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

[David Blaikie via cfe-commits]

================
@@ -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>(
+                  dyn_cast<llvm::MetadataAsValue>(DbgDeclare->getOperand(1))
+                      ->getMetadata());
+              if (MDNode->getType() == Type)
+                return;
+            }
+          }
+        }
+      }
+    }
+  }
+
+  // Find the correct location to insert a sequence of instructions to
+  // materialize Value on the stack.
+  auto SaveInsertionPoint = Builder.saveIP();
+  if (llvm::InvokeInst *Invoke = dyn_cast<llvm::InvokeInst>(Value))
+    Builder.SetInsertPoint(Invoke->getNormalDest()->begin());
+  else if (llvm::Instruction *Next = Value->getIterator()->getNextNode())
+    Builder.SetInsertPoint(Next);
+  else
+    Builder.SetInsertPoint(Value->getParent());
+  auto SaveDebugLoc = Builder.getCurrentDebugLocation();
+  llvm::DebugLoc DL = Value->getDebugLoc();
+  if (DL.get())
+    Builder.SetCurrentDebugLocation(DL);
+
+  llvm::AllocaInst *PseudoVar = Builder.CreateAlloca(Value->getType());
+  Address PseudoVarAddr(PseudoVar, Value->getType(),
+                        CharUnits::fromQuantity(PseudoVar->getAlign()));
+  llvm::LoadInst *Load = Builder.CreateLoad(PseudoVarAddr);
+  Value->replaceAllUsesWith(Load);
+  Builder.SetInsertPoint(Load);
+  Builder.CreateStore(Value, PseudoVarAddr);
+
+  // Emit debug info for materialized Value.
+  unsigned Line = Builder.getCurrentDebugLocation().getLine();
----------------
dwblaikie wrote:

Hmm - what breaks? I tried this with some hand-modified IR and it seems to produce the desired DWARF:
https://godbolt.org/z/n8a5Ef1Yj - `i` local has had the file/line removed, `j` local has not. Oh, and it even works if the name is removed - which might save a few bytes too? (I think if a name is going to be included, I'd expect it to be a reserved name, starting with `__`)

Ah, I see, you're also using the location for the DILocation of the debug intrinsic - I meant to refer only to the location (file, line, and column) of the DILocalVariable created by `createAutoVariable` (ie, I'm suggesting: ` DBuilder.createAutoVariable(LexicalBlockStack.back(), "", nullptr, 0, Type)`)

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