https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103995
Bug ID: 103995 Summary: conj() ignored with tree loop vectorizer Product: gcc Version: 11.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: michelemartone at users dot sourceforge.net Target Milestone: --- Created attachment 52173 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52173&action=edit conj() ignored with tree loop vectorizer -- reproducer Hi. With gcc-11.2.0 on x86-64 and given certain flags, this program (see attachment) computes wrong results. Specifically, it seems like a conj() is being ignored in a loop. The buggy.c file contains the impacted function, but also a reference one; their results are being compared in main.c. Reproduces on arch x86-64 with 11.2.0 with: gcc -c -O3 buggy.c && gcc -c -O0 main.c && gcc buggy.o main.o -o buggy && ./buggy Using ASAN: gcc -c -O3 -fsanitize=address -fno-omit-frame-pointer buggy.c && gcc -c -O3 -fsanitize=address -fno-omit-frame-pointer main.c && gcc buggy.o -lasan main.o -o buggy && ./buggy it does not reproduce. Reproducer flags with lower optimization are: gcc -c -O1 -ftree-loop-vectorize -ftree-slp-vectorize buggy.c && gcc -c -O0 main.c && gcc -O0 buggy.o main.o -o buggy && ./buggy This can be minimized further by deactivating the individual -O1 flags listed on https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/Optimize-Options.html#index-ftree-dse using (-fno-...), exception made for two flags: -ftree-ch and -ftree-forwprop. Using: -O0 -ftree-ch -ftree-forwprop -ftree-loop-vectorize -ftree-slp-vectorize is not enough -- seems like some implicit -O1 option is needed. Putting all the code in one file does not reproduce the problem. The Makefile has a recipe building and running the program with decreasing optimization level, stepwise. make # will print BUG on mismatch, OK otherwise; make expected NOT to fail on gcc-11.2.0 make dist # zip the whole make clean # clean It does not reproduce on x86-64 and 10.2.0. On aarch64 tried only with 11.0.0 and it does not reproduce.