[Bug libstdc++/85466] Performance is slow when doing 'branchless' conditional style math operations

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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.