[PATCH] glibc __builtin_add_overflow problem with GCC < 7

[Paul Eggert]

Stefan Liebler's recent glibc commit "s390x: Require GCC 7.1 or later to 
build glibc" is motivated by a bug with __builtin_add_overflow that he 
encountered on s390x. However, it appears that the underlying GCC bug

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78559

is platform-independent (I am not sure of this, and am being conservative).

In Gnulib, Bruno Haible worked around the bug today by updating intprops.h 
so that INT_ADD_WRAPV does not use __builtin_add_overflow in older GCCs. 
When Gnulib intprops.h is copied to glibc this should fix glibc modules like 
posix/regexec.c and time/mktime.c that are shared with Gnulib and use 
intprops.h and INT_ADD_WRAPV etc.

However, several glibc modules (e.g., malloc/malloc.c) are not shared with 
Gnulib and use __builtin_add_overflow directly, so they won't benefit from 
the intprops.h fix.

A simple workaround would be to change INSTALL to recommend GCC 7.1 or 
higher, as in the first attached patch (configure and sysdeps/s390/configure 
files would also need to be regenerated). This patch also rejects icc, as 
icc 2021.1 pretends to be GCC 10 but doesn't do proper integer overflow 
checking.

Alternatively, if requiring GCC 7.1 is too drastic, glibc could change all 
direct uses of __builtin_add_overflow etc. to use intprops.h and 
INT_ADD_WRAPV etc., as Adhemerval suggested recently 
<https://sourceware.org/pipermail/libc-alpha/2020-December/120855.html>. The 
first step in this would be to update intprops.h from Gnulib (which should 
be routine anyway), as in the second attached patch. We'd also need to 
change malloc/malloc.c etc.
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