https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81661
--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 2 Aug 2017, jakub at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81661 > > Jakub Jelinek <jakub at gcc dot gnu.org> changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |jakub at gcc dot gnu.org > > --- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> --- > So do we need some predicate that will tell us if it is safe to evaluate some > generic expression using force_gimple_operand* and bail out if it isn't > possible? Yes, see for example niter analysis expand_simple_operations which avoids to turn GIMPLE into GENERIC if that might trap. I suspect we're looking at a similar case here, just needs to be tracked down... > Even if gimplify_cond_expr tried harder around: > 3865 if (gimplify_ctxp->allow_rhs_cond_expr > 3866 /* If either branch has side effects or could trap, it > can't be > 3867 evaluated unconditionally. */ > 3868 && !TREE_SIDE_EFFECTS (then_) > 3869 && !generic_expr_could_trap_p (then_) > 3870 && !TREE_SIDE_EFFECTS (else_) > 3871 && !generic_expr_could_trap_p (else_)) > 3872 return gimplify_pure_cond_expr (expr_p, pre_p); > 3873 > 3874 tmp = create_tmp_var (type, "iftmp"); > and built for the case where gimplify_pure_cond_expr can't be used a SSA_NAME > and a PHI if we're in SSA, I guess the vast majority of callers of > force_gimple_operand* don't really expect that the sequence might need to be > split into several basic blocks. Yes, which is why we shouldn't really support this. Of course the real issue is that we can't emit a non-trapping PLUS_EXPR besides forcing unsigned arithmetic. That said, this would be a whole lot easier to the middle-end if we would have lowered -ftrapv operations and the actual arithmetic were just wrapping or undefined... which is AFAIK where we want to go anyway. Given we now have the overflow IFNs maybe we can try lowering this at gimplification time? (and clear flag_trapv afterwards)
