[llvm] r175576 - [ms-inline asm] Force the use of a base pointer if the MachineFunction includes

[Reid Kleckner]

I sent this patch but IMO it needs a fatal error:
http://llvm-reviews.chandlerc.com/D1317

I haven't figured out where it would go yet.


On Thu, Aug 8, 2013 at 10:56 AM, Chad Rosier <chad.rosier at gmail.com> wrote:

> That sounds like a reasonable step forward.  Inline assembly does have a
> diagnostics handler, so we should prefer the use of that over reporting the
> use of a fatal error.  Unfortunately, I'm off breaking other parts of the
> compiler, so I don't know if or when I would be able to get to this.
> Patches are welcome!
>
>  Chad
>
>
> On Thu, Aug 8, 2013 at 1:42 PM, Reid Kleckner <rnk at google.com> wrote:
>
>> gcc will try really hard to find a free register from which to address
>> your stack variables, starting with esp, and then falling back to any free
>> register.  If you clobber every register except for ebp *and* require stack
>> realignment, it errors out.
>>
>> If you remove any clobber from the following example, it will compile.
>>
>> $ cat realign_inline_asm.c
>> int foo(int a) {
>>   __attribute__((aligned(16))) int r = -1;
>>   asm volatile ("push %%edi\n\t"
>>                 "xor %%eax, %%eax\n\t"
>>                 "xor %%ebx, %%ebx\n\t"
>>                 "xor %%ecx, %%ecx\n\t"
>>                 "xor %%edx, %%edx\n\t"
>>                 "xor %%esi, %%esi\n\t"
>>                 "mov %0, %%eax\n\t"
>>                 "pop %%edi\n\t"
>>                 "xor %%edi, %%edi\n\t"
>>                 : "=m"(r) /* out */
>>                 : /* in */
>>                 : "eax", "ebx", "ecx", "edx", "esi", "edi", "esp");
>>   return r;
>> }
>>
>> $ gcc -m32 -c realign_inline_asm.c -S -o -  -mstackrealign
>>         .file   "realign_inline_asm.c"
>> realign_inline_asm.c: In function ‘foo’:
>> realign_inline_asm.c:3:3: error: can’t find a register in class
>> ‘GENERAL_REGS’ while reloading ‘asm’
>>
>> For now, at least, it would be nice to report a fatal error when we want
>> to use esi for the BP, but we have inline asm that clobbers it.  We already
>> have the clobber list with that info.
>>
>>
>> On Thu, Aug 8, 2013 at 7:39 AM, Chad Rosier <chad.rosier at gmail.com>wrote:
>>
>>>
>>> On Wed, Aug 7, 2013 at 8:38 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:
>>>
>>>> On Aug 7, 2013, at 5:17 PM, Reid Kleckner <rnk at google.com> wrote:
>>>>
>>>> > Wouldn't it be much better to use ebp for the base pointer and put
>>>> the pre-alignment SP into a local that could be spilled or register
>>>> allocated as necessary?  Is that just a matter of work?
>>>>
>>>> I think stack unwinders would expect to find the frame pointer in %ebp.
>>>>
>>>> /jakob
>>>
>>>
>>> Unfortunately, I can't think of a simple solution here.  How does gcc
>>> handle this case?  IIRC, gcc was the original motivating factor for forcing
>>> the use of the FP?
>>>
>>>  Chad
>>>
>>
>>
>
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