https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91987
--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> --- On Fri, 4 Oct 2019, jakub at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91987 > > --- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- > (In reply to rguent...@suse.de from comment #3) > > Ick. I'd say we should unconditionally guard the transform > > with the appropriate TREE_SIDE_EFFECTS check? > > See above, wouldn't that mean throwing the optimization away completely, > because COMPOUND_EXPRs with no side-effects in the rhs1 should be just > optimized into rhs2? > Do you mean unconditionally for all binary/comparison ops, or just the > shifts/rotates? > For languages where it is UB if the side-effects modify the other operands, I > don't see > anything wrong with continuing with the optimization. Well, AFAICS this "optimization" is purely for frontend folding since in the middle-end we have expressions not "obfuscated" with COMPOUND_EXPRs. So I wouldn't mind dropping it completely either... > The other C++17 rules are summarized in > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91415#c5 > (though that PR is also unfinished). > > > > One thing I'm worried about are the special cases where we enforce some > > > argument order, evaluate one argument before the other or vice versa. > > > For is_gimple_reg_type args it can be just a matter of forcing it into an > > > SSA_NAME or a new VAR_DECL, but if a function argument is a structure, > > > struct S { int a, b; } c = { 1, 2 }; > > > fun (c, (c.b = 3, 5); > > > and fun is one of the magic one that enforce operand ordering and the > > > first > > > argument needs to be sequenced before the second, what can we do? > > > > You need to emit an aggregate copy, don't you? Consider > > Probably, but aggregate copy of TREE_ADDRESSABLE aggregates might be a > problem. > For the arguments, I'm not planning to do anything myself, because I don't > understand it well, just wanted to raise possible issue. Likewise. I don't even _want_ to understand :P
