http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49820
--- Comment #15 from Agner Fog <agner at agner dot org> 2011-07-27 14:27:33 UTC --- How do you define "clever things"? Checking that a variable is within the allowed range is certainly a standard thing that every SW teacher tells you to do. I think it is reasonable to expect -Wall to do what it says and set a very high warning level. Optimizing away an overflow check is such a dangerous thing to do that it requires a warning. I think it may be wise to distinguish between optimizing away a whole branch or loop, versus just making calculations more efficient, e.g. simplifying expressions or making induction variables. If a branch can be optimized away then it is either violating the intentions of the programmer or the program has a logical error. A warning would be in place in either case. What I am suggesting is that optimizing away a branch should give a warning at a lower level than simplifying an arithmetic expression. I know this might be somewhat complicated to implement, but it would be useful for catching the situation where an overflow check is optimized away. Checking for overflow in a "safe" way is so complicated and tedious that it is practically never done (see https://www.securecoding.cert.org/confluence/display/seccode/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow ) Sorry for being persistent, but I think this issue has serious security implications.
