[Bug?] compiler warning with gcc >= 4.2

Hi,

while testing for an (probably) unrelated miscompilation bug, I got the following warning while compiling git:

gcc-4.2 -o sha1_file.o -c -g -O2 -Wall -DSHA1_HEADER='<openssl/sha.h>' -DETC_GITCONFIG='"/home/djpig/etc/gitconfig"' -DNO_STRLCPY sha1_file.c sha1_file.c: In function ‘check_packed_git_idx’: sha1_file.c:523: warning: assuming signed overflow does not occur when assuming that (X + c) < X is always false sha1_file.c:523: warning: assuming signed overflow does not occur when assuming that (X + c) < X is always false

This comes from the new -Wstrict-overflow which warns about the fact that with -fstrict-overflow, which is activated by default with -O2, the if clause referenced in the warning will be optimised away.

So I would be interested to know
a) if the compiler optimising this check away (which seems to be a check
   about whether signed overflow can occour) can lead to unwanted results
b) if not a), if it would make sense trying to suppress that warning, so
   that other people don't end up wondering the same as me

The used compiler was:
$ gcc-4.2 -v
Using built-in specs.
Target: hppa-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr
--disable-libssp --disable-werror --enable-checking=release
--build=hppa-linux-gnu --host=hppa-linux-gnu
--target=hppa-linux-gnuThread model: posix
gcc version 4.2.1 20070627 (prerelease) (Debian 4.2-20070627-1)

Gruesse,