https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68590
--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> --- > Instead it originates from > > (for cmp (ge le) > > (simplify > > (cmp @0 @0) > > (eq @0 @0))) > > hmm. Yes, we use @0 twice here. But we also use a matching capture, sth > we (unfortunately) don't record (yet). We _do_ have a load that we > compare so avoiding the redundant computation when we duplicate an > operand makes sense even when there is no side-effect present (that was > the objective of the change). Simplifying > > (le s1->length s1->length) > > to > > (eq save_expr <s1->length> save_expr <s1->length>) > > looks like a win. I beg to differ, s1->length has no TREE_SIDE_EFFECTS so it's a pessimization. Having no TREE_SIDE_EFFECTS *precisely* means that you are free to reuse the object as-is in the same expression any number of times. > Now simplifying that down to > > (save_expr <s1->length>, 1) > > is "correct" as well. And we can't just drop random save_exprs here because > of the comment in save_expr: > > /* This expression might be placed ahead of a jump to ensure that the > value was computed on both sides of the jump. So make sure it isn't > eliminated as dead. */ > TREE_SIDE_EFFECTS (t) = 1; > > not sure what this is refering to... it sounds confusing - if it was placed > there and did not have side-effects it better should have a user via a > data dependence. Having TREE_SIDE_EFFECTS on SAVE_EXPRs is required for GENERIC folding (of course not if your ultimate objective is to kill GENERIC folding...) because if you have something like: COND_EXPR <cond, op0 + op1, op0 - op2> and want to turn it into: COND_EXPR <cond, SAVE_EXPR <op0> + op1, SAVE_EXPR <op0> - op2> the SAVE_EXPR <op0> will create a variable that will be initialized once, in other words in only one of the arms and you don't know which one at compile time. So you need to create a COMPOUND_EXPR: COMPOUND_EXPR <SAVE_EXPR <op0>, COND_EXPR <cond, SAVE_EXPR <op0> + op1, SAVE_EXPR <op0> - op2>> to make sure it is evaluated ahead of the branch and TREE_SIDE_EFFECTS is required to keep the first operand. I'll note that the problematic change comes with neither a testcase nor a comment (and the original comment about TREE_SIDE_EFFECT is even still there) so it's little hard to understand where it comes from...
