https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78447
Bug ID: 78447 Summary: [7 Regression] wrong code when combine changes shifts by << 5 and >> 15 to >> 10 at -O Product: gcc Version: 7.0 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: --- Host: x86_64-pc-linux-gnu Target: x86_64-pc-linux-gnu Build: x86_64-pc-linux-gnu Created attachment 40099 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40099&action=edit reduced testcase Output: $ x86_64-pc-linux-gnu-gcc -O testcase.c $ ./a.out Aborted $ x86_64-pc-linux-gnu-gcc -v Using built-in specs. COLLECT_GCC=/repo/gcc-trunk/binary-latest/bin/x86_64-pc-linux-gnu-gcc COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-242583-checking-yes-rtl-df-extra-nobootstrap-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 --disable-bootstrap --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-242583-checking-yes-rtl-df-extra-nobootstrap-nographite-amd64 Thread model: posix gcc version 7.0.0 20161118 (experimental) (GCC) Before .combine: ... (insn 9 8 10 2 (parallel [ (set (reg:SI 108) (ashift:SI (reg/v:SI 105 [ xD.1799 ]) (const_int 5 [0x5]))) (clobber (reg:CC 17 flags)) ]) "testcase.c":13 529 {*ashlsi3_1} (expr_list:REG_DEAD (reg/v:SI 105 [ xD.1799 ]) (expr_list:REG_UNUSED (reg:CC 17 flags) (nil)))) ... (insn 11 10 12 2 (parallel [ (set (reg:HI 110) (lshiftrt:HI (subreg:HI (reg:SI 109) 0) (const_int 15 [0xf]))) (clobber (reg:CC 17 flags)) ]) "testcase.c":15 572 {*lshrhi3_1} (expr_list:REG_DEAD (reg:SI 109) (expr_list:REG_UNUSED (reg:CC 17 flags) (nil)))) (insn 12 11 13 2 (set (reg:QI 92 [ _11 ]) (subreg:QI (reg:HI 110) 0)) "testcase.c":15 85 {*movqi_internal} (expr_list:REG_DEAD (reg:HI 110) (nil))) ... .combine shows: ... Trying 9, 11 -> 12: ... Successfully matched this instruction: (set (reg:SI 110) (lshiftrt:SI (reg/v:SI 105 [ xD.1799 ]) (const_int 10 [0xa]))) Successfully matched this instruction: (set (subreg:SI (reg:QI 92 [ _11 ]) 0) (and:SI (reg:SI 110) (const_int 255 [0xff]))) allowing combination of insns 9, 11 and 12 original costs 4 + 4 + 4 = 12 replacement costs 4 + 4 = 8 Which is probably wrong. Tested revisions: r242657 - FAIL r242583 - FAIL r242510 - FAIL r242408 - OK