Re: Failures building glibc with mainline GCC

[Martin Sebor via Libc-alpha <libc-alpha@sourceware.org>]

On 8/3/21 11:21 AM, Joseph Myers wrote:
> On Tue, 3 Aug 2021, Martin Sebor via Gcc wrote:
> 
>> Yes, we know about that one.  What I'm asking for is the translation
>> units with the other warnings you showed with the latest GCC (including
>> the threader patches) on the other targets (including i686 and power).
>> I don't have an easy way to create them using my setup.
> 
> I've attached (all gzipped) hurdselect.i (i686-gnu, -O2 -Wall -Werror
> -Wno-parentheses):

Thanks.

> 
> hurdselect.c: In function '_hurd_select':
> hurdselect.c:555:7: error: 'ss' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> hurdselect.c:70:25: note: 'ss' was declared here

This is a false positive caused by the overly restrictive analysis
limits hardwired into GCC (like PR 85872).  I have a patch that
relaxes the limits and that avoids the warning in this case but
it's not yet ready.

> 
> and t.61.i (powerpc-linux-gnu, -O2 -Wall -Werror):
> 
> In file included from t.61.c:437:
> In function 'from_t_61_single',
>      inlined from 'gconv' at ../iconv/skeleton.c:568:15:
> ../iconv/loop.c:440:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
> In file included from t.61.c:437:
> ../iconv/loop.c: In function 'gconv':
> ../iconv/loop.c:382:17: note: at offset 2 into destination object 'bytebuf' of size 2

This is the same warning as Jeff noted on s390-linux-gnu and that
I explained in my reply:
   https://gcc.gnu.org/pipermail/gcc/2021-July/236943.html
There's nothing the warning alone can do to avoid triggering in these
cases (unrolling too many iterations of a loop).  The unroller does
have code to avoid doing this (infer_loop_bounds_from_undefined) but
it's not good enough.  Using -fno-aggressive-loop-optimizations
prevents the warning.

> and dl-tls.i (powerpc64-linux-gnu, -O2 -finline-limit=2000 -Wall -Werror;
> the -finline-limit=2000 option is necessary for the error and comes from
> sysdeps/powerpc/powerpc64/Makefile):
> 
> In file included from ../sysdeps/powerpc/dl-tls.c:20:
> In function '_dl_allocate_tls_init',
>      inlined from '_dl_allocate_tls' at ../elf/dl-tls.c:621:10:
> ../elf/dl-tls.c:529:10: error: array subscript -1 is outside array bounds of 'void[9223372036854775807]' [-Werror=array-bounds]
> ../elf/dl-tls.c:544:40: error: array subscript -1 is outside array bounds of 'void[9223372036854775807]' [-Werror=array-bounds]
> 

These warnings are reproducible even with a native x86_64 compiler
and the same option.  It's most likely due to the same root cause
as PR 101600 (the one in comment #0, not comment #2).  I don't have
a fix for it yet but it shouldn't be too hard.

Martin