[PATCH] D98218: [LSR] fix a issue that LoopStrengthReduction drop debug location unnecessarily

[Yuanbo Li via Phabricator via llvm-commits]

yuanboli233 added a comment.

As @jmorse pointed out, OptimizeLoopTermCond might move the newly created instruction around. I was working on an example to trigger an instruction move and the replacement of the conditional. Here I have an example, I show part of the code here:

  define void @foobar(i32 %n) !dbg !6 {
  bb.nph:
    %cond = icmp eq i32 %n, 0, !dbg !16
    call void @llvm.dbg.value(metadata i1 %cond, metadata !9, metadata !DIExpression()), !dbg !16
    %umax = select i1 %cond, i32 1, i32 %n, !dbg !17
    call void @llvm.dbg.value(metadata i32 %umax, metadata !11, metadata !DIExpression()), !dbg !17
    br label %bb, !dbg !18
  
  bb:                                               ; preds = %bb, %bb.nph
    %i.03 = phi i32 [ 0, %bb.nph ], [ %indvar.next, %bb ], !dbg !19
    call void @llvm.dbg.value(metadata i32 %i.03, metadata !13, metadata !DIExpression()), !dbg !19
    %indvar.next = add nuw nsw i32 %i.03, 1, !dbg !20
    call void @llvm.dbg.value(metadata i32 %indvar.next, metadata !14, metadata !DIExpression()), !dbg !20
    %exitcond = icmp eq i32 %indvar.next, %umax, !dbg !21
    call void @llvm.dbg.value(metadata i1 %exitcond, metadata !15, metadata !DIExpression()), !dbg !21
    br i1 %exitcond, label %return, label %bb, !dbg !22
  
  return:                                           ; preds = %bb
    ret void, !dbg !23
  }

is optimized to

  define void @foobar(i32 %n) !dbg !6 {
  bb.nph:
    call void @llvm.dbg.value(metadata i1 undef, metadata !9, metadata !DIExpression()), !dbg !16
    call void @llvm.dbg.value(metadata i32 undef, metadata !11, metadata !DIExpression()), !dbg !17
    br label %bb, !dbg !18
  
  bb:                                               ; preds = %bb, %bb.nph
    %i.03 = phi i32 [ 0, %bb.nph ], [ %indvar.next, %bb ], !dbg !19
    call void @llvm.dbg.value(metadata i32 %i.03, metadata !13, metadata !DIExpression()), !dbg !19
    %indvar.next = add nuw nsw i32 %i.03, 1, !dbg !20
    call void @llvm.dbg.value(metadata i32 %indvar.next, metadata !14, metadata !DIExpression()), !dbg !20
    call void @llvm.dbg.value(metadata i1 %scmp, metadata !15, metadata !DIExpression()), !dbg !21
    %scmp = icmp uge i32 %indvar.next, %n, !dbg !21
    br i1 %scmp, label %return, label %bb, !dbg !22
  
  return:                                           ; preds = %bb
    ret void, !dbg !23
  }

The %exitcond is replaced by %scmp,  and %scmp is moved to the second last instruction of the block. Maybe the debug location should probably still be kept, but I am not very sure about it. Even the debug location should not be kept, it probably should be dropped during the instruction move in the OptimizeLoopTermCond function. May I know your thoughts about this specific case?


Repository:
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https://reviews.llvm.org/D98218