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Date: Sun, 5 Jan 2020 15:05:41 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] math: move x86_64 fabs, fabsf to C with inline asm
On Sun, Jan 05, 2020 at 07:36:39PM +0300, Alexander Monakov wrote:
> ---
>
> Questions:
>
> Why are there amd64-specific fabs implementations in the first place?
> (Only) because GCC generated poor code for the generic C version?
I think so. It generates:
0000000000000000 <fabs>:
0: 66 48 0f 7e c0 movq %xmm0,%rax
5: 48 ba ff ff ff ff ff movabs $0x7fffffffffffffff,%rdx
c: ff ff 7f
f: 48 21 d0 and %rdx,%rax
12: 48 89 44 24 f8 mov %rax,-0x8(%rsp)
17: f2 0f 10 44 24 f8 movsd -0x8(%rsp),%xmm0
1d: c3 retq
> Do annotations for mask manipulation in the patch help? Any way to make
> them less ambiguous?
I think so. I like how you did individual asm statements with
dependency relationship between them so compiler could even schedule
them if it likes. I wonder if you could just write 0x7fffffffffffffff
as an operand and have the compiler load it, though.
Rich
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