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[BranchFolding] Salvage DBG_VALUE instructions from empty blocks
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Summary:
This patch will introduce copying of DBG_VALUE instructions
from an otherwise empty basic block to predecessor/successor
blocks in case the empty block is eliminated/bypassed. It
is currently only done in one identified situation in the
BranchFolding pass, before optimizing on empty block.
It can be seen as a light variant of the propagation done
by the LiveDebugValues pass, which unfortunately is executed
after the BranchFolding pass.

We only propagate (copy) DBG_VALUE instructions in a limited
number of situations:
 a) If the empty BB is the only predecessor of a successor
    we can copy the DBG_VALUE instruction to the beginning of
    the successor (because the DBG_VALUE instruction is always
    part of the flow between the blocks).
 b) If the empty BB is the only successor of a predecessor
    we can copy the DBG_VALUE instruction to the end of the
    predecessor (because the DBG_VALUE instruction is always
    part of the flow between the blocks). In this case we add
    the DBG_VALUE just before the first terminator (assuming
    that the terminators do not impact the DBG_VALUE).

A future solution, to handle more situations, could perhaps
be to run the LiveDebugValues pass before branch folding?

This fix is related to PR37234. It is expected to resolve
the problem seen, when applied together with the fix in
SelectionDAG from here: https://reviews.llvm.org/D46129

Reviewers: #debug-info, aprantl, rnk

Reviewed By: #debug-info, aprantl

Subscribers: ormris, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D46184

llvm-svn: 331183
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@bjope
bjope committed Apr 30, 2018
1 parent abafca6 commit 9a8483a
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46 changes: 46 additions & 0 deletions llvm/lib/CodeGen/BranchFolding.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1356,6 +1356,51 @@ static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
return DebugLoc();
}

static void copyDebugInfoToPredecessor(const TargetInstrInfo *TII,
MachineBasicBlock &MBB,
MachineBasicBlock &PredMBB) {
auto InsertBefore = PredMBB.getFirstTerminator();
for (MachineInstr &MI : MBB.instrs())
if (MI.isDebugValue()) {
TII->duplicate(PredMBB, InsertBefore, MI);
DEBUG(dbgs() << "Copied debug value from empty block to pred: " << MI);
}
}

static void copyDebugInfoToSuccessor(const TargetInstrInfo *TII,
MachineBasicBlock &MBB,
MachineBasicBlock &SuccMBB) {
auto InsertBefore = SuccMBB.SkipPHIsAndLabels(SuccMBB.begin());
for (MachineInstr &MI : MBB.instrs())
if (MI.isDebugValue()) {
TII->duplicate(SuccMBB, InsertBefore, MI);
DEBUG(dbgs() << "Copied debug value from empty block to succ: " << MI);
}
}

// Try to salvage DBG_VALUE instructions from an otherwise empty block. If such
// a basic block is removed we would lose the debug information unless we have
// copied the information to a predecessor/successor.
//
// TODO: This function only handles some simple cases. An alternative would be
// to run a heavier analysis, such as the LiveDebugValues pass, before we do
// branch folding.
static void salvageDebugInfoFromEmptyBlock(const TargetInstrInfo *TII,
MachineBasicBlock &MBB) {
assert(IsEmptyBlock(&MBB) && "Expected an empty block (except debug info).");
// If this MBB is the only predecessor of a successor it is legal to copy
// DBG_VALUE instructions to the beginning of the successor.
for (MachineBasicBlock *SuccBB : MBB.successors())
if (SuccBB->pred_size() == 1)
copyDebugInfoToSuccessor(TII, MBB, *SuccBB);
// If this MBB is the only successor of a predecessor it is legal to copy the
// DBG_VALUE instructions to the end of the predecessor (just before the
// terminators, assuming that the terminator isn't affecting the DBG_VALUE).
for (MachineBasicBlock *PredBB : MBB.predecessors())
if (PredBB->succ_size() == 1)
copyDebugInfoToPredecessor(TII, MBB, *PredBB);
}

bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
bool MadeChange = false;
MachineFunction &MF = *MBB->getParent();
Expand All @@ -1380,6 +1425,7 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
// optimized away.
if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
SameFunclet) {
salvageDebugInfoFromEmptyBlock(TII, *MBB);
// Dead block? Leave for cleanup later.
if (MBB->pred_empty()) return MadeChange;

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125 changes: 125 additions & 0 deletions llvm/test/DebugInfo/X86/PR37234.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,125 @@
; RUN: llc %s -o - | FileCheck %s

; This test case was generated by running
;
; clang++ -O2 -g PR37234.cpp -S -save-temps
; opt -O2 -S PR37234.bc -o PR37234.ll
;
; using the following PR37234.cpp program
;
; int main() {
; const char* buffer = "aaa";
; unsigned aa = 0;
; while (char c = *buffer++) {
; if (c == 'a')
; ++aa; // DexWatch('aa')
; }
; return aa;
; }


