https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80693
Bug ID: 80693 Summary: [6/7/8 Regression] wrong code with -O -fno-tree-coalesce-vars Product: gcc Version: 8.0 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: --- Created attachment 41335 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41335&action=edit reduced testcase Output: $ x86_64-pc-linux-gnu-gcc -O -fno-tree-coalesce-vars testcase.c $ ./a.out Aborted The .combine dump shows: ... Trying 18, 19 -> 20: Failed to match this instruction: (set (reg:HI 116) (and:HI (not:HI (reg/v:HI 112 [ u16_1D.1800 ])) (const_int 1 [0x1]))) Successfully matched this instruction: (set (reg:HI 117) (not:HI (reg/v:HI 112 [ u16_1D.1800 ]))) Successfully matched this instruction: (set (reg:HI 116) (and:HI (reg:HI 117) (const_int 1 [0x1]))) allowing combination of insns 18, 19 and 20 original costs 0 + 4 + 4 = 0 replacement costs 4 + 4 = 8 deferring deletion of insn with uid = 18. modifying insn i2 19: r117:HI=!r112:HI deferring rescan insn with uid = 19. modifying insn i3 20: {r116:HI=r117:HI&0x1;clobber flags:CC;} REG_UNUSED flags:CC REG_DEAD r117:HI deferring rescan insn with uid = 20. ... Trying 19 -> 20: Successfully matched this instruction: (parallel [ (set (reg:HI 116) (xor:HI (reg/v:HI 112 [ u16_1D.1800 ]) (const_int 1 [0x1]))) (clobber (reg:CC 17 flags)) ]) allowing combination of insns 19 and 20 original costs 4 + 4 = 8 replacement cost 4 deferring deletion of insn with uid = 19. modifying insn i3 20: {r116:HI=r112:HI^0x1;clobber flags:CC;} REG_UNUSED flags:CC deferring rescan insn with uid = 20. ... Which looks wrong, since it changes "r116:HI=!r112:HI&1" (can be 0 or 1) to "r116:HI=r112:HI^0x1" (can be anything). Also, the costs dump in the first replacement looks wrong: original costs 0 + 4 + 4 = 0 $ x86_64-pc-linux-gnu-gcc COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-247635-checking-yes-rtl-df-extra-nographite-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/8.0.0/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-247635-checking-yes-rtl-df-extra-nographite-amd64 Thread model: posix gcc version 8.0.0 20170505 (experimental) (GCC) Tested revisions: trunk r247635 - FAIL 7-branch r247545 - FAIL 6-branch r247359 - FAIL 5-branch r247456 - OK