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Date: Tue, 3 Aug 2021 22:27:35 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Stefan Kanthak <stefan.kanthak@...go.de>
Cc: musl@...ts.openwall.com
Subject: Re: [Patch] src/math/i386/remquo.s: remove conditional
branch, shorter bit twiddling
* Stefan Kanthak <stefan.kanthak@...go.de> [2021-08-01 17:59:52 +0200]:
> Halve the number of instructions (from 12 to 6) to fetch the
> (3-bit partial) quotient from the FPU flags C0:C3:C1, and
> perform its negation without conditional branch.
i haven't tested it but it looks good.
i think we should not tweak x87 asm code too much though.
it can introduce bugs and there are not many users of it.
i think only the size saving can justify keeping any i386
math code at all.
but i'm not against committing this.
thanks for the patch.
> --- -/math/i386/remquo.s
> +++ +/math/i386/remquo.s
> @@ -2,49 +2,44 @@
> .type remquof,@function
> remquof:
> mov 12(%esp),%ecx
> + mov 8(%esp),%eax
> + xor 4(%esp),%eax
> flds 8(%esp)
> flds 4(%esp)
> - mov 11(%esp),%dh
> - xor 7(%esp),%dh
> - jmp 1f
> + jmp 0f
>
> .global remquol
> .type remquol,@function
> remquol:
> mov 28(%esp),%ecx
> + mov 24(%esp),%eax
> + xor 12(%esp),%eax
> + cwtl
> fldt 16(%esp)
> fldt 4(%esp)
> - mov 25(%esp),%dh
> - xor 13(%esp),%dh
> - jmp 1f
> + jmp 0f
>
> .global remquo
> .type remquo,@function
> remquo:
> mov 20(%esp),%ecx
> + mov 16(%esp),%eax
> + xor 8(%esp),%eax
> fldl 12(%esp)
> fldl 4(%esp)
> - mov 19(%esp),%dh
> - xor 11(%esp),%dh
> +0: cltd
> 1: fprem1
> fnstsw %ax
> sahf
> jp 1b
> fstp %st(1)
> - mov %ah,%dl
> - shr %dl
> - and $1,%dl
> - mov %ah,%al
> - shr $5,%al
> - and $2,%al
> - or %al,%dl
> - mov %ah,%al
> - shl $2,%al
> - and $4,%al
> - or %al,%dl
> - test %dh,%dh
> - jns 1f
> - neg %dl
> -1: movsbl %dl,%edx
> - mov %edx,(%ecx)
> + adc %al,%al
> + shl $2,%ah
> + adc %al,%al
> + shl $5,%ah
> + adc %al,%al
> + and $7,%eax
> + xor %edx,%eax
> + sub %edx,%eax
> + mov %eax,(%ecx)
> ret
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