https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85466
James Greenhalgh <jgreenhalgh at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |jgreenhalgh at gcc dot gnu.org
--- Comment #3 from James Greenhalgh <jgreenhalgh at gcc dot gnu.org> ---
Created attachment 43988
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43988&action=edit
Reduced testcase
I believe this testcase shows the issue being reported here. Clang seems to
spot this is essentially a memset across the array, while GCC doesn't.
On AArch64 with Clang:
.LBB1_9: // =>This Inner Loop Header: Depth=1
stp q0, q0, [x8, #-16]
subs x20, x20, #8 // =8
add x8, x8, #32 // =32
b.ne .LBB1_9
On x86-64 with Clang:
.LBB1_9: # =>This Inner Loop Header: Depth=1
movups %xmm0, -144(%rax,%rcx,4)
movups %xmm0, -128(%rax,%rcx,4)
movups %xmm0, -112(%rax,%rcx,4)
movups %xmm0, -96(%rax,%rcx,4)
movups %xmm0, -80(%rax,%rcx,4)
movups %xmm0, -64(%rax,%rcx,4)
movups %xmm0, -48(%rax,%rcx,4)
movups %xmm0, -32(%rax,%rcx,4)
movups %xmm0, -16(%rax,%rcx,4)
movups %xmm0, (%rax,%rcx,4)
addq $40, %rcx
cmpq $100036, %rcx # imm = 0x186C4
jne .LBB1_9
GCC doesn't spot this.
On the other hand G++'s inlining of the various random number initialisation
routines really hammers Clang, which ends up emulating 128-bit arithmetic on
AArch64.
