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fix-149477-- Emit correct closure members for lambdas in CreateLimitedType #149674

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Fix an issue where local lambdas caused a crash when importing the std module in LLDB due to missing capture fields in debug info.
Now closures always emit all fields for lambdas by skipping the isImplicit check.
This unblocks libc++ from using local lambdas freely

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@llvmbot llvmbot added clang Clang issues not falling into any other category clang:codegen IR generation bugs: mangling, exceptions, etc. debuginfo labels Jul 19, 2025
@llvmbot
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llvmbot commented Jul 19, 2025

@llvm/pr-subscribers-clang

Author: Ayush Pareek (ayushpareek2003)

Changes

Fix an issue where local lambdas caused a crash when importing the std module in LLDB due to missing capture fields in debug info.
Now closures always emit all fields for lambdas by skipping the isImplicit check.
This unblocks libc++ from using local lambdas freely


Full diff: https://github.com/llvm/llvm-project/pull/149674.diff

1 Files Affected:

  • (modified) clang/lib/CodeGen/CGDebugInfo.cpp (+26)
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index e24c68ed02865..a8f9e3e028e9f 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -4187,6 +4187,32 @@ llvm::DICompositeType *CGDebugInfo::CreateLimitedType(const RecordType *Ty) {
   RegionMap[Ty->getDecl()].reset(RealDecl);
   TypeCache[QualType(Ty, 0).getAsOpaquePtr()].reset(RealDecl);
 
+  if (const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+    SmallVector<llvm::Metadata *, 16> EltTys;
+    const ASTRecordLayout &Layout = CGM.getContext().getASTRecordLayout(CXXRD);
+
+    for (const FieldDecl *Field : CXXRD->fields()) {
+      if (!CXXRD->isLambda() && Field->isImplicit())
+        continue;
+
+      llvm::DIType *FieldType = getOrCreateType(Field->getType(), DefUnit);
+      unsigned FieldLine = getLineNumber(Field->getLocation());
+      uint64_t FieldOffset = Layout.getFieldOffset(Field->getFieldIndex());
+      llvm::DIFile *FieldFile = DefUnit;
+
+      llvm::DIDerivedType *Elem = DBuilder.createMemberType(
+          RealDecl, Field->getName(), FieldFile, FieldLine,
+          CGM.getContext().getTypeSize(Field->getType()),
+          getTypeAlignIfRequired(Field->getType(), CGM.getContext()),
+          FieldOffset,
+          getAccessFlag(Field->getAccess()), FieldType);
+
+      EltTys.push_back(Elem);
+    }
+    // Attach the fields.
+    DBuilder.replaceArrays(RealDecl, DBuilder.getOrCreateArray(EltTys));
+  }
+
   if (const auto *TSpecial = dyn_cast<ClassTemplateSpecializationDecl>(RD))
     DBuilder.replaceArrays(RealDecl, llvm::DINodeArray(),
                            CollectCXXTemplateParams(TSpecial, DefUnit));

@llvmbot
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llvmbot commented Jul 19, 2025

@llvm/pr-subscribers-debuginfo

Author: Ayush Pareek (ayushpareek2003)

Changes

Fix an issue where local lambdas caused a crash when importing the std module in LLDB due to missing capture fields in debug info.
Now closures always emit all fields for lambdas by skipping the isImplicit check.
This unblocks libc++ from using local lambdas freely


Full diff: https://github.com/llvm/llvm-project/pull/149674.diff

1 Files Affected:

  • (modified) clang/lib/CodeGen/CGDebugInfo.cpp (+26)
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index e24c68ed02865..a8f9e3e028e9f 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -4187,6 +4187,32 @@ llvm::DICompositeType *CGDebugInfo::CreateLimitedType(const RecordType *Ty) {
   RegionMap[Ty->getDecl()].reset(RealDecl);
   TypeCache[QualType(Ty, 0).getAsOpaquePtr()].reset(RealDecl);
 
+  if (const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+    SmallVector<llvm::Metadata *, 16> EltTys;
+    const ASTRecordLayout &Layout = CGM.getContext().getASTRecordLayout(CXXRD);
+
+    for (const FieldDecl *Field : CXXRD->fields()) {
+      if (!CXXRD->isLambda() && Field->isImplicit())
+        continue;
+
+      llvm::DIType *FieldType = getOrCreateType(Field->getType(), DefUnit);
+      unsigned FieldLine = getLineNumber(Field->getLocation());
+      uint64_t FieldOffset = Layout.getFieldOffset(Field->getFieldIndex());
+      llvm::DIFile *FieldFile = DefUnit;
+
+      llvm::DIDerivedType *Elem = DBuilder.createMemberType(
+          RealDecl, Field->getName(), FieldFile, FieldLine,
+          CGM.getContext().getTypeSize(Field->getType()),
+          getTypeAlignIfRequired(Field->getType(), CGM.getContext()),
+          FieldOffset,
+          getAccessFlag(Field->getAccess()), FieldType);
+
+      EltTys.push_back(Elem);
+    }
+    // Attach the fields.
+    DBuilder.replaceArrays(RealDecl, DBuilder.getOrCreateArray(EltTys));
+  }
+
   if (const auto *TSpecial = dyn_cast<ClassTemplateSpecializationDecl>(RD))
     DBuilder.replaceArrays(RealDecl, llvm::DINodeArray(),
                            CollectCXXTemplateParams(TSpecial, DefUnit));

