https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96275
--- Comment #1 from Witold Baryluk <witold.baryluk+gcc at gmail dot com> ---
FYI.
clang trunk 12 / 76a0c0ee6ffa9c38485776921948d8f930109674, doesn't do that
either:
fillArray: # @fillArray
test dil, 31
jne .LBB0_8
test edi, edi
je .LBB0_8
vmovss xmm0, dword ptr [rdx] # xmm0 = mem[0],zero,zero,zero
mov eax, edi
cmp edi, 32
jae .LBB0_4
xor edx, edx
jmp .LBB0_7
.LBB0_4:
vbroadcastss ymm1, xmm0
mov edx, eax
xor edi, edi
and edx, -32
.LBB0_5: # =>This Inner Loop Header: Depth=1
vmulps ymm2, ymm1, ymmword ptr [rcx + 4*rdi]
vmulps ymm3, ymm1, ymmword ptr [rcx + 4*rdi + 32]
vmulps ymm4, ymm1, ymmword ptr [rcx + 4*rdi + 64]
vmulps ymm5, ymm1, ymmword ptr [rcx + 4*rdi + 96]
vmovups ymmword ptr [rsi + 4*rdi], ymm2
vmovups ymmword ptr [rsi + 4*rdi + 32], ymm3
vmovups ymmword ptr [rsi + 4*rdi + 64], ymm4
vmovups ymmword ptr [rsi + 4*rdi + 96], ymm5
add rdi, 32
cmp rdx, rdi
jne .LBB0_5
cmp rdx, rax
je .LBB0_8
.LBB0_7: # =>This Inner Loop Header: Depth=1
vmulss xmm1, xmm0, dword ptr [rcx + 4*rdx]
vmovss dword ptr [rsi + 4*rdx], xmm1
inc rdx
cmp rax, rdx
jne .LBB0_7
.LBB0_8:
vzeroupper
ret
the main inner loop is unrolled / pipelined more aggressively, and the fallback
code is simpler (just handle scalars scalarly), which is unrelated. But the
fallback code is still there.
Changing to different variations of the condition, like `if ((N/32)*32 == N)
{`, `if ((N % 32) == 0) {`, `if ((N & ~31u) == N) {`, `if ((N >> 5) << 5 == N)
{`, doesn't make any difference.
I tried with signed int, and unsigned int. Same effect.
Reassigning to N (after removing constness), i.e. `N = N & ~31u`, or `N = (N >>
5) << 5`, does appear to do something, but if it is inside the condition it is
already too late.
