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[Assignment Tracking] Fix fragments for assignments to variables smal…
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…ler than the alloca

Prior to this patch the trackAssignments function would attribute all stores to
an alloca to all variables linked to the alloca. This is wrong in the case
where the alloca contains variables which are smaller than the alloca, and
caused erroneous fragment information to be generated.

Now stores outside the variable bounds are discarded, and we check whether a
fragment is needed based on whether the store covers the entire variable as
opposed to whether it covers the entire alloca (except for variables of unknown
size).

Note that trackAssignments doesn't yet understand whole variables sitting at
anything other than offset 0 in an alloca - those variables are still tracked
using dbg.declares.

Fixes https://lab.llvm.org/buildbot/#/builders/70/builds/36007

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D147777
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@OCHyams
OCHyams committed Apr 9, 2023
1 parent 9f5951f commit e932d2e
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Showing 2 changed files with 90 additions and 2 deletions.
20 changes: 18 additions & 2 deletions llvm/lib/IR/DebugInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1835,16 +1835,32 @@ std::optional<AssignmentInfo> at::getAssignmentInfo(const DataLayout &DL,
return getAssignmentInfoImpl(DL, AI, SizeInBits);
}

/// Returns nullptr if the assignment shouldn't be attributed to this variable.
static CallInst *emitDbgAssign(AssignmentInfo Info, Value *Val, Value *Dest,
Instruction &StoreLikeInst,
const VarRecord &VarRec, DIBuilder &DIB) {
auto *ID = StoreLikeInst.getMetadata(LLVMContext::MD_DIAssignID);
assert(ID && "Store instruction must have DIAssignID metadata");
(void)ID;

bool StoreToWholeVariable = Info.StoreToWholeAlloca;
if (auto Size = VarRec.Var->getSizeInBits()) {
// Discard stores to bits outside this variable. NOTE: trackAssignments
// doesn't understand base expressions yet, so all variables that reach
// here are guaranteed to start at offset 0 in the alloca.
// TODO: Could we truncate the fragment expression instead of discarding
// the assignment?
if (Info.OffsetInBits + Info.SizeInBits > *Size)
return nullptr;
// FIXME: As noted above - only variables at offset 0 are handled
// currently.
StoreToWholeVariable = Info.OffsetInBits == /*VarOffsetInAlloca*/ 0 &&
Info.SizeInBits == *Size;
}

DIExpression *Expr =
DIExpression::get(StoreLikeInst.getContext(), std::nullopt);
if (!Info.StoreToWholeAlloca) {
if (!StoreToWholeVariable) {
auto R = DIExpression::createFragmentExpression(Expr, Info.OffsetInBits,
Info.SizeInBits);
assert(R.has_value() && "failed to create fragment expression");
Expand Down Expand Up @@ -1944,7 +1960,7 @@ void at::trackAssignments(Function::iterator Start, Function::iterator End,
auto *Assign =
emitDbgAssign(*Info, ValueComponent, DestComponent, I, R, DIB);
(void)Assign;
LLVM_DEBUG(errs() << " > INSERT: " << *Assign << "\n");
LLVM_DEBUG(if (Assign) errs() << " > INSERT: " << *Assign << "\n");
}
}
}
Expand Down
72 changes: 72 additions & 0 deletions llvm/test/DebugInfo/Generic/assignment-tracking/inline/shared-alloca.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,72 @@
; RUN: opt -S %s -passes=inline -o - \
; RUN: | FileCheck %s --implicit-check-not="call void @llvm.dbg"

;; The dbg.assign linked to the large alloca describes a variable sitting at
;; offset 0, size 64. Check:
;; A) an inlined store to the alloca outside of the bits 0-64 is not attributed
;; to the variable.
;; B) a fragment expression is not created if the inlined store is the size of
;; the variable.
;; C) a fragment expression is created if the inlined store is not the size of
;; the variable.