@llvmbot
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llvmbot commented Jul 19, 2025

@llvm/pr-subscribers-clang-codegen

Author: Ayush Pareek (ayushpareek2003)

Changes

Fix an issue where local lambdas caused a crash when importing the std module in LLDB due to missing capture fields in debug info.
Now closures always emit all fields for lambdas by skipping the isImplicit check.
This unblocks libc++ from using local lambdas freely


Full diff: https://github.com/llvm/llvm-project/pull/149674.diff

1 Files Affected:

  • (modified) clang/lib/CodeGen/CGDebugInfo.cpp (+26)
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index e24c68ed02865..a8f9e3e028e9f 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -4187,6 +4187,32 @@ llvm::DICompositeType *CGDebugInfo::CreateLimitedType(const RecordType *Ty) {
   RegionMap[Ty->getDecl()].reset(RealDecl);
   TypeCache[QualType(Ty, 0).getAsOpaquePtr()].reset(RealDecl);
 
+  if (const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+    SmallVector<llvm::Metadata *, 16> EltTys;
+    const ASTRecordLayout &Layout = CGM.getContext().getASTRecordLayout(CXXRD);
+
+    for (const FieldDecl *Field : CXXRD->fields()) {
+      if (!CXXRD->isLambda() && Field->isImplicit())
+        continue;
+
+      llvm::DIType *FieldType = getOrCreateType(Field->getType(), DefUnit);
+      unsigned FieldLine = getLineNumber(Field->getLocation());
+      uint64_t FieldOffset = Layout.getFieldOffset(Field->getFieldIndex());
+      llvm::DIFile *FieldFile = DefUnit;
+
+      llvm::DIDerivedType *Elem = DBuilder.createMemberType(
+          RealDecl, Field->getName(), FieldFile, FieldLine,
+          CGM.getContext().getTypeSize(Field->getType()),
+          getTypeAlignIfRequired(Field->getType(), CGM.getContext()),
+          FieldOffset,
+          getAccessFlag(Field->getAccess()), FieldType);
+
+      EltTys.push_back(Elem);
+    }
+    // Attach the fields.
+    DBuilder.replaceArrays(RealDecl, DBuilder.getOrCreateArray(EltTys));
+  }
+
   if (const auto *TSpecial = dyn_cast<ClassTemplateSpecializationDecl>(RD))
     DBuilder.replaceArrays(RealDecl, llvm::DINodeArray(),
                            CollectCXXTemplateParams(TSpecial, DefUnit));

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@Michael137 Michael137 left a comment

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Thanks for trying to fix that LLDB issue. Could you elaborate on what you think the root cause is and how this fixes it? Not saying it's impossible, but I'm a bit surprised that debug-info is involved in this crash at all since we're supposedly getting the decls from the std module.

Also, this patch fails to build for me locally. And when I fix that build failure the crash still reproduces.

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⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:
git-clang-format --diff HEAD~1 HEAD --extensions cpp -- clang/lib/CodeGen/CGDebugInfo.cpp
View the diff from clang-format here.
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index a8f9e3e02..081b038cc 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -4204,8 +4204,7 @@ llvm::DICompositeType *CGDebugInfo::CreateLimitedType(const RecordType *Ty) {
           RealDecl, Field->getName(), FieldFile, FieldLine,
           CGM.getContext().getTypeSize(Field->getType()),
           getTypeAlignIfRequired(Field->getType(), CGM.getContext()),
-          FieldOffset,
-          getAccessFlag(Field->getAccess()), FieldType);
+          FieldOffset, getAccessFlag(Field->getAccess()), FieldType);
 
       EltTys.push_back(Elem);
     }

@dwblaikie
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Some Clang test coverage would be nice - showing the change in behavior (& not a committed test, but to @Michael137's point - showing the difference in the resulting DWARF would be good to look through too)

@ayushpareek2003
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According to my understanding, this issue happens because when a standard library header like uses a function-local lambda, the compiler generates a closure type for that lambda
sometimes this closure type gets added to the debug info without including its captured variables.

when LLDB runs import std and tries to inspect types that indirectly depend on that header (for ex- std::list), it needs the debug info for all these

I made CreateLimitedType keep all fields for lambdas, so their captures aren’t skipped by the isImplicit check

am I missing something here or am I looking at this from the wrong angle?

@dwblaikie
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According to my understanding, this issue happens because when a standard library header like uses a function-local lambda, the compiler generates a closure type for that lambda sometimes this closure type gets added to the debug info without including its captured variables.

when LLDB runs import std and tries to inspect types that indirectly depend on that header (for ex- std::list), it needs the debug info for all these

I made CreateLimitedType keep all fields for lambdas, so their captures aren’t skipped by the isImplicit check

am I missing something here or am I looking at this from the wrong angle?

I think we need some more detail - generally we try not to emit definitions of classes (implicitly, including the closure type of a lambda) if the definition isn't needed (essentially, if the code doesn't do something that would fail to compile if the class were declared but not defined, we skip the definition in the DWARF).

Most issues that could apply to lambda closure types would apply to local class types - so mostly any bug that would apply to a lambda, would apply to a local class type - so I'm suspicious of any change that's lambda specific like this one. Feels like we might not know enough about the problem yet.

Can you provide a specific example? (source code, compilation commands, platform, debugger, debugger commands, failure mode/crash/etc)

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