this PR the issue is that we reject (valid) code such as
> _Alignas (long long) long long foo;
> with -m32, because we trip this condition:
> 
>    alignas_align = 1U << declspecs->align_log;
>    if (alignas_align < TYPE_ALIGN_UNIT (type))
>      {
>        if (name)
>          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>                    "alignment of %qE", name);
> 
> and error later on, since alignas_align is 4 (correct, see PR52023 for
> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> is wrong here as that won't give us minimal alignment required.
> In c_sizeof_or_alignof_type we already have the code to compute such
> minimal alignment so I just moved the code to a separate function
> and used that instead of TYPE_ALIGN_UNIT.
> 
> Note that the test is run only on i?86 and x86_64, because we can't (?)
> easily determine which target requires what alignment.
> 
> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
> powerpc64-unknown-linux-gnu, ok for trunk?

I should add that I checked what clang does on my testcase, and with this
patch both compilers accept and reject the same sort of testcases,
both with -m32 and -m64.

	Marek
Richard Guenther - May 6, 2014, 8:04 a.m.
On Mon, May 5, 2014 at 10:27 PM, Marek Polacek <polacek@redhat.com> wrote:
> In this PR the issue is that we reject (valid) code such as
> _Alignas (long long) long long foo;
> with -m32, because we trip this condition:
>
>    alignas_align = 1U << declspecs->align_log;
>    if (alignas_align < TYPE_ALIGN_UNIT (type))
>      {
>        if (name)
>          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>                    "alignment of %qE", name);
>
> and error later on, since alignas_align is 4 (correct, see PR52023 for
> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> is wrong here as that won't give us minimal alignment required.
> In c_sizeof_or_alignof_type we already have the code to compute such
> minimal alignment so I just moved the code to a separate function
> and used that instead of TYPE_ALIGN_UNIT.

Hmm, but isn't TYPE_ALIGN_UNIT wrong then?

Richard.

