[Bug rtl-optimization/82024] New: [8 Regression] wrong code with -Og -fgcse-sm -frerun-cse-after-loop --param=max-combine-insns=3

["zsojka at seznam dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82024

            Bug ID: 82024
           Summary: [8 Regression] wrong code with -Og -fgcse-sm
                    -frerun-cse-after-loop --param=max-combine-insns=3
           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: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu

Created attachment 42079
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42079&action=edit
reduced testcase

Output:
$ x86_64-pc-linux-gnu-gcc -Og -fgcse-sm -frerun-cse-after-loop
--param=max-combine-insns=3 testcase.c
$ ./a.out 
Aborted

$ 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-251387-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-251387-checking-yes-rtl-df-extra-nographite-amd64
Thread model: posix
gcc version 8.0.0 20170828 (experimental) (GCC) 


.combine does:
...
Trying 13, 12 -> 14:
...
Successfully matched this instruction:
(set (reg:DI 106)
    (const_int 0 [0]))
Successfully matched this instruction:
(set (reg:DI 105)
    (plus:DI (reg/v:DI 101 [ eD.1821 ])
        (reg:DI 103)))
allowing combination of insns 12, 13 and 14
original costs 4 + 4 + 0 = 0
replacement costs 4 + 4 = 8
deferring deletion of insn with uid = 12.
modifying insn i2    13: r106:DI=0
deferring rescan insn with uid = 13.
modifying insn i3    14: r105:DI=r101:DI+r103:DI
      REG_DEAD r101:DI
      REG_DEAD r103:DI
deferring rescan insn with uid = 14.
...
Trying 14, 13 -> 20:
Successfully matched this instruction:
(set (reg:DI 105)
    (plus:DI (reg/v:DI 101 [ eD.1821 ])
        (reg:DI 103)))
allowing combination of insns 13, 14 and 20
original costs 4 + 4 + 6 = 14
replacement cost 4
deferring rescan insn with uid = 20.
deferring deletion of insn with uid = 14.
deferring deletion of insn with uid = 13.
modifying insn i3    20: r105:DI=r101:DI+r103:DI
      REG_DEAD r103:DI
      REG_DEAD r101:DI
deferring rescan insn with uid = 20.
...


which results in:
...
(insn 17 16 20 2 (parallel [
            (set (reg/v:DI 100 [ <retval> ])
                (plus:DI (reg:DI 107 [ _2 ])
                    (reg:DI 105)))
            (clobber (reg:CC 17 flags))
        ]) "testcase.c":16 218 {*adddi_1}
     (expr_list:REG_DEAD (reg:DI 107 [ _2 ])
        (expr_list:REG_DEAD (reg:DI 105)
            (expr_list:REG_UNUSED (reg:CC 17 flags)
                (nil)))))
(insn 20 17 25 2 (set (reg:DI 105)
        (plus:DI (reg/v:DI 101 [ eD.1821 ])
            (reg:DI 103))) "testcase.c":18 214 {*leadi}
     (expr_list:REG_DEAD (reg:DI 103)
        (expr_list:REG_DEAD (reg/v:DI 101 [ eD.1821 ])
            (nil))))
...

insn 17 uses r105, which is now set in insn 20, which appears after it