https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97623
Bug ID: 97623 Summary: Extremely slow O2 compile (>>O(n^2)) Product: gcc Version: 9.3.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: wsnyder at wsnyder dot org Target Milestone: --- Created attachment 49461 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49461&action=edit slow.cpp The attached program compiles very slowly with -O2. It is fast with clang (FWIW). This is an shortened version of a routine taking > 10 minutes. $ gcc --version gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 time g++ -O2 -c slow.cpp 6.457 (average) time g++ -Os -c slow.cpp 0.236 (average) time g++ -O2 -DNO_BOTTOM -c slow.cpp 0.560 (average) time g++ -O2 -DNO_TOP -c slow.cpp 0.702 (average) time g++ -O2 -DNO_FINAL -c slow.cpp 6.271 (average) time clang++ -O2 -c slow.cpp 0.210 (average) Note Os is much faster. The defines remove the top half of the function, or bottom half, note the time is crazy faster, one would expect the whole program to be only slightly slower than the sum of these. Removing a single statement from the bottom (-DNO_FINAL) also makes an enormous 0.21 second difference, so some algorithm is not scaling correctly. Would greatly appreciate some indication of 1. a compiler flag to bypass the bad optimization if possible, and 2. a hint as to what operator or how to avoid this as this is generated code so there's some flexibility for a workaround. Thanks.