>
> Note that the test is run only on i?86 and x86_64, because we can't (?)
> easily determine which target requires what alignment.
>
> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
> powerpc64-unknown-linux-gnu, ok for trunk?
>
> 2014-05-05  Marek Polacek  <polacek@redhat.com>
>
>         PR c/61053
> c-family/
>         * c-common.c (min_align_of_type): New function factored out from...
>         (c_sizeof_or_alignof_type): ...here.
>         * c-common.h (min_align_of_type): Declare.
> c/
>         * c-decl.c (grokdeclarator): Use min_align_of_type instead of
>         TYPE_ALIGN_UNIT.
> testsuite/
>         * gcc.dg/pr61053.c: New test.
>
> diff --git gcc/c-family/c-common.c gcc/c-family/c-common.c
> index 0ad955d..6d4440e 100644
> --- gcc/c-family/c-common.c
> +++ gcc/c-family/c-common.c
> @@ -4931,6 +4931,26 @@ c_common_get_alias_set (tree t)
>    return -1;
>  }
>
> +/* Return the least alignment required for type TYPE.  */
> +
> +unsigned int
> +min_align_of_type (tree type)
> +{
> +  unsigned int align = TYPE_ALIGN (type);
> +  align = MIN (align, BIGGEST_ALIGNMENT);
> +#ifdef BIGGEST_FIELD_ALIGNMENT
> +  align = MIN (align, BIGGEST_FIELD_ALIGNMENT);
> +#endif
> +  unsigned int field_align = align;
> +#ifdef ADJUST_FIELD_ALIGN
> +  tree field = build_decl (UNKNOWN_LOCATION, FIELD_DECL, NULL_TREE,
> +                          type);
> +  field_align = ADJUST_FIELD_ALIGN (field, field_align);
> +#endif
> +  align = MIN (align, field_align);
> +  return align / BITS_PER_UNIT;
> +}
> +
>  /* Compute the value of 'sizeof (TYPE)' or '__alignof__ (TYPE)', where
>     the IS_SIZEOF parameter indicates which operator is being applied.
>     The COMPLAIN flag controls whether we should diagnose possibly
> @@ -5009,21 +5029,7 @@ c_sizeof_or_alignof_type (location_t loc,
>                                 size_int (TYPE_PRECISION (char_type_node)
>                                           / BITS_PER_UNIT));
>        else if (min_alignof)
> -       {
> -         unsigned int align = TYPE_ALIGN (type);
> -         align = MIN (align, BIGGEST_ALIGNMENT);
> -#ifdef BIGGEST_FIELD_ALIGNMENT
> -         align = MIN (align, BIGGEST_FIELD_ALIGNMENT);
> -#endif
> -         unsigned int field_align = align;
> -#ifdef ADJUST_FIELD_ALIGN
> -         tree field = build_decl (UNKNOWN_LOCATION, FIELD_DECL, NULL_TREE,
> -                                  type);
> -         field_align = ADJUST_FIELD_ALIGN (field, field_align);
> -#endif
> -         align = MIN (align, field_align);
> -         value = size_int (align / BITS_PER_UNIT);
> -       }
> +       value = size_int (min_align_of_type (type));
>        else
>         value = size_int (TYPE_ALIGN_UNIT (type));
>      }
> diff --git gcc/c-family/c-common.h gcc/c-family/c-common.h
> index 57b7dce..d34d2bb 100644
> --- gcc/c-family/c-common.h
> +++ gcc/c-family/c-common.h
> @@ -758,6 +758,7 @@ extern tree c_wrap_maybe_const (tree, bool);
>  extern tree c_save_expr (tree);
>  extern tree c_common_truthvalue_conversion (location_t, tree);
>  extern void c_apply_type_quals_to_decl (int, tree);
> +extern unsigned int min_align_of_type (tree);
>  extern tree c_sizeof_or_alignof_type (location_t, tree, bool, bool, int);
>  extern tree c_alignof_expr (location_t, tree);
>  /* Print an error message for invalid operands to arith operation CODE.
> diff --git gcc/c/c-decl.c gcc/c/c-decl.c
> index 6e7c589..9f7419a 100644
> --- gcc/c/c-decl.c
> +++ gcc/c/c-decl.c
> @@ -5931,7 +5931,7 @@ grokdeclarator (const struct c_declarator *declarator,
>        else if (declspecs->align_log != -1)
>         {
>           alignas_align = 1U << declspecs->align_log;
> -         if (alignas_align < TYPE_ALIGN_UNIT (type))
> +         if (alignas_align < min_align_of_type (type))
>             {
>               if (name)
>                 error_at (loc, "%<_Alignas%> specifiers cannot reduce "
> diff --git gcc/testsuite/gcc.dg/pr61053.c gcc/testsuite/gcc.dg/pr61053.c
> index e69de29..e3db534 100644
> --- gcc/testsuite/gcc.dg/pr61053.c
> +++ gcc/testsuite/gcc.dg/pr61053.c
> @@ -0,0 +1,75 @@
> +/* PR c/61053 */
> +/* { dg-do compile { target i?86-*-* x86_64-*-* } } */
> +/* { dg-options "-std=c11 -pedantic-errors" } */
> +
> +_Alignas (char) char cc;
> +_Alignas (short int) char cs;
> +_Alignas (int) char ci;
> +_Alignas (long int) char cl;
> +_Alignas (long long int) char cll;
> +_Alignas (float) char cf;
> +_Alignas (double) char cd;
> +_Alignas (long double) char cld;
> +
> +_Alignas (char) short int sc; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) short int ss;
> +_Alignas (int) short int si;
> +_Alignas (long int) short int sl;
> +_Alignas (long long int) short int sll;
> +_Alignas (float) short int sf;
> +_Alignas (double) short int sd;
> +_Alignas (long double) short int sld;
> +
> +_Alignas (char) int ic; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) int is; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (int) int ii;
> +_Alignas (long int) int il;
> +_Alignas (long long int) int ill;
> +_Alignas (float) int if_;
> +_Alignas (double) int id;
> +_Alignas (long double) int ild;
> +
> +_Alignas (char) long int lic; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) long int lis; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (int) long int lii; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (long int) long int lil;
> +_Alignas (long long int) long int lill;
> +_Alignas (float) long int lif; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (double) long int lid;
> +_Alignas (long double) long int lild;
> +
> +_Alignas (char) long long int llic; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) long long int llis; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (int) long long int llii; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (long int) long long int llil;
> +_Alignas (long long int) long long int llill;
> +_Alignas (float) long long int llif; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (double) long long int llid;
> +_Alignas (long double) long long int llild;
> +
> +_Alignas (char) float fc; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) float fs; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (int) float fi;
> +_Alignas (long int) float fl;
> +_Alignas (long long int) float fll;
> +_Alignas (float) float ff;
> +_Alignas (double) float fd;
> +_Alignas (long double) float fld;
> +
> +_Alignas (char) double dc; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) double ds; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (int) double di; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (long int) double dl;
> +_Alignas (long long int) double dll;
> +_Alignas (float) double df; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (double) double dd;
> +_Alignas (long double) double dld;
> +
> +_Alignas (char) long double ldc; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (short int) long double lds; /* { dg-error "cannot reduce alignment" } */
> +_Alignas (int) long double ldi; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (long int) long double ldl; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (long long int) long double ldll; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (float) long double ldf; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (double) long double ldd; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
> +_Alignas (long double) long double ldld;
>
>         Marek
Jakub Jelinek - May 6, 2014, 8:14 a.m.
On Tue, May 06, 2014 at 10:04:33AM +0200, Richard Biener wrote:
> On Mon, May 5, 2014 at 10:27 PM, Marek Polacek <polacek@redhat.com> wrote:
> > In this PR the issue is that we reject (valid) code such as
> > _Alignas (long long) long long foo;
> > with -m32, because we trip this condition:
> >
> >    alignas_align = 1U << declspecs->align_log;
> >    if (alignas_align < TYPE_ALIGN_UNIT (type))
> >      {
> >        if (name)
> >          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
> >                    "alignment of %qE", name);
> >
> > and error later on, since alignas_align is 4 (correct, see PR52023 for
> > why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> > is wrong here as that won't give us minimal alignment required.
> > In c_sizeof_or_alignof_type we already have the code to compute such
> > minimal alignment so I just moved the code to a separate function
> > and used that instead of TYPE_ALIGN_UNIT.
> 
> Hmm, but isn't TYPE_ALIGN_UNIT wrong then?

