https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78262
Bug ID: 78262 Summary: [7 Regression] wrong code with -fschedule-insns Product: gcc Version: 7.0 Status: UNCONFIRMED Keywords: wrong-code Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: zsojka at seznam dot cz Target Milestone: --- Host: x86_64-pc-linux-gnu Target: x86_64-pc-linux-gnu Created attachment 39996 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39996&action=edit reduced testcase Output: $ x86_64-pc-linux-gnu-gcc -O -fschedule-insns testcase.c $ ./a.out Aborted At the assembly level, the problem seems to be: # testcase.c:12: u128_0 = u128_0 << 127 | u128_0 >> 1; mov rax, rcx # tmp99, _2 shrd rcx, rbx, 1 # _2, _2, shrd rbx, rax, 1 # _2, tmp99, # testcase.c:13: u128_0 >>= (u8)u128_0; shrd rcx, rbx, cl # _5, _5, _2 shr rbx, cl # _5, _2 ^^^ the last shrd modifies rcx (and thus cl), which is used in the last shr instruction - but it should use the original, unmodified value. This might be a duplicate of some other bug that I cannot find now. $ x86_64-pc-linux-gnu-gcc -v Using built-in specs. COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-241953-checking-yes-rtl-df-extra-nographite-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++ --enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra --without-cloog --without-ppl --without-isl --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld --with-as=/usr/bin/x86_64-pc-linux-gnu-as --disable-libstdcxx-pch --prefix=/repo/gcc-trunk//binary-trunk-241953-checking-yes-rtl-df-extra-nographite-amd64 Thread model: posix gcc version 7.0.0 20161108 (experimental) (GCC)