Re: Bug with -ftree-vrp?

[Brian Dessent <brian@dessent.net>]

Alexey Neyman wrote:

> The problem is that with -O2, the second conditional gets optimized
> away. However, such optimization results in a misbehavior for the
> minimum number representable in 32 bits, 0x80000000. The option which
> triggers this optimization is -ftree-vrp.

Signed integer overflow is undefined in ISO C.  If v = INT_MIN then v =
-v is undefined, and hence the compiler can assume this won't occur and
optimize away the test since a conformant program does not invoke
undefined behavior.  Yes, there is some debate around the legitimacy of
this optimization because "traditional" C has more or less held that
signed integer overflow is perfectly defined on all two's compliment
machines (i.e. everywhere.)  But the standard leaves it undefined and so
gcc is within its right to do this optimization, so say the language
lawyers.

You may be interested in the options -fno-strict-overflow, -fwrapv, and
-Wstrict-overflow.

Brian