No, after all, you don't want to change __alignof__ (long long), that is
pretty essential part of ABI.

	Jakub
Marek Polacek - May 6, 2014, 8:58 a.m.
On Tue, May 06, 2014 at 10:14:42AM +0200, Jakub Jelinek wrote:
> > Hmm, but isn't TYPE_ALIGN_UNIT wrong then?
> 
> No, after all, you don't want to change __alignof__ (long long), that is
> pretty essential part of ABI.

Yeah, as I understand things, for x86_64 __alignof__ (T) returns
*preferred* alignment which is specified for each type in the ABI
(Figure 3.1: Scalar Types), and is the same as sizeof (T).  But
_Alignof (T) (thus also _Alignas (T)) is the *least* alignment for
a type -- and they can differ if the type is in a struct/union.
Please correct me if I'm mistaken.

	Marek
Joseph S. Myers - May 7, 2014, 5:15 p.m.
On Mon, 5 May 2014, Marek Polacek wrote:

> In this PR the issue is that we reject (valid) code such as
> _Alignas (long long) long long foo;
> with -m32, because we trip this condition:
> 
>    alignas_align = 1U << declspecs->align_log;
>    if (alignas_align < TYPE_ALIGN_UNIT (type))
>      {
>        if (name)
>          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>                    "alignment of %qE", name);
> 
> and error later on, since alignas_align is 4 (correct, see PR52023 for
> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> is wrong here as that won't give us minimal alignment required.
> In c_sizeof_or_alignof_type we already have the code to compute such
> minimal alignment so I just moved the code to a separate function
> and used that instead of TYPE_ALIGN_UNIT.
> 
> Note that the test is run only on i?86 and x86_64, because we can't (?)
> easily determine which target requires what alignment.
> 
> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
> powerpc64-unknown-linux-gnu, ok for trunk?

