[clang] [llvm] Add option to generate additional debug info for expression dereferencing pointer to pointers. (PR #81545)

[David Blaikie via llvm-commits]

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@@ -5636,6 +5636,84 @@ void CGDebugInfo::EmitExternalVariable(llvm::GlobalVariable *Var,
   Var->addDebugInfo(GVE);
 }
 
+void CGDebugInfo::EmitPseudoVariable(CGBuilderTy &Builder,
+                                     llvm::Instruction *Value, QualType Ty) {
+  // Only when -g2 or above is specified, debug info for variables will be
+  // generated.
+  if (CGM.getCodeGenOpts().getDebugInfo() <=
+      llvm::codegenoptions::DebugLineTablesOnly)
+    return;
+
+  llvm::DIFile *Unit = Builder.getCurrentDebugLocation()->getFile();
+  llvm::DIType *Type = getOrCreateType(Ty, Unit);
+
+  // Check if Value is already a declared variable and has debug info, in this
+  // case we have nothing to do. Clang emits declared variable as alloca, and
+  // it is loaded upon use, so we identify such pattern here.
+  if (llvm::LoadInst *Load = dyn_cast<llvm::LoadInst>(Value)) {
+    llvm::Value *Var = Load->getPointerOperand();
+    if (llvm::Metadata *MDValue = llvm::ValueAsMetadata::getIfExists(Var)) {
+      if (llvm::Value *DbgValue = llvm::MetadataAsValue::getIfExists(
+              CGM.getLLVMContext(), MDValue)) {
+        for (llvm::User *U : DbgValue->users()) {
+          if (llvm::CallInst *DbgDeclare = dyn_cast<llvm::CallInst>(U)) {
+            if (DbgDeclare->getCalledFunction()->getIntrinsicID() ==
+                    llvm::Intrinsic::dbg_declare &&
+                DbgDeclare->getArgOperand(0) == DbgValue) {
+              // There can be implicit type cast applied on a variable if it is
+              // an opaque ptr, in this case its debug info may not match the
+              // actual type of object being used as in the next instruction, so
+              // we will need to emit a pseudo variable for type-casted value.
+              llvm::DILocalVariable *MDNode = dyn_cast<llvm::DILocalVariable>(
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dwblaikie wrote:

Given the next line unconditionally dereferences this pointer, this should be a `cast` instead of a `dyn_cast`, I think?

https://github.com/llvm/llvm-project/pull/81545