[Bug rtl-optimization/94613] New: combine: Wrong code due to splitting a simplified IF_THEN_ELSE

[krebbel at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94613

            Bug ID: 94613
           Summary: combine: Wrong code due to splitting a simplified
                    IF_THEN_ELSE
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krebbel at gcc dot gnu.org
  Target Milestone: ---

Created attachment 48284
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48284&action=edit
autoreduced testcase

When compiling the attached testcase for IBM Z GCC optimizes an OR operation
away during combine leading to wrong code being generated:

cc1plus -O3 -march=z14 t.ii -quiet -o t.s

try_combine is invoked for the following 3 insns:

i1: 28: r111 = r110 | r158
i2: 31: r115 = r111 - r94
i3: 36: r122 = (r110 == 0 ? r115 : r177)

i3 becomes the following after inserting i1 and i2

36: r122 = (r110 == 0 ? r110 | r158 - r94 : r177)

simplification omits ORing r110 since it is 0 anyway:

36: r122 = (r110 == 0 ? r158 - r94 : r177)

after not being able to recognize the insn it gets split into parts again:

28: deleted
31: r115 = r158 - r94
36: r122 = (r110 == 0 ? r115 : r177)