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Message-ID: <CF1B278A1B1D40F68C8C33CC16CC5E8D@H270>
Date: Sat, 7 Aug 2021 15:12:14 +0200
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: "Rich Felker" <dalias@...c.org>
Cc: "Alexander Monakov" <amonakov@...ras.ru>,
"Szabolcs Nagy" <nsz@...t70.net>,
<musl@...ts.openwall.com>
Subject: Re: [Patch] src/math/i386/remquo.s: remove conditional branch, shorter bit twiddling
Rich Felker <dalias@...c.org> wrote:
> On Fri, Aug 06, 2021 at 07:23:19PM +0200, Stefan Kanthak wrote:
>> Rich Felker <dalias@...c.org> wrote:
>> > The path forward for all the math asm is moving it to inline asm in C
>> > files, with no flow control or bit/register shuffling in the asm, only
>> > using asm for the single instructions.
I'd even go a step further and put all those short functions into header
files to give the compiler a chance to properly them in context.
And then get rid of all the __builtin_* bloat GCC unfortunately carries
with it...
>> > I haven't read the mul trick here in detail but I believe it should be
>> > duplicable with plain C * operator.
>>
>> It is.
>
> Great! Does it improve the code generation?
Of course: the same 3 instructions Alexander showed here:
remquol:
fldt 16(%esp)
fldt 4(%esp)
.L2:
#APP
# 19 "remquol.c" 1
fprem1; fnstsw %ax
# 0 "" 2
#NO_APP
testb $4, %ah
jne .L2
fstp %st(1)
andl $17152, %eax #
movzbl 15(%esp), %ecx # this should be movb 15(%esp), %cl
imull $9502720, %eax, %eax #
shrl $29, %eax #
movl %eax, %edx
negl %edx
xorb 27(%esp), %cl
cmovs %edx, %eax
movl 28(%esp), %edx
movl %eax, (%edx)
ret
The code to flip the sign is but not as short as in the assembly version
I posted; the following change may give shorter code:
int sign = (*cx^*cy) >> 7;
qbits += sign;
qbits ^= sign;
>> > I really do not want to review/merge asm changes that keep this kind
>> > of complex logic in asm when there's no strong motivation for it (like
>> > fixing an actual bug, vs just reducing size or improving speed). The
>> > risk to reward ratio is just not reasonable.
>>
>> Final patch attached!
>
> Thanks. I don't mind making any final tweaks needed, if necessary.
Do so as you will and wish: I don't mind^W^Wappreciate if someone can
improve code I've written.
Stefan
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