On 04/26/2017 05:31 AM, Richard Biener wrote:
The following removes the third state we had apart from signed integer overflow wrapping and being undefined. It makes signed integer overflow undefined, consistently at all optimization levels. -fno-strict-overflow stays as a backward compatible way to avoid optimizations that rely on signed integer overflow being undefined by making it wrapping (this is also the reason of using !flag_wrapv in POINTER_TYPE_OVERFLOW_UNDEFINED rather than a new option, for now). Surprisingly there's no UBSAN integer overflow testsuite fallout, foldings that happen before instrumentation (which is done after into-SSA) and rely on signed integer overflow being undefined will cause false negatives. If that turns out to be a problem the flag_strict_overflow flag can be re-introduced (not that this would be my preference) and it can be unset after UBSAN instrumentation is finished. The main motivation for aliasing -fstrict-overflow to -f[no-]wrapv is that with -fno-strict-overflow (and thus -O1 at the moment) you get the worst of both worlds, you can't optimize based on the undefinedness but you also cannot rely on wrapping behavior (to know that re-association will not introduce undefined behavior). Using -fwrapv for -fno-strict-overflow makes it clear what the semantics are. Bootstrapped and tested on x86_64-unknown-linux-gnu. I opened PR80525 for the appearant mishandling of (a + 1) && (a + 1) with -Wlogical-op when overflow is undefined. If there are no further comments I plan to install this after 7.1 is released. I consider the Ada FE change obvious. The next step is to get rid of all that ugly -Wstrict-overflow code in VRP. strict-overflow warnings from folding were already detoriating with moving stuff to match.pd where it isn't easy to preserve those. Ripping those out can be done later, it's not blocking other stuff, and eventually somebody picks up -Wstrict-overflow to warn for some cases from the FEs. changes.html/porting_to.html will need to have instructions how to use ubsan to get at the real problems in code.
This all sounds good to me. jeff
