Re: assuming signed overflow does not occur

[Florian Weimer <fw at deneb dot enyo dot de>]

* Bruce Korb:

> I know about all these theoretical possibilities of numbers behaving
> in strange ways when arithmetic optimizations assume that signed
> overflow won't occur when they actually might. Yep, it creates subtle
> bugs. The warning is worthwhile. Still and all:
>
>     485         tvdiff = abs_tval(sub_tval(timetv, tvlast));
>     486         if (tvdiff.tv_sec != 0) {
>
> systime.c: In function 'step_systime':
> systime.c:486:5: warning: assuming signed overflow does not occur when
> simplifying conditional to constant [-Wstrict-overflow]
>
> What possible optimization might be going on to cause an overflow
> problem when I simply want to know if the "tv_sec" field is zero or
> not? (BTW, in current source, "tvdiff" is a structure that is returned
> by abs_tval())

This usually happens after inlining and other optimizations.  You'll
have to look at GIMPLE dumps to figure out what is going on.

(This is more a question for the gcc-help list.)