bug#49261: Segfault during loadup

[Paul Eggert]

On 7/11/21 8:25 AM, Eli Zaretskii wrote:

> >    lisp.h:251:18: warning: unsigned conversion from 'long long int' to 
> > 'uintptr_t' {aka 'unsigned int'} changes value from '2305843009213693951' to 
> > '4294967295' [-Woverflow]
> >      251 | # define VALMASK (USE_LSB_TAG ? - (1 << GCTYPEBITS) : VAL_MAX)
> >         |                  ^
> >    alloc.c:4767:24: note: in expansion of macro 'VALMASK'
> >     4767 |       uintptr_t mask = VALMASK;
> >         |                        ^~~~~~~
> I tried to fix this on master, please take a look.

Yes that GCC warning was bogus, and your pacification of GCC is valid 
now that we no longer tag the MSB of pointers. Still, there should be a 
simpler way to pacify GCC so I installed a further fix that I hope does 
that (see first attached patch). This fix simply uses a cast (uintptr_t) 
VALMASK to pacify GCC; if GCC issues a bogus warning even for that cast, 
we could substitute (uintptr_t) (VALMASK & UINTPTR_MAX) though this is 
starting to get a little ridiculous.

The version of GCC that I tried (11.1.1 20210531 (Red Hat 11.1.1-3)) 
don't warn about the original code, so perhaps the bogus warning that 
you saw is a GCC bug that's been fixed in later GCC versions. However, 
GCC 11.1.1 does warn about some other stuff so I installed the remaining 
patches to pacify it. Some of these patches fix real (albeit unlikely) 
bugs in Emacs. Some work around what are evidently flaws in GCC 11.1.1. 
Oh well.

0001-Pacify-gcc-Woverflow-more-nicely.patch
Description: Text Data

0002-Pacify-gcc-11.1.1-Wanalyzer-null-argument.patch
Description: Text Data

0003-Pacify-gcc-11.1.1-Wanalyzer-possible-null-dereferenc.patch
Description: Text Data

0004-Pacify-gcc-11.1.1-Wclobbered.patch
Description: Text Data

0005-Port-test-module-to-glibc-2.33.patch
Description: Text Data