OK, though I'm not sure if the "lp64" conditions are right in the testcase 
(i.e. if x32 has the same peculiarity as -m32 here, which is what's 
implied by the use of "lp64").
H.J. Lu - May 7, 2014, 6:31 p.m.
On Wed, May 7, 2014 at 10:15 AM, Joseph S. Myers
<joseph@codesourcery.com> wrote:
> On Mon, 5 May 2014, Marek Polacek wrote:
>
>> In this PR the issue is that we reject (valid) code such as
>> _Alignas (long long) long long foo;
>> with -m32, because we trip this condition:
>>
>>    alignas_align = 1U << declspecs->align_log;
>>    if (alignas_align < TYPE_ALIGN_UNIT (type))
>>      {
>>        if (name)
>>          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>>                    "alignment of %qE", name);
>>
>> and error later on, since alignas_align is 4 (correct, see PR52023 for
>> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
>> is wrong here as that won't give us minimal alignment required.
>> In c_sizeof_or_alignof_type we already have the code to compute such
>> minimal alignment so I just moved the code to a separate function
>> and used that instead of TYPE_ALIGN_UNIT.
>>
>> Note that the test is run only on i?86 and x86_64, because we can't (?)
>> easily determine which target requires what alignment.
>>
>> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
>> powerpc64-unknown-linux-gnu, ok for trunk?
>
> OK, though I'm not sure if the "lp64" conditions are right in the testcase

It should be !ia32 instead of lp64.

> (i.e. if x32 has the same peculiarity as -m32 here, which is what's
> implied by the use of "lp64").
>

Alignments of long long and long double on x32 are the same as x86-64.
Marek Polacek - May 8, 2014, 6:19 p.m.
On Wed, May 07, 2014 at 11:31:38AM -0700, H.J. Lu wrote:
> > OK, though I'm not sure if the "lp64" conditions are right in the testcase
> 
> It should be !ia32 instead of lp64.

Ok, I changed lp64 to ! { ia32 } and committed the patch now.

	Marek
Marek Polacek - May 25, 2014, 9:30 a.m.
On Mon, May 05, 2014 at 10:27:03PM +0200, Marek Polacek wrote:
> In this PR the issue is that we reject (valid) code such as
> _Alignas (long long) long long foo;
> with -m32, because we trip this condition:
> 
>    alignas_align = 1U << declspecs->align_log;
>    if (alignas_align < TYPE_ALIGN_UNIT (type))
>      {
>        if (name)
>          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>                    "alignment of %qE", name);
> 
> and error later on, since alignas_align is 4 (correct, see PR52023 for
> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> is wrong here as that won't give us minimal alignment required.
> In c_sizeof_or_alignof_type we already have the code to compute such
> minimal alignment so I just moved the code to a separate function
> and used that instead of TYPE_ALIGN_UNIT.
> 
> Note that the test is run only on i?86 and x86_64, because we can't (?)
> easily determine which target requires what alignment.
> 
> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
> powerpc64-unknown-linux-gnu, ok for trunk?

Can I backport this one to 4.9?

	Marek
Marek Polacek - June 2, 2014, 6:52 a.m.
Ping.

On Sun, May 25, 2014 at 11:30:51AM +0200, Marek Polacek wrote:
> On Mon, May 05, 2014 at 10:27:03PM +0200, Marek Polacek wrote:
> > In this PR the issue is that we reject (valid) code such as
> > _Alignas (long long) long long foo;
> > with -m32, because we trip this condition:
> > 
> >    alignas_align = 1U << declspecs->align_log;
> >    if (alignas_align < TYPE_ALIGN_UNIT (type))
> >      {
> >        if (name)
> >          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
> >                    "alignment of %qE", name);
> > 
> > and error later on, since alignas_align is 4 (correct, see PR52023 for
> > why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> > is wrong here as that won't give us minimal alignment required.
> > In c_sizeof_or_alignof_type we already have the code to compute such
> > minimal alignment so I just moved the code to a separate function
> > and used that instead of TYPE_ALIGN_UNIT.
> > 
> > Note that the test is run only on i?86 and x86_64, because we can't (?)
> > easily determine which target requires what alignment.
> > 
> > Regtested/bootstrapped on x86_64-unknown-linux-gnu and
> > powerpc64-unknown-linux-gnu, ok for trunk?
> 
> Can I backport this one to 4.9?

