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CarlosAlbertoEncisoCarlosAlbertoEnciso
committedOct 1, 2018
[DebugInfo][Dexter] Incorrect DBG_VALUE after MCP dead copy instruction removal.
When MachineCopyPropagation eliminates a dead 'copy', its associated debug information becomes invalid. as the recorded register has been removed. It causes the debugger to display wrong variable value. Differential Revision: https://reviews.llvm.org/D52614 llvm-svn: 343445
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5 files changed

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‎llvm/include/llvm/CodeGen/MachineInstr.h

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@@ -1544,6 +1544,10 @@ class MachineInstr
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/// Scan instructions following MI and collect any matching DBG_VALUEs.
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void collectDebugValues(SmallVectorImpl<MachineInstr *> &DbgValues);
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/// Find all DBG_VALUEs immediately following this instruction that point
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/// to a register def in this instruction and point them to \p Reg instead.
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void changeDebugValuesDefReg(unsigned Reg);
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private:
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/// If this instruction is embedded into a MachineFunction, return the
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/// MachineRegisterInfo object for the current function, otherwise

‎llvm/lib/CodeGen/MachineCSE.cpp

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@@ -181,12 +181,8 @@ bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
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LLVM_DEBUG(dbgs() << "Coalescing: " << *DefMI);
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LLVM_DEBUG(dbgs() << "*** to: " << *MI);
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// Collect matching debug values.
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SmallVector<MachineInstr *, 2> DbgValues;
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DefMI->collectDebugValues(DbgValues);
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// Propagate SrcReg to debug value instructions.
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for (auto *DBI : DbgValues)
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DBI->getOperand(0).setReg(SrcReg);
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// Update matching debug values.
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DefMI->changeDebugValuesDefReg(SrcReg);
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// Propagate SrcReg of copies to MI.
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MO.setReg(SrcReg);

‎llvm/lib/CodeGen/MachineCopyPropagation.cpp

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Original file line numberDiff line numberDiff line change
@@ -598,6 +598,11 @@ void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
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LLVM_DEBUG(dbgs() << "MCP: Removing copy due to no live-out succ: ";
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MaybeDead->dump());
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assert(!MRI->isReserved(MaybeDead->getOperand(0).getReg()));
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// Update matching debug values.
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assert(MaybeDead->isCopy());
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MaybeDead->changeDebugValuesDefReg(MaybeDead->getOperand(1).getReg());
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MaybeDead->eraseFromParent();
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Changed = true;
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++NumDeletes;

‎llvm/lib/CodeGen/MachineInstr.cpp

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Original file line numberDiff line numberDiff line change
@@ -2092,3 +2092,13 @@ void MachineInstr::collectDebugValues(
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DbgValues.push_back(&*DI);
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}
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}
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void MachineInstr::changeDebugValuesDefReg(unsigned Reg) {
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// Collect matching debug values.
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SmallVector<MachineInstr *, 2> DbgValues;
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collectDebugValues(DbgValues);
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// Propagate Reg to debug value instructions.
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for (auto *DBI : DbgValues)
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DBI->getOperand(0).setReg(Reg);
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}

‎llvm/test/CodeGen/MIR/X86/pr38773.mir

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,105 @@
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# RUN: llc -o - %s -mtriple=x86_64-- -run-pass=machine-cp | FileCheck %s
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# When MachineCopyPropagation eliminates a dead 'copy', its associated debug
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# information becomes invalid. as the recorded register has been removed.
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# It causes the debugger to display wrong variable value.
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#
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# When in the debugger, on the line "return read1;", the value of "read1"
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# is reported as '4', where it should be '1'.
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#
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# MIR generated with:
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# clang -S -g -O2 -emit-llvm pr38773.cpp -o pr38773.ll -mllvm
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# llc pr38773.ll -stop-after=tailduplication -simplify-mir
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#
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# // pr38773.cpp
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# int main() {
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# volatile int foo = 4;
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# int read1 = foo;
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# int read2 = foo;
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#
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# switch ((read1 == 4) ? 3 : 1) {
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# case 1:
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# read1 *= read2;
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# break;
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# case 3:
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# read1 /= read2;
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# break;
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# }
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#
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# return read1;
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# }
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#
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# Update the register for the '@llvm.dbg.value' associated with 'read1', when
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# the 'copy' is removed, to be the 'source' register.
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--- |
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; ModuleID = 'pr38773.ll'
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source_filename = "pr38773.cpp"
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target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-pc-linux-gnu"
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define dso_local i32 @main() local_unnamed_addr !dbg !7 {
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entry:
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%foo = alloca i32, align 4
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store volatile i32 4, i32* %foo, align 4
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%foo.0.foo.0. = load volatile i32, i32* %foo, align 4
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%foo.0.foo.0.6 = load volatile i32, i32* %foo, align 4
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%cmp = icmp eq i32 %foo.0.foo.0., 4
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br i1 %cmp, label %sw.bb1, label %sw.bb
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sw.bb: ; preds = %entry
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%mul = mul nsw i32 %foo.0.foo.0.6, %foo.0.foo.0.
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br label %sw.epilog
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sw.bb1: ; preds = %entry
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%div = sdiv i32 4, %foo.0.foo.0.6
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call void @llvm.dbg.value(metadata i32 %div, metadata !12, metadata !DIExpression()), !dbg !13
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br label %sw.epilog
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sw.epilog: ; preds = %sw.bb1, %sw.bb
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%read1.0 = phi i32 [ %div, %sw.bb1 ], [ %mul, %sw.bb ]
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call void @llvm.dbg.value(metadata i32 %read1.0, metadata !12, metadata !DIExpression()), !dbg !13
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ret i32 %read1.0
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}
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declare void @llvm.dbg.value(metadata, metadata, metadata) #0
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!llvm.dbg.cu = !{!0}
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!llvm.module.flags = !{!3, !4, !5}
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!llvm.ident = !{!6}
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!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 8.0.0 (trunk 343183)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
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!1 = !DIFile(filename: "pr38773.cpp", directory: ".")
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!2 = !{}
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!3 = !{i32 2, !"Dwarf Version", i32 4}
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!4 = !{i32 2, !"Debug Info Version", i32 3}
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!5 = !{i32 1, !"wchar_size", i32 4}
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!6 = !{!"clang version 8.0.0 (trunk 343183)"}
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!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, retainedNodes: !11)
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!8 = !DISubroutineType(types: !9)
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!9 = !{!10}
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!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
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!11 = !{!12}
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!12 = !DILocalVariable(name: "read1", scope: !7, file: !1, line: 3, type: !10)
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!13 = !DILocation(line: 3, column: 7, scope: !7)
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...
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---
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name: main
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body: |
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bb.2.sw.bb1:
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liveins: $ecx
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$eax = MOV32ri 4
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$edx = MOV32r0 implicit-def dead $eflags
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IDIV32r killed renamable $ecx, implicit-def $eax, implicit-def dead $edx, implicit-def dead $eflags, implicit $eax, implicit killed $edx
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renamable $ecx = COPY $eax
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; CHECK: IDIV32r killed renamable $ecx
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; CHECK-NEXT: DBG_VALUE debug-use $eax, debug-use $noreg, !12, !DIExpression(), debug-location !13
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DBG_VALUE debug-use $ecx, debug-use $noreg, !12, !DIExpression(), debug-location !13
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$eax = COPY killed renamable $ecx
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RET 0, $eax
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...

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