; CHECK-LABEL: # %bb.{{.*}}:
; CHECK: #DEBUG_VALUE: main:aa <- 0
; CHECK: #DEBUG_VALUE: main:aa <- $[[REG:[0-9a-z]+]]
; CHECK: jmp .LBB0_1
; CHECK: .LBB0_3:
; CHECK: #DEBUG_VALUE: main:aa <- $[[REG]]
; CHECK: incl %[[REG]]
; CHECK: #DEBUG_VALUE: main:aa <- $[[REG]]
; CHECK: .LBB0_1:
; CHECK: #DEBUG_VALUE: main:aa <- $[[REG]]
; CHECK: je .LBB0_4
; CHECK: # %bb.{{.*}}:
; CHECK: #DEBUG_VALUE: main:aa <- $[[REG]]
; CHECK: jne .LBB0_1
; CHECK: jmp .LBB0_3
; CHECK: .LBB0_4:
; CHECK: #DEBUG_VALUE: main:aa <- $[[REG]]
; CHECK: retq

source_filename = "PR37234.cpp"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@.str = private unnamed_addr constant [4 x i8] c"aaa\00", align 1

; Function Attrs: norecurse nounwind readonly uwtable
define dso_local i32 @main() local_unnamed_addr #0 !dbg !7 {
entry:
call void @llvm.dbg.value(metadata i32 0, metadata !16, metadata !DIExpression()), !dbg !19
br label %while.cond.outer, !dbg !20

while.cond.outer: ; preds = %if.then, %entry
%buffer.0.ph = phi i8* [ %incdec.ptr, %if.then ], [ getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i64 0, i64 0), %entry ]
%aa.0.ph = phi i32 [ %inc, %if.then ], [ 0, %entry ]
br label %while.cond, !dbg !21

while.cond: ; preds = %while.cond.outer, %while.cond
%buffer.0 = phi i8* [ %incdec.ptr, %while.cond ], [ %buffer.0.ph, %while.cond.outer ], !dbg !22
call void @llvm.dbg.value(metadata i32 %aa.0.ph, metadata !16, metadata !DIExpression()), !dbg !19
call void @llvm.dbg.value(metadata i8* %buffer.0, metadata !12, metadata !DIExpression()), !dbg !23
%incdec.ptr = getelementptr inbounds i8, i8* %buffer.0, i64 1, !dbg !21
call void @llvm.dbg.value(metadata i8* %incdec.ptr, metadata !12, metadata !DIExpression()), !dbg !23
%0 = load i8, i8* %buffer.0, align 1, !dbg !24, !tbaa !25
call void @llvm.dbg.value(metadata i8 %0, metadata !18, metadata !DIExpression()), !dbg !28
switch i8 %0, label %while.cond [
i8 0, label %while.end
i8 97, label %if.then
], !dbg !20

if.then: ; preds = %while.cond
%inc = add i32 %aa.0.ph, 1, !dbg !29
call void @llvm.dbg.value(metadata i32 %inc, metadata !16, metadata !DIExpression()), !dbg !19
br label %while.cond.outer, !dbg !29

while.end: ; preds = %while.cond
call void @llvm.dbg.value(metadata i32 %aa.0.ph, metadata !16, metadata !DIExpression()), !dbg !19
call void @llvm.dbg.value(metadata i32 %aa.0.ph, metadata !16, metadata !DIExpression()), !dbg !19
call void @llvm.dbg.value(metadata i32 %aa.0.ph, metadata !16, metadata !DIExpression()), !dbg !19
call void @llvm.dbg.value(metadata i32 %aa.0.ph, metadata !16, metadata !DIExpression()), !dbg !19
ret i32 %aa.0.ph, !dbg !32
}

; Function Attrs: nounwind readnone speculatable
declare void @llvm.dbg.value(metadata, metadata, metadata) #1

attributes #0 = { norecurse nounwind readonly uwtable }
attributes #1 = { nounwind readnone speculatable }

!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!3, !4, !5}
!llvm.ident = !{!6}

!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 7.0.0 (trunk 330946) (llvm/trunk 330952)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
!1 = !DIFile(filename: "PR37234.cpp", directory: "")
!2 = !{}
!3 = !{i32 2, !"Dwarf Version", i32 4}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = !{i32 1, !"wchar_size", i32 4}
!6 = !{!"clang version 7.0.0 (trunk 330946) (llvm/trunk 330952)"}
!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !11)
!8 = !DISubroutineType(types: !9)
!9 = !{!10}
!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!11 = !{!12, !16, !18}
!12 = !DILocalVariable(name: "buffer", scope: !7, file: !1, line: 2, type: !13)
!13 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !14, size: 64)
!14 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !15)
!15 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
!16 = !DILocalVariable(name: "aa", scope: !7, file: !1, line: 3, type: !17)
!17 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
!18 = !DILocalVariable(name: "c", scope: !7, file: !1, line: 4, type: !15)
!19 = !DILocation(line: 3, column: 18, scope: !7)
!20 = !DILocation(line: 4, column: 9, scope: !7)
!21 = !DILocation(line: 4, column: 32, scope: !7)
!22 = !DILocation(line: 0, scope: !7)
!23 = !DILocation(line: 2, column: 21, scope: !7)
!24 = !DILocation(line: 4, column: 25, scope: !7)
!25 = !{!26, !26, i64 0}
!26 = !{!"omnipotent char", !27, i64 0}
!27 = !{!"Simple C++ TBAA"}
!28 = !DILocation(line: 4, column: 21, scope: !7)
!29 = !DILocation(line: 6, column: 25, scope: !30)
!30 = distinct !DILexicalBlock(scope: !31, file: !1, line: 5, column: 21)
!31 = distinct !DILexicalBlock(scope: !7, file: !1, line: 4, column: 36)
!32 = !DILocation(line: 9, column: 1, scope: !7)

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