https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84234
Bug ID: 84234
Summary: #pragma omp declare simd is ignored
Product: gcc
Version: 7.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc.account at lemaitre dot re
Target Milestone: ---
Created attachment 43344
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43344&action=edit
Simple example showing the bug: gcc -O3 -fopenmp-simd
When I try to use #pragma omp declare simd on a forward declaration, it seems
to be ignored during vectorization at the call site.
ex:
#pragma omp declare simd
float add2(float a, float b);
void ADD2() {
for (int i = 0; i < 1024; i++) {
A[i] = add2(A[i], B[i]);
}
}
is compiled into:
ADD2:
.LFB2:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
xorl %ebx, %ebx
.p2align 4,,10
.p2align 3
.L19:
movss B(%rbx), %xmm1
addq $4, %rbx
movss A-4(%rbx), %xmm0
call add2
movss %xmm0, A-4(%rbx)
cmpq $4096, %rbx
jne .L19
popq %rbx
.cfi_def_cfa_offset 8
ret
.cfi_endproc
where
#pragma omp declare simd
float __attribute((noinline)) add1(float a, float b) {
return a+b;
}
void ADD1() {
for (int i = 0; i < 1024; i++) {
A[i] = add1(A[i], B[i]);
}
}
is compiled into:
ADD1:
.LFB1:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
xorl %ebx, %ebx
.p2align 4,,10
.p2align 3
.L15:
movaps A(%rbx), %xmm0
addq $16, %rbx
movaps B-16(%rbx), %xmm1
call _ZGVbN4vv_add1
movaps %xmm0, A-16(%rbx)
cmpq $4096, %rbx
jne .L15
popq %rbx
.cfi_def_cfa_offset 8
ret
.cfi_endproc
When the function has no definition, the compiler doesn't use the vectorized
variant of the function.
This also happens if one tries to give the definition of the function, but
defines the symbol as weak.
This is really annoying as we have to put the definition of such a function
within the same translation unit as it uses, with all problems that might
occur.
This bug is present on all gcc versions I tested, namely: GCC 4.9 x86, GCC 5.5
x86, GCC 6.4 x86, GCC 7.3 x86 and GCC trunk x86 (from godbolt.org).
On other architectures, the pragma seems to be always ignored, even when a
definition is available (GCC 7.2 ARM, GCC 6.3 AARCH64, GCC 6.3 PPC64).
For information, this works as expected on ICC.
