[llvm-dev] Incompatible x86 calling convention for __m64 parameters and returns.

[Hal Finkel via llvm-dev]

On 03/09/2017 09:51 AM, Brad Conroy via llvm-dev wrote:
> On x86 (32 bit) GCC passes mmx parameters in mm0, mm1 and mm2 and returns in mm0.
> According to comments in X86CallingConv.td, llvm supposedly does the same:
>
>>   // MMX vector types are always returned in MM0. If the target doesn't have
>>   // MM0, it doesn't support these vector types.
>>   CCIfType<[x86mmx], CCAssignToReg<[MM0]>>,
> and
>
>>   // The first 3 __m64 vector arguments are passed in mmx registers if the
>>   // call is not a vararg call.
>>   CCIfNotVarArg<CCIfType<[x86mmx],
>>                 CCAssignToReg<[MM0, MM1, MM2]>>>,
> However in practice, llvm is actually passing parameters on the stack and returning them
> in the eax:edx register pair as if they were long long or unsigned long long.
>
> This goes back to at least clang-3.0, but I couldn't find a bug report on it... maybe because
> mmx intrinsics are rarely used, so it may have no real world implications.

You're probably right about the intrinsics being rarely used (at this 
point), but filing a bug report sounds like the right thing to do.

  -Hal

>
> R,
> Brad Conroy
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory