[PATCH] D48721: Patch to fix pragma metadata for do-while loops

[Bjorn Pettersson via Phabricator via cfe-commits]

bjope added a comment.

I tried running

  /clang -cc1 -O3 -funroll-loops -S -emit-llvm pragma-do-while-unroll.cpp -o - -mllvm -print-after-all

and I get this

  ...
  !2 = distinct !{!2, !3}
  !3 = !{!"llvm.loop.unroll.count", i32 3}
  !4 = !{!5, !5, i64 0}
  !5 = !{!"int", !6, i64 0}
  !6 = !{!"omnipotent char", !7, i64 0}
  !7 = !{!"Simple C/C++ TBAA"}
  !8 = distinct !{!8, !9}
  !9 = !{!"llvm.loop.unroll.count", i32 5}
  *** IR Dump After Combine redundant instructions ***
  ; Function Attrs: nounwind
  define i32 @test(i32* %a, i32 %n) local_unnamed_addr #0 {
  entry:
    br label %do.body, !llvm.loop !2
  
  do.body:                                          ; preds = %do.body, %entry
    %i.0 = phi i32 [ 0, %entry ], [ %inc, %do.body ]
    %sum.0 = phi i32 [ 0, %entry ], [ %add5, %do.body ]
    %0 = zext i32 %i.0 to i64
    %arrayidx = getelementptr inbounds i32, i32* %a, i64 %0
    %1 = load i32, i32* %arrayidx, align 4, !tbaa !4
    %add = add nsw i32 %1, 1
    store i32 %add, i32* %arrayidx, align 4, !tbaa !4
    %add5 = add nsw i32 %sum.0, %add
    %inc = add nuw nsw i32 %i.0, 1
    %cmp = icmp slt i32 %inc, %n
    br i1 %cmp, label %do.body, label %do.end, !llvm.loop !2
  
  do.end:                                           ; preds = %do.body
    br label %do.body6, !llvm.loop !8
  
  do.body6:                                         ; preds = %do.body6, %do.end
    %i.1 = phi i32 [ 0, %do.end ], [ %inc15, %do.body6 ]
    %sum.1 = phi i32 [ %add5, %do.end ], [ %add14, %do.body6 ]
    %2 = zext i32 %i.1 to i64
    %arrayidx8 = getelementptr inbounds i32, i32* %a, i64 %2
    %3 = load i32, i32* %arrayidx8, align 4, !tbaa !4
    %add9 = add nsw i32 %3, 1
    store i32 %add9, i32* %arrayidx8, align 4, !tbaa !4
    %add14 = add nsw i32 %sum.1, %add9
    %inc15 = add nuw nsw i32 %i.1, 1
    %cmp17 = icmp slt i32 %inc15, %n
    br i1 %cmp17, label %do.body6, label %do.end18, !llvm.loop !8
  
  do.end18:                                         ; preds = %do.body6
    ret i32 %add14
  }
  *** IR Dump After Simplify the CFG ***
  ; Function Attrs: nounwind
  define i32 @test(i32* %a, i32 %n) local_unnamed_addr #0 {
  entry:
    br label %do.body, !llvm.loop !2
  
  do.body:                                          ; preds = %do.body, %entry
    %i.0 = phi i32 [ 0, %entry ], [ %inc, %do.body ]
    %sum.0 = phi i32 [ 0, %entry ], [ %add5, %do.body ]
    %0 = zext i32 %i.0 to i64
    %arrayidx = getelementptr inbounds i32, i32* %a, i64 %0
    %1 = load i32, i32* %arrayidx, align 4, !tbaa !4
    %add = add nsw i32 %1, 1
    store i32 %add, i32* %arrayidx, align 4, !tbaa !4
    %add5 = add nsw i32 %sum.0, %add
    %inc = add nuw nsw i32 %i.0, 1
    %cmp = icmp slt i32 %inc, %n
    br i1 %cmp, label %do.body, label %do.body6, !llvm.loop !8
  
  do.body6:                                         ; preds = %do.body, %do.body6
    %i.1 = phi i32 [ %inc15, %do.body6 ], [ 0, %do.body ]
    %sum.1 = phi i32 [ %add14, %do.body6 ], [ %add5, %do.body ]
    %2 = zext i32 %i.1 to i64
    %arrayidx8 = getelementptr inbounds i32, i32* %a, i64 %2
    %3 = load i32, i32* %arrayidx8, align 4, !tbaa !4
    %add9 = add nsw i32 %3, 1
    store i32 %add9, i32* %arrayidx8, align 4, !tbaa !4
    %add14 = add nsw i32 %sum.1, %add9
    %inc15 = add nuw nsw i32 %i.1, 1
    %cmp17 = icmp slt i32 %inc15, %n
    br i1 %cmp17, label %do.body6, label %do.end18, !llvm.loop !8
  
  do.end18:                                         ; preds = %do.body6
    ret i32 %add14
  }
  ...

So up until simplifyCFG I think things look pretty good with different loop-metadata for the two loops. But when simplifyCFG is tranforming

    br i1 %cmp, label %do.body, label %do.end, !llvm.loop !2
  
  do.end:                                           ; preds = %do.body
    br label %do.body6, !llvm.loop !8

into

  br i1 %cmp, label %do.body, label %do.body6, !llvm.loop !8

we get incorrect metadata for one branch target.

Is the fault that the metadata only should be put on the back edge, not the branch in the preheader?


Repository:
  rC Clang

https://reviews.llvm.org/D48721