https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85466
--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to James Greenhalgh from comment #3) > Created attachment 43988 [details] > 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. I bet clang is clever on the undefinedness I pointed out. > 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.
