https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89597
Bug ID: 89597
Summary: Inconsistent vector calling convention on windows with
Clang and MSVC
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: yyc1992 at gmail dot com
Target Milestone: ---
For 256bit and 512bit vector return values, Clang and MSVC always returns them
in the corresponding registers even without `__vectorcall`. GCC, however,
returns the value as reference. Together with the missing support of
`__vectorcall`[1], this means that the code GCC generate for functions that
returns vector value is not compatible with any other compilers. The problem
does not exist for 128bit vectors.
Test case.
```
typedef double vdouble __attribute__((vector_size(32)));
vdouble f(vdouble x, vdouble y)
{
return x + y;
}
```
GCC compiles this to,
```
f:
vmovapd (%r8), %ymm0
vaddpd (%rdx), %ymm0, %ymm0
movq %rcx, %rax
vmovapd %ymm0, (%rcx)
vzeroupper
ret
```
Clang compiles this to,
```
f:
vmovapd (%rcx), %ymm0
vaddpd (%rdx), %ymm0, %ymm0
retq
```
Given the stack alignment issue[2], I wonder if this can be fixed now without
breaking anyone's code. (i.e. everyone that's using it is probably broken
anyway due to the other bug...)
Disclaimer. I did all my test with clang. I believe MSVC behaves the same from
the compiled result I got from someone else and I don't have MSVC to personally
test it.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89485
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54412
