https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85376
Bug ID: 85376 Summary: [8 Regression] wrong code with -Og -fno-dce -fgcse -fno-tree-ccp -fno-tree-copy-prop Product: gcc Version: 8.0.1 Status: UNCONFIRMED Keywords: wrong-code Severity: normal Priority: P3 Component: rtl-optimization 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 43918 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43918&action=edit reduced testcase Output: $ x86_64-pc-linux-gnu-gcc -Og -fno-dce -fgcse -fno-tree-ccp -fno-tree-copy-prop testcase.c -Wno-psabi $ ./a.out Aborted At the assembly level: ... # testcase.c:16: q[7] >>= __builtin_add_overflow (0xfffffff0, __builtin_ffs (n), &s[5]); mov eax, 0 # tmp201, bsf edx, eax # _28, tmp201 mov ecx, -16 # tmp204, add eax, ecx # tmp203, tmp204 mov DWORD PTR [rsp+404], ecx # s, tmp204 # testcase.c:17: t = __builtin_ffs (g[7]); bsf eax, DWORD PTR g[rip+28] # _38, g lea ecx, [rcx+15] # tmp206, cmove eax, ecx # tmp206,, _38 add eax, 1 # _38, # testcase.c:18: e *= __builtin_sub_overflow (o, t, &f); mov ecx, 0 # _41, sub dl, al # tmp208, _38 jb .L7 #, ... "sub dl, al" is using edx set in "bsf edx, eax", which is not related (and even undefined because eax == 0). Use of ecx is correct, even though it does not look so on the first sight ("lea ecx, [rcx+15]" just sets ecx=-1 with affecting the flags) $ 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-259340-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/8.0.1/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 --disable-bootstrap --with-cloog --with-ppl --with-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-259340-checking-yes-rtl-df-extra-nobootstrap-amd64 Thread model: posix gcc version 8.0.1 20180412 (experimental) (GCC)