https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81291
--- Comment #1 from Zdenek Sojka <zsojka at seznam dot cz> --- Created attachment 41673 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41673&action=edit testcase needing less compiler flags $ powerpc-unknown-linux-gnu-gcc -Og -fgcse -fweb testcase.c $ qemu-ppc ./a.out qemu: uncaught target signal 6 (Aborted) - core dumped Aborted $ powerpc-unknown-linux-gnu-gcc -v Using built-in specs. COLLECT_GCC=/repo/gcc-trunk//binary-latest-powerpc/bin/powerpc-unknown-linux-gnu-gcc COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-249846-checking-yes-rtl-df-extra-nographite-powerpc/bin/../libexec/gcc/powerpc-unknown-linux-gnu/8.0.0/lto-wrapper Target: powerpc-unknown-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 --with-sysroot=/usr/powerpc-unknown-linux-gnu --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --target=powerpc-unknown-linux-gnu --with-ld=/usr/bin/powerpc-unknown-linux-gnu-ld --with-as=/usr/bin/powerpc-unknown-linux-gnu-as --disable-libstdcxx-pch --prefix=/repo/gcc-trunk//binary-trunk-249846-checking-yes-rtl-df-extra-nographite-powerpc Thread model: posix gcc version 8.0.0 20170630 (experimental) (GCC) The situations seems to be the same as in comment #0: $ cat testcase.c.259.init-regs ... (insn 73 99 75 5 (parallel [ (set (reg:SI 185 [+4 ]) (minus:SI (const_int 0 [0]) (reg:SI 179 [ _5+4 ]))) (set (reg:SI 76 ca) (leu:SI (reg:SI 179 [ _5+4 ]) (const_int 0 [0]))) ]) "testcase.c":12 104 {subfsi3_carry} (expr_list:REG_UNUSED (reg:SI 76 ca) (nil))) ... (insn 79 78 80 5 (parallel [ (set (reg:SI 187 [+4 ]) (plus:SI (reg:SI 189 [+4 ]) (const_int 1 [0x1]))) (set (reg:SI 76 ca) (geu:SI (reg:SI 189 [+4 ]) (const_int -1 [0xffffffffffffffff]))) ]) "testcase.c":14 79 {*addsi3_imm_carry_pos} (expr_list:REG_DEAD (reg:SI 189 [+4 ]) (nil))) (insn 80 79 102 5 (parallel [ (set (reg:SI 186) (plus:SI (reg:SI 185 [+4 ]) (reg:SI 76 ca))) (clobber (reg:SI 76 ca)) ]) "testcase.c":14 85 {addsi3_carry_in_0} (expr_list:REG_DEAD (reg:SI 185 [+4 ]) (expr_list:REG_DEAD (reg:SI 76 ca) (expr_list:REG_UNUSED (reg:SI 76 ca) (nil))))) ... $ cat testcase.c.261.combine ... Trying 73 -> 80: Failed to match this instruction: (parallel [ (set (reg:SI 186) (const_int 1 [0x1])) (clobber (reg:SI 76 ca)) ]) Successfully matched this instruction: (set (reg:SI 186) (const_int 1 [0x1])) allowing combination of insns 73 and 80 original costs 0 + 4 = 0 replacement cost 4 deferring deletion of insn with uid = 73. deferring deletion of insn with uid = 73. modifying insn i3 80: r186:SI=0x1 deferring rescan insn with uid = 80. ... But the analysis I did in comment #0 is not correct, since I misunderstood some of the semantics from the dumps.