Re: Bug report: strtod drops LSB

[Rich Felker <dalias@libc.org>]

On Wed, Sep 25, 2019 at 09:32:35AM -0500, Anastasios wrote:
> Hello,
> 
> Consider this program, strtod.c:
> 
>     #include <stdlib.h>
>     #include <stdio.h>
> 
>     int main()
>     {
>         printf("%lf\n", strtod("283686952306183", NULL));
>     }
> 
> With current musl master from Git:
> 
>     $ musl-gcc -static strtod.c -o a.musl
>     $ ./a.musl
>     283686952306176.000000
> 
> By comparison, with glibc:
> 
>     $ gcc -static strtod.c -o a.glibc
>     $ ./a.glibc
>     283686952306183.000000
> 
> The correct binary representation of this float is
> 
>     0x42f0203040506070
> 
> but musl strtod produces
> 
>     0x42f0203040506000
> 
> i.e., it fails to set the LSB. I examined this while ruling out printf as the cause.

I can't reproduce this. My test program for strtod shows, for the
input "283686952306183":

d:  283686952306183 [0x1.020304050607p+48] [42f0203040506070]

I suspect you miscompiled musl, possibly by passing in CFLAGS (perhaps
from defaults in your environment?) that break floating point
semantics. We test for and refuse to build if __FAST_MATH__ is
defined, but GCC only defines it if you use -ffast-math, not if you
manually enable one or more of the individual broken options that
-ffast-math enables.

Alternatively, it's possible that you have a broken compiler version
that miscompiles floating point code.

Rich