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Robert Lougher
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Remove debug location from common tail when tail-merging
The branch folding pass tail merges blocks into a common-tail. However, the tail retains the debug information from one of the original inputs to the merge (chosen randomly). This is a problem for sampled-based PGO, as hits on the common-tail will be attributed to whichever block was chosen, irrespective of which path was actually taken to the common-tail. This patch fixes the issue by nulling the debug location for the common-tail. Differential Revision: https://reviews.llvm.org/D25742 llvm-svn: 285093
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3 files changed

+85
-8
lines changed

3 files changed

+85
-8
lines changed

llvm/lib/CodeGen/BranchFolding.cpp

Lines changed: 5 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -720,8 +720,6 @@ bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
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SameTails[commonTailIndex].getTailStartPos();
721721
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
722722

723-
// If the common tail includes any debug info we will take it pretty
724-
// randomly from one of the inputs. Might be better to remove it?
725723
DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
726724
<< maxCommonTailLength);
727725

@@ -898,6 +896,11 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
898896
// Recompute common tail MBB's edge weights and block frequency.
899897
setCommonTailEdgeWeights(*MBB);
900898

899+
// Remove the original debug location from the common tail.
900+
for (auto &MI : *MBB)
901+
if (!MI.isDebugValue())
902+
MI.setDebugLoc(DebugLoc());
903+
901904
// MBB is common tail. Adjust all other BB's to jump to this one.
902905
// Traversal must be forwards so erases work.
903906
DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()

llvm/test/DebugInfo/COFF/local-variables.ll

Lines changed: 4 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -63,11 +63,9 @@
6363
; ASM: .cv_loc 2 1 5 3 # t.cpp:5:3
6464
; ASM: callq capture
6565
; ASM: leaq 36(%rsp), %rcx
66-
; ASM: [[inline_site2_end:\.Ltmp.*]]:
66+
; ASM: [[else_end:\.Ltmp.*]]:
6767
; ASM: .LBB0_3: # %if.end
68-
; ASM: .cv_loc 0 1 15 5 # t.cpp:15:5
6968
; ASM: callq capture
70-
; ASM: [[else_end:\.Ltmp.*]]:
7169
; ASM: .cv_loc 0 1 17 1 # t.cpp:17:1
7270
; ASM: nop
7371
; ASM: addq $56, %rsp
@@ -101,7 +99,7 @@
10199
; ASM: .long 116 # TypeIndex
102100
; ASM: .short 0 # Flags
103101
; ASM: .asciz "v"
104-
; ASM: .cv_def_range [[inline_site2]] [[inline_site2_end]], "E\021O\001\000\0000\000\000\000"
102+
; ASM: .cv_def_range [[inline_site2]] [[else_end]], "E\021O\001\000\0000\000\000\000"
105103
; ASM: .short 4430 # Record kind: S_INLINESITE_END
106104

107105
; OBJ: Subsection [
@@ -159,7 +157,7 @@
159157
; OBJ: LocalVariableAddrRange {
160158
; OBJ: OffsetStart: .text+0x2D
161159
; OBJ: ISectStart: 0x0
162-
; OBJ: Range: 0x24
160+
; OBJ: Range: 0x1F
163161
; OBJ: }
164162
; OBJ: }
165163
; OBJ: InlineSite {
@@ -200,7 +198,7 @@
200198
; OBJ: ChangeLineOffset: 1
201199
; OBJ: ChangeCodeOffset: 0x35
202200
; OBJ: ChangeCodeOffsetAndLineOffset: {CodeOffset: 0xD, LineOffset: 1}
203-
; OBJ: ChangeCodeLength: 0xA
201+
; OBJ: ChangeCodeLength: 0xF
204202
; OBJ: ]
205203
; OBJ: }
206204
; OBJ: Local {

llvm/test/DebugInfo/X86/tail-merge.ll

Lines changed: 76 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,76 @@
1+
; RUN: llc %s -mtriple=x86_64-unknown-unknown -use-unknown-locations=true -o - | FileCheck %s
2+
3+
; Generated with "clang -gline-tables-only -c -emit-llvm -o - | opt -sroa -S"
4+
; from source:
5+
;
6+
; extern int foo(int);
7+
; extern int bar(int);
8+
;
9+
; int test(int a, int b) {
10+
; if(b)
11+
; a += foo(a);
12+
; else
13+
; a += bar(a);
14+
; return a;
15+
; }
16+
17+
; When tail-merging the debug location of the common tail should be removed.
18+
19+
; CHECK-LABEL: test:
20+
; CHECK: movl %edi, [[REG:%.*]]
21+
; CHECK: testl %esi, %esi
22+
; CHECK: je [[ELSE:.LBB[0-9]+_[0-9]+]]
23+
; CHECK: .loc 1 6 10
24+
; CHECK: callq foo
25+
; CHECK: jmp [[TAIL:.LBB[0-9]+_[0-9]+]]
26+
; CHECK: [[ELSE]]:
27+
; CHECK: .loc 1 8 10
28+
; CHECK: callq bar
29+
; CHECK: [[TAIL]]:
30+
; CHECK: .loc 1 0 0
31+
; CHECK: addl [[REG]], %eax
32+
; CHECK: .loc 1 9 3
33+
34+
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
35+
target triple = "x86_64-unknown-linux-gnu"
36+
37+
define i32 @test(i32 %a, i32 %b) !dbg !6 {
38+
entry:
39+
%tobool = icmp ne i32 %b, 0, !dbg !8
40+
br i1 %tobool, label %if.then, label %if.else, !dbg !8
41+
42+
if.then: ; preds = %entry
43+
%call = call i32 @foo(i32 %a), !dbg !9
44+
%add = add nsw i32 %a, %call, !dbg !10
45+
br label %if.end, !dbg !11
46+
47+
if.else: ; preds = %entry
48+
%call1 = call i32 @bar(i32 %a), !dbg !12
49+
%add2 = add nsw i32 %a, %call1, !dbg !13
50+
br label %if.end
51+
52+
if.end: ; preds = %if.else, %if.then
53+
%a.addr.0 = phi i32 [ %add, %if.then ], [ %add2, %if.else ]
54+
ret i32 %a.addr.0, !dbg !14
55+
}
56+
57+
declare i32 @foo(i32)
58+
declare i32 @bar(i32)
59+
60+
!llvm.dbg.cu = !{!0}
61+
!llvm.module.flags = !{!3, !4}
62+
63+
!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "", isOptimized: false, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2)
64+
!1 = !DIFile(filename: "test.c", directory: "")
65+
!2 = !{}
66+
!3 = !{i32 2, !"Dwarf Version", i32 4}
67+
!4 = !{i32 2, !"Debug Info Version", i32 3}
68+
!6 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 4, type: !7, isLocal: false, isDefinition: true, scopeLine: 4, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
69+
!7 = !DISubroutineType(types: !2)
70+
!8 = !DILocation(line: 5, column: 6, scope: !6)
71+
!9 = !DILocation(line: 6, column: 10, scope: !6)
72+
!10 = !DILocation(line: 6, column: 7, scope: !6)
73+
!11 = !DILocation(line: 6, column: 5, scope: !6)
74+
!12 = !DILocation(line: 8, column: 10, scope: !6)
75+
!13 = !DILocation(line: 8, column: 7, scope: !6)
76+
!14 = !DILocation(line: 9, column: 3, scope: !6)

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