https://bugs.llvm.org/show_bug.cgi?id=40844
Bug ID: 40844
Summary: Unneeded stack alignment on x86
Product: libraries
Version: 7.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
Ref https://bugs.llvm.org/show_bug.cgi?id=40843. The useless stack operations
are likely caused by a different issue...
Reproducible with C code,
```
typedef double _vdouble __attribute__((vector_size(32)));
typedef struct {
_vdouble x;
int y;
} vdouble;
vdouble f(vdouble x, vdouble y)
{
vdouble res = {x.x + y.x, x.y + y.y};
return res;
}
```
Compiling with clang with `-mavx` emits very similar code to GCC
```
f: # @f
# %bb.0:
pushq %rbp
movq %rsp, %rbp
andq $-32, %rsp
subq $32, %rsp
vmovapd 16(%rbp), %ymm0
vaddpd 80(%rbp), %ymm0, %ymm0
vmovapd %ymm0, (%rdi)
movl 112(%rbp), %eax
addl 48(%rbp), %eax
movl %eax, 32(%rdi)
movq %rdi, %rax
movq %rbp, %rsp
popq %rbp
vzeroupper
retq
```
except that it emits 6 additional instructions for stack realignment without
actually using the stack pointer anywhere else in the function.
Seems to be a x86 backend issue since I can reproduce even without enabling avx
(so nothing in the backend should require 32bit alignment, only the frontend
should) and I can't reproduce on arm or aarch64.
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