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[DeadStoreElimination] Salvage debug info from dead insts
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According to `llvm-dwarfdump --statistics` this salvages 43 additional
unique source variables in a stage2 build of clang. It increases the
size of the .debug_loc section by 0.002% (or 2864 bytes).

Differential Revision: https://reviews.llvm.org/D43220

llvm-svn: 325035
vedantk committed Feb 13, 2018
1 parent f381e94 commit 35fc103
Showing 2 changed files with 43 additions and 0 deletions.
3 changes: 3 additions & 0 deletions llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
Original file line number Diff line number Diff line change
@@ -115,6 +115,9 @@ deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
Instruction *DeadInst = NowDeadInsts.pop_back_val();
++NumFastOther;

// Try to preserve debug information attached to the dead instruction.
salvageDebugInfo(*DeadInst);

// This instruction is dead, zap it, in stages. Start by removing it from
// MemDep, which needs to know the operands and needs it to be in the
// function.
40 changes: 40 additions & 0 deletions llvm/test/Transforms/DeadStoreElimination/debuginfo.ll
Original file line number Diff line number Diff line change
@@ -0,0 +1,40 @@
; RUN: opt < %s -debugify -basicaa -dse -S | FileCheck %s

target datalayout = "E-p:64:64:64-a0:0:8-f32:32:32-f64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-v64:64:64-v128:128:128"

declare noalias i8* @malloc(i32)

declare void @test_f()

define i32* @test_salvage() {
; CHECK-LABEL: @test_salvage()
; CHECK-NEXT: malloc
; CHECK-NEXT: bitcast
; CHECK-NEXT: call void @test_f()
; CHECK-NEXT: store i32 0, i32* %P

; Check that all four original local variables have their values preserved.
; CHECK-NEXT: call void @llvm.dbg.value(metadata i8* %p, metadata !8, metadata !DIExpression()), !dbg !14
; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata !10, metadata !DIExpression()), !dbg !15
; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata !11, metadata !DIExpression(DW_OP_deref)), !dbg !18
; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata !13, metadata !DIExpression(DW_OP_deref, DW_OP_plus_uconst, 1, DW_OP_stack_value)), !dbg !19
%p = tail call i8* @malloc(i32 4)
%P = bitcast i8* %p to i32*
%DEAD = load i32, i32* %P
%DEAD2 = add i32 %DEAD, 1
store i32 %DEAD2, i32* %P
call void @test_f()
store i32 0, i32* %P
ret i32* %P
}

; CHECK: !8 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 1, type: !9)
; CHECK: !10 = !DILocalVariable(name: "2", scope: !5, file: !1, line: 2, type: !9)
; CHECK: !11 = !DILocalVariable(name: "3", scope: !5, file: !1, line: 3, type: !12)
; CHECK: !13 = !DILocalVariable(name: "4", scope: !5, file: !1, line: 4, type: !12)
; CHECK-DAG: !14 = !DILocation(line: 1, column: 1, scope: !5)
; CHECK-DAG: !15 = !DILocation(line: 2, column: 1, scope: !5)
; CHECK-DAG: !18 = !DILocation(line: 3, column: 1, scope: !5)
; CHECK-DAG: !19 = !DILocation(line: 4, column: 1, scope: !5)
; CHECK-DAG: !16 = !DILocation(line: 6, column: 1, scope: !5)
; CHECK-DAG: !17 = !DILocation(line: 7, column: 1, scope: !5)

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