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Message-ID: <5BF8FB2FE1AA418393E6091F7F8AFC14@H270>
Date: Wed, 11 Dec 2019 13:33:44 +0100
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: "Szabolcs Nagy" <nsz@...t70.net>
Cc: <musl@...ts.openwall.com>
Subject: Re: [PATCH] fmax(), fmaxf(), fmaxl(), fmin(), fminf(), fminl() simplified
"Szabolcs Nagy" <nsz@...t70.net> wrote:
>* Stefan Kanthak <stefan.kanthak@...go.de> [2019-12-11 10:55:29 +0100]:
>> Still more optimisations/simplifications in the math subtree.
>>
>> JFTR: I'm NOT subscribed to your mailing list, so CC: me in replies!
>>
>> --- -/src/math/fmax.c
>> +++ +/src/math/fmax.c
>> @@ -3,11 +3,9 @@
>> double fmax(double x, double y)
>> {
>> - if (isnan(x))
>> + if (x != x)
>
> these two are not equivalent for snan input, but we dont care
> about snan, nor the compiler by default, so the compiler can
> optimize one to the other (although musl uses explicit int
> arithmetics instead of __builtin_isnan so it's a bit harder).
The latter behaviour was my reason to use (x != x) here: I attempt
to replace as many function calls as possible with "normal" code,
and also try to avoid transfers to/from FPU/SSE registers to/from
integer registers if that does not result in faster/shorter code.
> in any case the two are equivalent for practical purposes and
> using isnan better documents the intention, you should change
> the isnan definition if you think it's not efficient.
>
>> return y;
>> - if (isnan(y))
>> - return x;
>> /* handle signed zeros, see C99 Annex F.9.9.2 */
>> - if (signbit(x) != signbit(y))
>> + if (x == y)
>> return signbit(x) ? y : x;
>> return x < y ? y : x;
>
> nice trick, but the fenv behaviour is not right.
--- -/src/math/fmax.c
+++ +/src/math/fmax.c
@@ -3,11 +3,9 @@
double fmax(double x, double y)
{
- if (isnan(x))
+ if (x != x)
return y;
- if (isnan(y))
+ if (y != y)
return x;
/* handle signed zeros, see C99 Annex F.9.9.2 */
- if (signbit(x) != signbit(y))
+ if (x == y)
return signbit(x) ? y : x;
return x < y ? y : x;
}
> you should run any such change through libc-test
> git://repo.or.cz/libc-test and look for regressions.
I already told Rich that I neither use an OS nor a compiler/assembler
where musl or libc-test can be built.
Stefan
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