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[GVNHoist] Merge DebugLoc metadata on hoisted instructions
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Authored by twoh on Jan 31 2017, 11:47 PM.

Details

Summary

When instructions are hoisted, current implementation keeps DebugLoc metadata of the instruction that chosen as Repl (and its GEP operand if Repl is a load or a store). However, DebugLoc metadata should be updated to the 'merged' location across all hoisted instructions. See the following example code:

 1:  typedef struct {
 2:    int a[10];
 3:  } S1;
 4: 
 5:  extern S1 *s1[10];
 6: 
 7:  void foo(int x, int y, int i) {
 8:    if (y)
 9:      s1[i]->a[i] = x + y;
10:    else
11:      s1[i]->a[i] = x;
12:  }

Below is LLVM IR representation of the program before gvn-hoist:

%struct.S1 = type { [10 x i32] }
@s1 = external local_unnamed_addr global [10 x %struct.S1*], align 16

define void @foo(i32 %x, i32 %y, i32 %i) !dbg !4 {
entry:
  %tobool = icmp ne i32 %y, 0, !dbg !8
  br i1 %tobool, label %if.then, label %if.else, !dbg !10

if.then:                                          ; preds = %entry
  %add = add nsw i32 %x, %y, !dbg !11
  %idxprom = sext i32 %i to i64, !dbg !12
  %arrayidx = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !12
  %0 = load %struct.S1*, %struct.S1** %arrayidx, align 8, !dbg !12, !tbaa !13
  %a = getelementptr inbounds %struct.S1, %struct.S1* %0, i32 0, i32 0, !dbg !17
  br label %if.end, !dbg !12

if.else:                                          ; preds = %entry
  %idxprom3 = sext i32 %i to i64, !dbg !18
  %arrayidx4 = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom3, !dbg !18
  %1 = load %struct.S1*, %struct.S1** %arrayidx4, align 8, !dbg !18, !tbaa !13
  %a5 = getelementptr inbounds %struct.S1, %struct.S1* %1, i32 0, i32 0, !dbg !19
  br label %if.end

if.end:                                           ; preds = %if.else, %if.then
  %a5.sink = phi [10 x i32]* [ %a5, %if.else ], [ %a, %if.then ]
  %.sink = phi i32 [ %x, %if.else ], [ %add, %if.then ]
  %idxprom6 = sext i32 %i to i64
  %arrayidx7 = getelementptr inbounds [10 x i32], [10 x i32]* %a5.sink, i64 0, i64 %idxprom6
  store i32 %.sink, i32* %arrayidx7, align 4, !tbaa !20
  ret void, !dbg !22
}

where

!11 = !DILocation(line: 9, column: 18, scope: !9)
!12 = !DILocation(line: 9, column: 5, scope: !9)
!18 = !DILocation(line: 11, column: 5, scope: !9)
!19 = !DILocation(line: 11, column: 9, scope: !9)

. And below is after gvn-hoist:

define void @foo(i32 %x, i32 %y, i32 %i) !dbg !4 {
entry:
  %tobool = icmp ne i32 %y, 0, !dbg !8
  %idxprom = sext i32 %i to i64, !dbg !10
  %0 = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !10
  %1 = load %struct.S1*, %struct.S1** %0, align 8, !dbg !10, !tbaa !11
  br i1 %tobool, label %if.then, label %if.else, !dbg !15

if.then:                                          ; preds = %entry
  %add = add nsw i32 %x, %y, !dbg !16
  %arrayidx = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !10
  %a = getelementptr inbounds %struct.S1, %struct.S1* %1, i32 0, i32 0, !dbg !17
  br label %if.end, !dbg !10

if.else:                                          ; preds = %entry
  %arrayidx4 = getelementptr inbounds [10 x %struct.S1*], [10 x %struct.S1*]* @s1, i64 0, i64 %idxprom, !dbg !18
  %a5 = getelementptr inbounds %struct.S1, %struct.S1* %1, i32 0, i32 0, !dbg !19
  br label %if.end

if.end:                                           ; preds = %if.else, %if.then
  %a5.sink = phi [10 x i32]* [ %a5, %if.else ], [ %a, %if.then ]
  %.sink = phi i32 [ %x, %if.else ], [ %add, %if.then ]
  %arrayidx7 = getelementptr inbounds [10 x i32], [10 x i32]* %a5.sink, i64 0, i64 %idxprom
  store i32 %.sink, i32* %arrayidx7, align 4, !tbaa !20
  ret void, !dbg !22
}

As you see, loads and their GEPs have been hosited from if.then/if.else block to entry block. However, DebugLoc metadata of these new instructions are still same as the instructions in if.then block, as they are moved/cloned from if.then block. This may result incorrect stepping and imprecise sample profile result.

Event Timeline

twoh created this revision.Jan 31 2017, 11:47 PM
twoh edited the summary of this revision. (Show Details)Jan 31 2017, 11:47 PM
sebpop edited edge metadata.Feb 1 2017, 6:05 AM

Thanks Taewook for fixing this!
The patch looks good to me.
I cannot approve the patch, and I would like someone more knowledgeable with debug info to have a look as well.

sebpop accepted this revision.Feb 6 2017, 1:02 PM

The change looks simple enough. Please commit.

This revision is now accepted and ready to land.Feb 6 2017, 1:02 PM
twoh closed this revision.Feb 6 2017, 2:16 PM
twoh added a comment.Feb 6 2017, 2:18 PM

@sebpop Thank you for your review!