;; Test A:
; CHECK: %0 = alloca %"struct.llvm::detail::DenseMapPair", i32 0, align 8, !DIAssignID ![[ID1:[0-9]+]]
; CHECK: call void @llvm.dbg.assign(metadata i1 undef, metadata ![[#]], metadata !DIExpression(), metadata ![[ID1]], metadata ptr %0, metadata !DIExpression())

;; Test B:
;; CHECK: store i64 1, ptr %0, align 4, !DIAssignID ![[ID2:[0-9]+]]
;; CHECK: call void @llvm.dbg.assign(metadata i64 1, metadata ![[#]], metadata !DIExpression(), metadata ![[ID2]], metadata ptr %0, metadata !DIExpression())

;; Test C:
;; CHECK: store i32 2, ptr %0, align 4, !DIAssignID ![[ID3:[0-9]+]]
;; CHECK: call void @llvm.dbg.assign(metadata i32 2, metadata ![[#]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32), metadata ![[ID3]], metadata ptr %0, metadata !DIExpression())

%"struct.llvm::detail::DenseMapPair" = type { %"struct.std::pair" }
%"struct.std::pair" = type { ptr, %"class.llvm::SmallVector" }
%"class.llvm::SmallVector" = type { %"class.llvm::SmallVectorImpl", %"struct.llvm::SmallVectorStorage" }
%"class.llvm::SmallVectorImpl" = type { %"class.llvm::SmallVectorTemplateBase" }
%"class.llvm::SmallVectorTemplateBase" = type { %"class.llvm::SmallVectorTemplateCommon" }
%"class.llvm::SmallVectorTemplateCommon" = type { %"class.llvm::SmallVectorBase" }
%"class.llvm::SmallVectorBase" = type { ptr, i64, i64 }
%"struct.llvm::SmallVectorStorage" = type { [40 x i8] }

define void @_Z6verifyv() {
entry:
%0 = alloca %"struct.llvm::detail::DenseMapPair", i32 0, align 8, !DIAssignID !5
call void @llvm.dbg.assign(metadata i1 undef, metadata !6, metadata !DIExpression(), metadata !5, metadata ptr %0, metadata !DIExpression()), !dbg !14
call void @_ZN4llvm6detail12DenseMapPairIP4SCEVNS_11SmallVectorI6FoldIDLj40EEEEC2ERKS7_(ptr %0)
ret void
}

define linkonce_odr void @_ZN4llvm6detail12DenseMapPairIP4SCEVNS_11SmallVectorI6FoldIDLj40EEEEC2ERKS7_(ptr %this) {
entry:
%second.i = getelementptr %"struct.std::pair", ptr %this, i64 0, i32 1
store i32 0, ptr %second.i, align 4
%test = getelementptr %"struct.std::pair", ptr %this, i64 0, i32 0
store i64 1, ptr %test
store i32 2, ptr %test
ret void
}

declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata)

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

!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 17.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, globals: !2, imports: !2, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "reduce.cpp", directory: "")
!2 = !{}
!3 = !{i32 2, !"Debug Info Version", i32 3}
!4 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
!5 = distinct !DIAssignID()
!6 = !DILocalVariable(name: "a", scope: !7, file: !1, line: 10, type: !12)
!7 = distinct !DILexicalBlock(scope: !8, file: !1, line: 10, column: 3)
!8 = distinct !DILexicalBlock(scope: !9, file: !1, line: 10, column: 3)
!9 = distinct !DISubprogram(name: "verify", linkageName: "_Z6verifyv", scope: !1, file: !1, line: 9, type: !10, scopeLine: 9, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
!10 = !DISubroutineType(types: !11)
!11 = !{null}
!12 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !13, size: 64)
!13 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "SCEV", file: !1, line: 3, flags: DIFlagFwdDecl | DIFlagNonTrivial, identifier: "_ZTS4SCEV")
!14 = !DILocation(line: 0, scope: !7)

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