	Marek
Jeff Law - June 4, 2014, 7:17 a.m.
On 06/02/14 00:52, Marek Polacek wrote:
> Ping.
>
> On Sun, May 25, 2014 at 11:30:51AM +0200, Marek Polacek wrote:
>> On Mon, May 05, 2014 at 10:27:03PM +0200, Marek Polacek wrote:
>>> In this PR the issue is that we reject (valid) code such as
>>> _Alignas (long long) long long foo;
>>> with -m32, because we trip this condition:
>>>
>>>     alignas_align = 1U << declspecs->align_log;
>>>     if (alignas_align < TYPE_ALIGN_UNIT (type))
>>>       {
>>>         if (name)
>>>           error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>>>                     "alignment of %qE", name);
>>>
>>> and error later on, since alignas_align is 4 (correct, see PR52023 for
>>> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
>>> is wrong here as that won't give us minimal alignment required.
>>> In c_sizeof_or_alignof_type we already have the code to compute such
>>> minimal alignment so I just moved the code to a separate function
>>> and used that instead of TYPE_ALIGN_UNIT.
>>>
>>> Note that the test is run only on i?86 and x86_64, because we can't (?)
>>> easily determine which target requires what alignment.
>>>
>>> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
>>> powerpc64-unknown-linux-gnu, ok for trunk?
>>
>> Can I backport this one to 4.9?
Yes, backporting to 4.9 is fine.
jeff

Patch

diff --git gcc/c-family/c-common.c gcc/c-family/c-common.c
index 0ad955d..6d4440e 100644
--- gcc/c-family/c-common.c
+++ gcc/c-family/c-common.c
@@ -4931,6 +4931,26 @@  c_common_get_alias_set (tree t)
   return -1;
 }
 
