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<p class="MsoNormal">Hi all,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Recently, Reid Kleckner found an ABI incompatibility between clang and GCC in the way vector parameters are passed on 32-bit x86.
<o:p></o:p></p>
<p class="MsoNormal">(This is documented in PR21510.)<o:p></o:p></p>
<p class="MsoNormal"><br>
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. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.
<o:p></o:p></p>
<p class="MsoNormal">Still, XMM0-XMM2 looks like the common choice, and I think the current clang behavior should be considered a bug.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.
<o:p></o:p></p>
<p class="MsoNormal">On the other hand, not changing it would mean continued ABI incompatibility with GCC.
<o:p></o:p></p>
<p class="MsoNormal">(This only applies to _m128 and _m256. Making the _m512 behavior GCC-compatible should be painless).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Any thoughts on this, especially from OS/libraries people, will be very appreciated.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal"> Michael<o:p></o:p></p>
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