[PATCH] D48765: [X86] The TEST instruction is eliminated when BSF/TZCNT is used

[Ivan Kulagin via Phabricator via llvm-commits]

ikulagin added a comment.

> We need tests cases that use tzcnt too.

I can add the test for CTTZ, but in the case of using CTTZ the TEST instruction
can't be eliminated because it produces three basic blocks as follows:

  bb.0:
    liveins: $edi
    %2:gr32 = COPY $edi
    %3:gr32 = MOV32ri 32
    TEST32rr %2:gr32, %2:gr32, implicit-def $eflags
    JE_1 %bb.2, implicit $eflags
    JMP_1 %bb.1
  bb.1.cond.false:
    %0:gr32 = BSF32rr %2:gr32, implicit-def dead $eflags
  bb.2.cond.end:
    %1:gr32 = PHI %3:gr32, %bb.0, %0:gr32, %bb.1
    TEST32rr %2:gr32, %2:gr32, implicit-def $eflags
    %4:gr32 = MOV32ri -1
    %5:gr32 = CMOVE32rr %4:gr32, %1:gr32, implicit $eflags
    %6:gr32 = ADD32ri8 %5:gr32, 6, implicit-def dead $eflags
    $eax = COPY %6:gr32
    RET 0, $eax

We should analyze the bb.0 and bb.1.cond.false to eliminate the TEST instruction followed by the CMOV.
Does it make sence to implement such analysis and transformation or it will suffice to eliminate
the TEST when CTTZ_ZERO_UNDEF is used?


Repository:
  rL LLVM

https://reviews.llvm.org/D48765