+/* Return the least alignment required for type TYPE.  */
+
+unsigned int
+min_align_of_type (tree type)
+{
+  unsigned int align = TYPE_ALIGN (type);
+  align = MIN (align, BIGGEST_ALIGNMENT);
+#ifdef BIGGEST_FIELD_ALIGNMENT
+  align = MIN (align, BIGGEST_FIELD_ALIGNMENT);
+#endif
+  unsigned int field_align = align;
+#ifdef ADJUST_FIELD_ALIGN
+  tree field = build_decl (UNKNOWN_LOCATION, FIELD_DECL, NULL_TREE,
+			   type);
+  field_align = ADJUST_FIELD_ALIGN (field, field_align);
+#endif
+  align = MIN (align, field_align);
+  return align / BITS_PER_UNIT;
+}
+
 /* Compute the value of 'sizeof (TYPE)' or '__alignof__ (TYPE)', where
    the IS_SIZEOF parameter indicates which operator is being applied.
    The COMPLAIN flag controls whether we should diagnose possibly
@@ -5009,21 +5029,7 @@  c_sizeof_or_alignof_type (location_t loc,
 				size_int (TYPE_PRECISION (char_type_node)
 					  / BITS_PER_UNIT));
       else if (min_alignof)
-	{
-	  unsigned int align = TYPE_ALIGN (type);
-	  align = MIN (align, BIGGEST_ALIGNMENT);
-#ifdef BIGGEST_FIELD_ALIGNMENT
-	  align = MIN (align, BIGGEST_FIELD_ALIGNMENT);
-#endif
-	  unsigned int field_align = align;
-#ifdef ADJUST_FIELD_ALIGN
-	  tree field = build_decl (UNKNOWN_LOCATION, FIELD_DECL, NULL_TREE,
-				   type);
-	  field_align = ADJUST_FIELD_ALIGN (field, field_align);
-#endif
-	  align = MIN (align, field_align);
-	  value = size_int (align / BITS_PER_UNIT);
-	}
+	value = size_int (min_align_of_type (type));
       else
 	value = size_int (TYPE_ALIGN_UNIT (type));
     }
diff --git gcc/c-family/c-common.h gcc/c-family/c-common.h
index 57b7dce..d34d2bb 100644
--- gcc/c-family/c-common.h
+++ gcc/c-family/c-common.h
@@ -758,6 +758,7 @@  extern tree c_wrap_maybe_const (tree, bool);
 extern tree c_save_expr (tree);
 extern tree c_common_truthvalue_conversion (location_t, tree);
 extern void c_apply_type_quals_to_decl (int, tree);
+extern unsigned int min_align_of_type (tree);
 extern tree c_sizeof_or_alignof_type (location_t, tree, bool, bool, int);
 extern tree c_alignof_expr (location_t, tree);
 /* Print an error message for invalid operands to arith operation CODE.
diff --git gcc/c/c-decl.c gcc/c/c-decl.c
index 6e7c589..9f7419a 100644
--- gcc/c/c-decl.c
+++ gcc/c/c-decl.c
@@ -5931,7 +5931,7 @@  grokdeclarator (const struct c_declarator *declarator,
       else if (declspecs->align_log != -1)
 	{
 	  alignas_align = 1U << declspecs->align_log;
-	  if (alignas_align < TYPE_ALIGN_UNIT (type))
+	  if (alignas_align < min_align_of_type (type))
 	    {
 	      if (name)
 		error_at (loc, "%<_Alignas%> specifiers cannot reduce "
diff --git gcc/testsuite/gcc.dg/pr61053.c gcc/testsuite/gcc.dg/pr61053.c
index e69de29..e3db534 100644
--- gcc/testsuite/gcc.dg/pr61053.c
+++ gcc/testsuite/gcc.dg/pr61053.c
@@ -0,0 +1,75 @@ 
+/* PR c/61053 */
+/* { dg-do compile { target i?86-*-* x86_64-*-* } } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+_Alignas (char) char cc;
+_Alignas (short int) char cs;
+_Alignas (int) char ci;
+_Alignas (long int) char cl;
+_Alignas (long long int) char cll;
+_Alignas (float) char cf;
+_Alignas (double) char cd;
+_Alignas (long double) char cld;
+
+_Alignas (char) short int sc; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) short int ss;
+_Alignas (int) short int si;
+_Alignas (long int) short int sl;
+_Alignas (long long int) short int sll;
+_Alignas (float) short int sf;
+_Alignas (double) short int sd;
+_Alignas (long double) short int sld;
+
+_Alignas (char) int ic; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) int is; /* { dg-error "cannot reduce alignment" } */
+_Alignas (int) int ii;
+_Alignas (long int) int il;
+_Alignas (long long int) int ill;
+_Alignas (float) int if_;
+_Alignas (double) int id;
+_Alignas (long double) int ild;
+
+_Alignas (char) long int lic; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) long int lis; /* { dg-error "cannot reduce alignment" } */
+_Alignas (int) long int lii; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (long int) long int lil;
+_Alignas (long long int) long int lill;
+_Alignas (float) long int lif; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (double) long int lid;
+_Alignas (long double) long int lild;
+
+_Alignas (char) long long int llic; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) long long int llis; /* { dg-error "cannot reduce alignment" } */
+_Alignas (int) long long int llii; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (long int) long long int llil;
+_Alignas (long long int) long long int llill;
+_Alignas (float) long long int llif; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (double) long long int llid;
+_Alignas (long double) long long int llild;
+
+_Alignas (char) float fc; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) float fs; /* { dg-error "cannot reduce alignment" } */
+_Alignas (int) float fi;
+_Alignas (long int) float fl;
+_Alignas (long long int) float fll;
+_Alignas (float) float ff;
+_Alignas (double) float fd;
+_Alignas (long double) float fld;
+
+_Alignas (char) double dc; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) double ds; /* { dg-error "cannot reduce alignment" } */
+_Alignas (int) double di; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (long int) double dl;
+_Alignas (long long int) double dll;
+_Alignas (float) double df; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (double) double dd;
+_Alignas (long double) double dld;
+
+_Alignas (char) long double ldc; /* { dg-error "cannot reduce alignment" } */
+_Alignas (short int) long double lds; /* { dg-error "cannot reduce alignment" } */
+_Alignas (int) long double ldi; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (long int) long double ldl; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (long long int) long double ldll; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (float) long double ldf; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (double) long double ldd; /* { dg-error "cannot reduce alignment" "" { target { lp64 } } } */
+_Alignas (long double) long double ldld;