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[DebugInfo] Avoid adding too much indirection to pointer-valued variables
This patch addresses PR41675, where a stack-pointer variable is dereferenced too many times by its location expression, presenting a value on the stack as the pointer to the stack. The difference between a stack *pointer* DBG_VALUE and one that refers to a value on the stack, is currently the indirect flag. However the DWARF backend will also try to guess whether something is a memory location or not, based on whether there is any computation in the location expression. By simply prepending the stack offset to existing expressions, we can accidentally convert a register location into a memory location, which introduces a suprise (and unintended) dereference. The solution is to add DW_OP_stack_value whenever we add a DIExpression computation to a stack *pointer*. It's an implicit location computed on the expression stack, thus needs to be flagged as a stack_value. For the edge case where the offset is zero and the location could be a register location, DIExpression::prepend will still generate opcodes, and thus DW_OP_stack_value must still be added. Differential Revision: https://reviews.llvm.org/D63429 llvm-svn: 364736
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‎llvm/include/llvm/IR/DebugInfoMetadata.h

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2464,6 +2464,10 @@ class DIExpression : public MDNode {
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/// Return whether this is an implicit location description.
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bool isImplicit() const;
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/// Return whether the location is computed on the expression stack, meaning
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/// it cannot be a simple register location.
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bool isComplex() const;
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/// Append \p Ops with operations to apply the \p Offset.
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static void appendOffset(SmallVectorImpl<uint64_t> &Ops, int64_t Offset);
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‎llvm/lib/CodeGen/PrologEpilogInserter.cpp

Lines changed: 11 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1200,6 +1200,16 @@ void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &MF,
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MI.getOperand(0).setIsDebug();
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const DIExpression *DIExpr = MI.getDebugExpression();
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// If we have a direct DBG_VALUE, and its location expression isn't
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// currently complex, then adding an offset will morph it into a
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// complex location that is interpreted as being a memory address.
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// This changes a pointer-valued variable to dereference that pointer,
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// which is incorrect. Fix by adding DW_OP_stack_value.
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unsigned PrependFlags = DIExpression::ApplyOffset;
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if (!MI.isIndirectDebugValue() && !DIExpr->isComplex())
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PrependFlags |= DIExpression::StackValue;
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// If we have DBG_VALUE that is indirect and has a Implicit location
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// expression need to insert a deref before prepending a Memory
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// location expression. Also after doing this we change the DBG_VALUE
@@ -1211,8 +1221,7 @@ void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &MF,
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// Make the DBG_VALUE direct.
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MI.getOperand(1).ChangeToRegister(0, false);
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}
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DIExpr =
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DIExpression::prepend(DIExpr, DIExpression::ApplyOffset, Offset);
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DIExpr = DIExpression::prepend(DIExpr, PrependFlags, Offset);
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MI.getOperand(3).setMetadata(DIExpr);
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continue;
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}

‎llvm/lib/IR/DebugInfoMetadata.cpp

Lines changed: 21 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -926,6 +926,27 @@ bool DIExpression::isImplicit() const {
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return false;
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}
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bool DIExpression::isComplex() const {
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if (!isValid())
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return false;
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if (getNumElements() == 0)
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return false;
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// If there are any elements other than fragment or tag_offset, then some
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// kind of complex computation occurs.
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for (const auto &It : expr_ops()) {
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switch (It.getOp()) {
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case dwarf::DW_OP_LLVM_tag_offset:
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case dwarf::DW_OP_LLVM_fragment:
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continue;
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default: return true;
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}
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}
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return false;
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}
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Optional<DIExpression::FragmentInfo>
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DIExpression::getFragmentInfo(expr_op_iterator Start, expr_op_iterator End) {
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for (auto I = Start; I != End; ++I)

