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Date: Sat, 21 Nov 2015 21:11:22 +0100
Subject: Re: for information, gcc-4.2.3 miscompiles musl math

On Sat, Nov 21, 2015 at 02:25:48PM -0500, Rich Felker wrote:
> > like "sin(8.000000) = 21.709544".
> > Looks like the argument reduction logic has changed in a way
> > which is not compatible with gcc-4.2.3.
> Are you using configure or a hand-written config.mak? configure sets
> up a big hammer, -ffloat-store, when -fexcess-precision=standard is
> not supported (i.e. on old gcc), which hopefully suffices to make this
> code work, but it's possible it doesn't always do the job.

I was using configure.

Here are the compilation flags which were used while building with 4.2.3:
(the command line has been run through "fmt" here)

gcc -march=i486 -std=c99 -nostdinc -ffreestanding -ffloat-store
-frounding-math -D_XOPEN_SOURCE=700 -I./arch/i386 -I./src/internal
-I./include  -Os -pipe -fomit-frame-pointer -fno-unwind-tables
-fno-asynchronous-unwind-tables -Wa,--noexecstack -march=i486
-mtune=generic -Werror=implicit-function-declaration -Werror=implicit-int
-Werror=pointer-sign -Werror=pointer-arith -fno-stack-protector  -c -o
src/math/sin.o src/math/sin.c

Yes, -ffloat-store is present but apparently did not suffice.

> > I do not notice any problems while compiling musl with gcc-5.2, nor
> > have a compelling reason to insist on using gcc-4.2.3 (somebody else
> > might have though, gcc-4.2.3 is the last one under gpl 2).
> I thought 4.2.1 was the last.

I think Wikipedia says so as well but The Source is the truth. It is
4.2.3 and nothing else. It is also in fact remarkably better than 4.2.1
and I could build with it mostly everything I needed, modulo features
newer than the compiler.

> If you don't want to look into it further yourself I'll see if someone
> else interested in old toolchains can or try to get around to it
> myself. I really don't want to introduce more hacks for these broken
> compilers though. If people really still want to use them, we should

It was not my intention to fix, only to warn. On the other side, I have a
convenient environment to play with multiple versions of everything,
I can run a test if you will want me to.

> probably just find a cheap way to fix the compiler, like patching it
> not to perform any optimizations whatsoever on floating point
> expressions.

This would surely be cleaner from the musl perspective, but also a much
more expensive procedure (rebuild the compiler) than e.g. patching musl.

Anyway, probably a fix is not worth spending any time on, until somebody
explicitly asks for support of 4.2.3.

Nevertheless, thanks for taking this into consideration Rich.


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