http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58359
--- Comment #8 from Anatoly Sinyavin <a.sinyavin at samsung dot com> --- (In reply to Marc Glisse from comment #7) > (In reply to Anatoly Sinyavin from comment #3) > > So I suggest processing __builtin_unreachable immediately after "cfg" pass > > (cfg buiding). > > That seems awfully early. Don't we want to at least wait until after VRP1, > maybe even DOM1 or later? __builtin_unreachable should be made not to hurt > other optimizations too much, but it shouldn't be made useless either. I changed my opinion and offered new solution couple of weeks ago. New patch was proposed in 'gcc-patc...@gcc.gnu.org' (Subject: "PR __builtin_unreachable prevents vectorization/58359 / bug fix / discussion") I attached this new patch here also (patch_and_test_results.tar.bz2). Body of this letter: >>> Hello colleagues, Bug fix for PR __builtin_unreachable prevents vectorization/58359 __builtin_unreachable is processed by "fab" optimization pass (fold all builtins / tree-ssa-ccp.c) in optimize_unreachable function. This pass is executed after vectorization so vectorizer gets __builtin_unreachable and it can't handle. We need to fold __builtin_unreachable before vectorization. I recommended (see comments & patches in 58359) to do it immediately after "cfg" pass (cfg buiding) but it's bad solution because it prevents some optimizations. For instance "gcc.dg/builtin-unreachable-2.c" failed. // part of gcc.dg/builtin-unreachable-2.c if (i > 1) __builtin_unreachable(); if (i > 1) foo (); // user expresses fact that "i > 1" // is always false so call can be removed User can express "always false" conditions via __builtin_unreachable so gcc can use this info for optimization purposes. I think good place is "ifcvt" pass. I have created two patches to fix this problem. The first patch (new_bug_fix_58359_builit_unreachable.patch) just moves functionality of optimize_unreachable from "fab" pass to "ifcvt" pass. The second patch (new_bug_fix_58359_builit_unreachable.AGGRESSIVE.patch) is more aggressive variant. Origininal implementation of optimize_unreachable doesn't delete basic block if there is FORCED_LABEL, non debug statemnt, or call function before __built_unreachable in this basic block. I think we can't delete basic block if it contains some statement X before __built_unreachable. This statement X can potentially transfer control from this basic block and can't return. It's possible in two cases: if statement X is procedure call (without return) or assembler instruction. (See also __built_unreachable description) Test cases: - test in PR 58359 - existing test in gcc test suit "gcc.dg/builtin-unreachable-2.c" Test report: - compiler without my changes original.log - compiler with "new_bug_fix_58359_builit_unreachable_2.patch" builtin_non_aggressive.log - compiler with "new_bug_fix_58359_builit_unreachable_2_aggressive.patch" builtin_aggressive.log There are no regressions Attached file: patch_and_test_results.tar.bz2 conatins patches & test results Many thanks, Anatoly S <<<