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PATCH: Properly generate X32 IE sequence

Message ID CAMe9rOpehoDqxVwBFA7oOpKp3+BvJW+DJcZ6svDu2Wz8Wb5LeA@mail.gmail.com
State New
Headers show

Commit Message

H.J. Lu March 11, 2012, 9:24 p.m. UTC 
On Sun, Mar 11, 2012 at 11:21 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Sun, Mar 11, 2012 at 7:16 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>
>>>>>>>        * config/i386/i386.c (ix86_decompose_address): Disallow fs:(reg)
>>>>>>>        if Pmode != word_mode.
>>>>>>>        (legitimize_tls_address): Call gen_tls_initial_exec_x32 if
>>>>>>>        Pmode == SImode for x32.
>>>>>>>
>>>>>>>        * config/i386/i386.md (UNSPEC_TLS_IE_X32): New.
>>>>>>>        (tls_initial_exec_x32): Likewise.
>>>>>>
>>>>>> Nice solution!
>>>>>>
>>>>>> OK for mainline.
>>>>>
>>>>> Done.
>>>>>
>>>>>> BTW: Did you investigate the issue with memory aliasing?
>>>>>>
>>>>>
>>>>> It isn't a problem since it is wrapped in UNSPEC_TLS_IE_X32
>>>>> which loads address of the TLS symbol.
>>>>>
>>>>> Thanks.
>>>>>
>>>>
>>>> Since we must use reg64 in %fs:(%reg) memory operand like
>>>>
>>>> movq x@gottpoff(%rip),%reg64;
>>>> mov %fs:(%reg64),%reg
>>>>
>>>> this patch optimizes x32 TLS IE load and store by wrapping
>>>> %reg64 inside of UNSPEC when Pmode == SImode.  OK for
>>>> trunk?
>>>
>>> I think we should just scrap all these complications and go with the
>>> idea of clearing MASK_TLS_DIRECT_SEG_REFS.
>>>
>>
>> I will give it a try.
>
> You can also revert:
>
>>>>>>>        * config/i386/i386.c (ix86_decompose_address): Disallow fs:(reg)
>>>>>>>        if Pmode != word_mode.
>
> then, since this part is handled later in the function.
>

Here is the patch which is equivalent to clearing MASK_TLS_DIRECT_SEG_REFS
when Pmode != word_mode.  We need to keep

          else if (Pmode == SImode)
            {
              /* Always generate
                        movl %fs:0, %reg32
                        addl xgottpoff(%rip), %reg32
                 to support linker IE->LE optimization and avoid
                 fs:(%reg32) as memory operand.  */
              dest = gen_reg_rtx (Pmode);
              emit_insn (gen_tls_initial_exec_x32 (dest, x));
              return dest;
            }

to support linker IE->LE optimization.  TARGET_TLS_DIRECT_SEG_REFS only affects
TLS LE access and fs:(%reg) is only generated by combine.

So the main impact of disabling TARGET_TLS_DIRECT_SEG_REFS is to disable
fs:immediate memory operand for TLS LE access, which doesn't have any problems
to begin with.

I would prefer to keep TARGET_TLS_DIRECT_SEG_REFS and disable only
fs:(%reg), which is generated by combine.
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Patch

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index b101922..1ffcc85 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -11478,6 +11478,7 @@  ix86_decompose_address (rtx addr, struct
ix86_address *out)

 	    case UNSPEC:
 	      if (XINT (op, 1) == UNSPEC_TP
+		  && Pmode == word_mode
 	          && TARGET_TLS_DIRECT_SEG_REFS
 	          && seg == SEG_DEFAULT)