http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49820
--- Comment #12 from Agner Fog <agner at agner dot org> 2011-07-25 14:21:52 UTC --- No the behavior is not predictable when it sometimes warns about ignoring overflow, and sometimes not. Please add a warning when it optimizes away an overflow check after the abs function. Unsafe optimizations are sometimes good, sometimes causing hard-to-find bugs. The programmer can't always predict what kind of optimizations the compiler makes. A warning feature is the best way to enable the programmer to check if the compiler does the right thing. The programmer can then turn off specific warnings after verifying that the optimizations are OK.
