[LLVMdev] ABI incompatability when passing vector parameters on 32-bit x86

[Bob Wilson]

I agree with Reid and do not think we should change the ABI for Darwin.

> On Dec 22, 2014, at 10:37 PM, Kuperstein, Michael M <michael.m.kuperstein at intel.com> wrote:
> 
> Ping?
>  
> Given that nobody objected so far, I’m considering changing it for all platforms, but I’d really like to get some feedback before I make an ABI-breaking change. ;-)
>   <>
> Michael
>  
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Kuperstein, Michael M
> Sent: Monday, December 15, 2014 16:43
> To: LLVMdev at cs.uiuc.edu
> Subject: [LLVMdev] ABI incompatability when passing vector parameters on 32-bit x86
>  
> Hi all,
>  
> Recently, Reid Kleckner found an ABI incompatibility between clang and GCC in the way vector parameters are passed on 32-bit x86.
> (This is documented in PR21510.)
> 
> Specifically, GCC uses XMM0-XMM2 to pass the first 3 __m128 parameters, and the rest are passed on the stack. Clang passes an additional parameter by register, using XMM0-XMM3. The same applies to __m256 with YMM0-2 vs. YMM0-3. In theory, it would apply to __m512 as well, but currently clang doesn’t support passing __m512 in x86 mode at all. ICC has the same behavior as GCC, and it seems that MSVC in 32-bit mode only *allows* up to 3 vector parameters per function (when not using __vectorcall), and these 3 are passed in XMM0-XMM2, which is closer to the GCC behavior. 
>  
> Unfortunately, it seems like there is no ABI specification to support either behavior as “correct”: while the x32 (“ILP32”) ABI explicitly specifies XMM0-XMM2, the latest version of the i386 psABI is too old to contain any useful information.
> Still, XMM0-XMM2 looks like the common choice, and I think the current clang behavior should be considered a bug.
>  
> The problem is that, regardless of whether it’s a bug or not, this behavior has been in place for many years, and changing it would mean breaking ABI compatibility with older clang versions.
> On the other hand, not changing it would mean continued ABI incompatibility with GCC.
> (This only applies to _m128 and _m256. Making the _m512 behavior GCC-compatible should be painless).
>  
> Reid (and I hope I’m not misrepresenting him here) suggested leaving the behavior as-is on platforms where clang is the system compiler (Darwin and BSD) and changing it elsewhere. However, I’m afraid interpreting the calling convention differently (compatible / incompatible with GCC) on different platforms may be confusing to end-uses.
>  
> Any thoughts on this, especially from OS/libraries people, will be very appreciated.
>  
> Thanks,
>    Michael
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