[Bug c/38470] value range propagation (VRP) would improve -Wsign-compare

["DeusExSophismata at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38470

David Stone <DeusExSophismata at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |DeusExSophismata at gmail
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--- Comment #10 from David Stone <DeusExSophismata at gmail dot com> 2012-03-24 06:52:04 UTC ---
Simple self-contained test case:


unsigned f (int x) {
    return (x >= 0) ? x : -x;
}
int main () {
    return 0;
}


It seems like we could at least add a simple improvement that just checks for
simple comparisons to 0. That probably catches most code (I often do one set of
calculations for non-negative values, and another set for negative values) and
avoids a lot of problems of trying to track complex calculations for the
signedness of the result. I wouldn't be perfect, but it would be better than it
currently is.

Being able to detect cases like this is also an optimization opportunity,
because division with positive integers is simpler than division with negative
integers, for example. We could reuse code that detects that optimization
opportunity for this warning.