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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/66623] Unsafe FP math reduction used in strict math mode Date: Mon, 22 Jun 2015 11:30:00 -0000 [thread overview] Message-ID: <bug-66623-4-zFnprLKnUT@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-66623-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66623 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |wrong-code Status|UNCONFIRMED |ASSIGNED Last reconfirmed| |2015-06-22 Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot gnu.org Ever confirmed|0 |1 --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- So this boils down to the vectorizer disabling the associative math test if it does outer-loop vectorization: nested_cycle = (loop != LOOP_VINFO_LOOP (loop_vinfo)); reduc_stmt = vect_force_simple_reduction (loop_vinfo, phi, !nested_cycle, &double_reduc); but I agree with your assessment that even with outer loop vectorization it is not safe. A patch as simple as Index: gcc/tree-vect-loop.c =================================================================== --- gcc/tree-vect-loop.c (revision 224723) +++ gcc/tree-vect-loop.c (working copy) @@ -2610,14 +2610,14 @@ vect_is_simple_reduction_1 (loop_vec_inf /* Check that it's ok to change the order of the computation. Generally, when vectorizing a reduction we change the order of the computation. This may change the behavior of the program in some - cases, so we need to check that this is ok. One exception is when - vectorizing an outer-loop: the inner-loop is executed sequentially, - and therefore vectorizing reductions in the inner-loop during - outer-loop vectorization is safe. */ + cases, so we need to check that this is ok. Even when + vectorizing an outer-loop and the inner-loop is executed sequentially, + vectorizing reductions in the inner-loop during outer-loop vectorization + is _not_ safe as the final reduction in the outer loop associates + the inner loop results. */ /* CHECKME: check for !flag_finite_math_only too? */ - if (SCALAR_FLOAT_TYPE_P (type) && !flag_associative_math - && check_reduction) + if (SCALAR_FLOAT_TYPE_P (type) && !flag_associative_math) { /* Changing the order of operations changes the semantics. */ if (dump_enabled_p ()) @@ -2625,8 +2625,7 @@ vect_is_simple_reduction_1 (loop_vec_inf "reduction: unsafe fp math optimization: "); return NULL; } - else if (INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_TRAPS (type) - && check_reduction) + else if (INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_TRAPS (type)) { /* Changing the order of operations changes the semantics. */ if (dump_enabled_p ()) @@ -2634,7 +2633,7 @@ vect_is_simple_reduction_1 (loop_vec_inf "reduction: unsafe int math optimization: "); return NULL; } - else if (SAT_FIXED_POINT_TYPE_P (type) && check_reduction) + else if (SAT_FIXED_POINT_TYPE_P (type)) { /* Changing the order of operations changes the semantics. */ if (dump_enabled_p ()) Might have some testsuite fallout though.
next parent reply other threads:[~2015-06-22 11:30 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <bug-66623-4@http.gcc.gnu.org/bugzilla/> 2015-06-22 11:30 ` rguenth at gcc dot gnu.org [this message] 2015-06-22 11:37 ` rguenth at gcc dot gnu.org 2021-06-08 14:16 ` rguenth at gcc dot gnu.org
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