‎llvm/test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir

Lines changed: 130 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,130 @@
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# RUN: llc %s -x mir -o - -mtriple=x86_64-unknown-unknown -run-pass=prologepilog | FileCheck %s
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#
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# Check when the DBG_VALUE on a stack slot below (for var "c") has its stack
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# slot replaced with $rsp and a complex expression, it has DW_OP_stack_value
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# added. A direct reference to the stack slot is considered to be the _address_
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# of that stack slot, wheras its contents would be an indirect DBG_VALUE.
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#
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# Check too that for the same DBG_VALUE inst, with an indirect reference to
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# the stack slot, we do _not_ get DW_OP_plus_uconst added. This expression
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# should remain indirect, referring to the contents of the stack slot.
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#
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# CHECK: ![[VAR:[0-9]+]] = !DILocalVariable(name: "c"
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# CHECK: ![[VAR2:[0-9]+]] = !DILocalVariable(name: "asdf"
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# CHECK: ![[VAR3:[0-9]+]] = !DILocalVariable(name: "bees"
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#
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# CHECK: LEA64r $rsp
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# CHECK-NEXT: DBG_VALUE $rsp, $noreg, ![[VAR]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}, DW_OP_stack_value)
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# CHECK-NEXT: DBG_VALUE $rsp, $noreg, ![[VAR2]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}, DW_OP_stack_value, DW_OP_LLVM_fragment, 1, 2)
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# CHECK-NEXT: DBG_VALUE $rsp, $noreg, ![[VAR3]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}, DW_OP_LLVM_tag_offset, 0, DW_OP_stack_value)
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# CHECK-NEXT: DBG_VALUE 1834104526
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# CHECK-NEXT: MOV64mr
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# CHECK-NEXT: DBG_VALUE $rsp, 0, ![[VAR]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}})
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--- |
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; ModuleID = 'out.ll'
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source_filename = "abc.c"
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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-unknown-linux-gnu"
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@b = common dso_local local_unnamed_addr global i32* null, align 8, !dbg !0
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@a = common dso_local local_unnamed_addr global i32 0, align 4, !dbg !6
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; Function Attrs: nounwind uwtable
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define dso_local i32 @main() local_unnamed_addr !dbg !14 {
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entry:
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%l_1081 = alloca i32, align 4
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%0 = bitcast i32* %l_1081 to i8*, !dbg !20
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call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0), !dbg !20
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call void @llvm.dbg.value(metadata i32 1834104526, metadata !18, metadata !DIExpression()), !dbg !21
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call void @llvm.dbg.value(metadata i32* %l_1081, metadata !19, metadata !DIExpression()), !dbg !21
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store i32* %l_1081, i32** @b, align 8, !dbg !22, !tbaa !23
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store i32 9, i32* @a, align 4, !dbg !27, !tbaa !28
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store i32 9, i32* %l_1081, align 4, !dbg !30, !tbaa !28
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%call = call i32 (...) @optimize_me_not(), !dbg !31
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call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0), !dbg !32
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ret i32 0, !dbg !32
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}
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; Function Attrs: argmemonly nounwind
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declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture)
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declare dso_local i32 @optimize_me_not(...) local_unnamed_addr
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; Function Attrs: argmemonly nounwind
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declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture)
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; Function Attrs: nounwind readnone speculatable
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declare void @llvm.dbg.value(metadata, metadata, metadata)
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!llvm.dbg.cu = !{!2}
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!llvm.module.flags = !{!10, !11, !12}
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!llvm.ident = !{!13}
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!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
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!1 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 2, type: !9, isLocal: false, isDefinition: true)
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!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
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!3 = !DIFile(filename: "abc.c", directory: ".")
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!4 = !{}
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!5 = !{!6, !0}
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!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
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!7 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
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!8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
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!9 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !8, size: 64)
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!10 = !{i32 2, !"Dwarf Version", i32 4}
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!11 = !{i32 2, !"Debug Info Version", i32 3}
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!12 = !{i32 1, !"wchar_size", i32 4}
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!13 = !{!"clang"}
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!14 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 3, type: !15, scopeLine: 3, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !17)
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!15 = !DISubroutineType(types: !16)
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!16 = !{!8}
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!17 = !{!18, !19, !33, !34}
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!18 = !DILocalVariable(name: "l_1081", scope: !14, file: !3, line: 4, type: !8)
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!19 = !DILocalVariable(name: "c", scope: !14, file: !3, line: 5, type: !9)
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!20 = !DILocation(line: 4, column: 3, scope: !14)
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!21 = !DILocation(line: 0, scope: !14)
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!22 = !DILocation(line: 6, column: 5, scope: !14)
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!23 = !{!24, !24, i64 0}
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!24 = !{!"any pointer", !25, i64 0}
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!25 = !{!"omnipotent char", !26, i64 0}
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!26 = !{!"Simple C/C++ TBAA"}
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!27 = !DILocation(line: 7, column: 10, scope: !14)
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!28 = !{!29, !29, i64 0}
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!29 = !{!"int", !25, i64 0}
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!30 = !DILocation(line: 7, column: 6, scope: !14)
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!31 = !DILocation(line: 8, column: 3, scope: !14)
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!32 = !DILocation(line: 9, column: 1, scope: !14)
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!33 = !DILocalVariable(name: "asdf", scope: !14, file: !3, line: 4, type: !8)
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!34 = !DILocalVariable(name: "bees", scope: !14, file: !3, line: 4, type: !8)
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...
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---
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name: main
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alignment: 4
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tracksRegLiveness: true
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frameInfo:
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maxAlignment: 4
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hasCalls: true
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stack:
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- { id: 0, name: l_1081, type: default, offset: 0, size: 4, alignment: 4,
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callee-saved-register: '', callee-saved-restored: true,
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debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
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body: |
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bb.0.entry:
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renamable $rax = LEA64r %stack.0.l_1081, 1, $noreg, 0, $noreg
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DBG_VALUE %stack.0.l_1081, $noreg, !19, !DIExpression(), debug-location !21
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DBG_VALUE %stack.0.l_1081, $noreg, !33, !DIExpression(DW_OP_LLVM_fragment, 1, 2), debug-location !21
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DBG_VALUE %stack.0.l_1081, $noreg, !34, !DIExpression(DW_OP_LLVM_tag_offset, 0), debug-location !21
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DBG_VALUE 1834104526, $noreg, !18, !DIExpression(), debug-location !21
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MOV64mr $rip, 1, $noreg, @b, $noreg, killed renamable $rax, debug-location !22 :: (store 8 into @b, !tbaa !23)
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DBG_VALUE %stack.0.l_1081, 0, !19, !DIExpression(), debug-location !21
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MOV32mi $rip, 1, $noreg, @a, $noreg, 9, debug-location !27 :: (store 4 into @a, !tbaa !28)
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MOV32mi %stack.0.l_1081, 1, $noreg, 0, $noreg, 9, debug-location !30 :: (store 4 into %ir.l_1081, !tbaa !28)
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ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !31
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dead $eax = MOV32r0 implicit-def dead $eflags, implicit-def $al, debug-location !31
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CALL64pcrel32 @optimize_me_not, csr_64, implicit $rsp, implicit $ssp, implicit $al, implicit-def $rsp, implicit-def $ssp, implicit-def dead $eax, debug-location !31
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ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !31
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$eax = MOV32r0 implicit-def dead $eflags, debug-location !32
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RET 0, $eax, debug-location !32
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...

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