Failures building glibc with mainline GCC

[Jeff Law]

On 7/30/2021 10:19 AM, Aldy Hernandez via Libc-alpha wrote:
> There's a new jump threader in GCC which is much more aggressive, and
> may trigger latent problems with other warning passes, especially
> -Warray-bounds, -Woverflow, and -Wuninitialized.
[ ... ]
Ugh.  First attempt got blocked as message was slightly too big.

I think this is pretty generic as I've seen it on multiple ports and 
Joseph mentioned them as well.

With an s390-linux-gnu (not s390x!) cross compiler you should be able to 
trigger:

bash-5.1# s390-linux-gnu-gcc -std=gnu99 -O2 -Wall -mlong-double-128 *.i
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: warning: writing 1 byte into a region of size 0 
[-Wstringop-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


I don't know if it's a real failure or a false positive.  I haven't even 
bisected, but I suspect the new threader is the triggering change.  
Ideally the threader threaded a path we hadn't previously and by some 
chain of events exposed a out of bounds write that needs to be fixed.




Jeff
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