[PATCH] D32999: [UnreachableBlockElim] Check return value of constrainRegClass().

[Mikael Holmén via Phabricator via llvm-commits]

uabelho added a comment.

Not sure who is code owner of IfConversion.cpp so I added the last three persons who changed
anything there as reviwers.

I found this problem on my out-of-tree target. I've tried to make a reproducer also on Hexagon
but I failed.

My out-of-tree test case looks like:

#RUN: llc -O0 -mtriple phoenix -mcpu=phoenixV -o - %s -run-pass=unreachable-mbb-elimination | FileCheck %s

- | define void @foo() { bb0: %_tmp0 = load i16*, i16** undef, align 1 br label %bb1

  bb1:                                              ; preds = %bb0 %_tmp4 = phi i16* [ %_tmp0, %bb0 ] %_tmp5 = icmp eq i16* %_tmp4, undef br label %bb2

  bb2:                                              ; preds = %bb1 ret void }

...
---

name:            foo
alignment:       0
exposesReturnsTwice: false
noVRegs:         false
legalized:       false
regBankSelected: false
selected:        false
tracksRegLiveness: true
registers:

- { id: 0, class: rn }
- { id: 1, class: anh_0_7 }
- { id: 2, class: rn }
- { id: 3, class: anh_0_7 }

frameInfo:

  isFrameAddressTaken: false
  isReturnAddressTaken: false
  hasStackMap:     false
  hasPatchPoint:   false
  stackSize:       0
  offsetAdjustment: 0
  maxAlignment:    0
  adjustsStack:    false
  hasCalls:        true
  maxCallFrameSize: 0
  hasOpaqueSPAdjustment: false
  hasVAStart:      false
  hasMustTailInVarArgFunc: false

body:             |

  bb.0:
    successors: %bb.1
  
    %2 = IMPLICIT_DEF
    %0 = mv_r16_rmod1_ar16 killed %2, 0, _, 0, implicit %dc0, implicit %dc1, implicit %ac0, implicit %ac1
    brr_uncond %bb.1
  
  bb.1:
    successors: %bb.2
  
    %1 = PHI %0, %bb.0
    %3 = IMPLICIT_DEF
    cmp_a16_a16 %1, killed %3, 0, _, 0, implicit-def %ccreg
    brr_uncond %bb.2
  
  bb.2:
    rets 0, _, 0, implicit-def dead %sp, implicit %sp, implicit %cuc

...

1. The important part here is that we do not use %0 directly in the cmp since %0
2. has regclass rn and the cmp only accepts anh_0_7. So we need a copy from %0
3. to %1, and %1 has regclass anh_0_7

#CHECK: registers:
#CHECK:   - { id: 0, class: rn }
#CHECK:   - { id: 1, class: anh_0_7 }

#CHECK:   bb.0:
#CHECK:     successors: %bb.1
#CHECK:     %0 = mv_r16_rmod1_ar16 killed %2

#CHECK:   bb.1:
#CHECK:     %1 = COPY %0
#CHECK:     cmp_a16_a16 %1, killed %3


https://reviews.llvm.org/D32999