Aggressive optimization breaks the code and removes checks

[mkurdej]

Bugzilla Link	22822
Resolution	INVALID
Resolved on	Mar 06, 2015 12:13
Version	3.6
OS	All
Attachments	Minimal example of the code causing the problem
CC	@dwblaikie,@hfinkel

Extended Description

The attached code is miscompiled at optimization levels -Os and above and the checks in checked_add function are apparently ignored.
Reproduced on FreeBSD and Windows with clang 3.5.0 (release=r217039) and 3.6.0(tags/RELEASE_360/final).

[dwblaikie]

Looks like undefined behavior to me.

signed integer overflow (as happens at the beginning of the call to checked_add when passed max_int) is undefined. Since your program unconditionally executes that addition, the behavior of the entire program is undefined - nothing it does, even before the point of the addition, is guaranteed by the C++ standard.