Add option to generate additional debug info for expression dereferencing pointer to pointers. (#81545)

Such expression does not correspond to a variable in the source code
thus does not have a debug location. When the user collects perf data on
the program, if the intermediate memory load instruction is sampled, it
could not be attributed to any variable/class member, which causes the
sampling results to be under-counted.
This patch adds an option `-fdebug_info_for_pointer_type` to generate a
psuedo variable and its debug info for intermediate expression with
pointer dereferencing, so that perf data collected on the instruction of
that expression can be attributed to the correct class member.

This is a prototype so comments are needed.

1 files changed, 84 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 1713f70..5f6f911 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -5737,6 +5737,90 @@ 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::DebugLoc SaveDebugLoc = Builder.getCurrentDebugLocation();
+  if (!SaveDebugLoc.get())
+    return;
+
+  llvm::DIFile *Unit = SaveDebugLoc->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 = cast<llvm::DILocalVariable>(
+                  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());
+  llvm::DebugLoc DL = Value->getDebugLoc();
+  if (DL.get())
+    Builder.SetCurrentDebugLocation(DL);
+  else if (!Builder.getCurrentDebugLocation().get())
+    Builder.SetCurrentDebugLocation(SaveDebugLoc);
+
+  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();
+  unsigned Column = Builder.getCurrentDebugLocation().getCol();
+  llvm::DILocalVariable *D = DBuilder.createAutoVariable(
+      LexicalBlockStack.back(), "", nullptr, 0, Type, false,
+      llvm::DINode::FlagArtificial);
+  llvm::DILocation *DIL =
+      llvm::DILocation::get(CGM.getLLVMContext(), Line, Column,
+                            LexicalBlockStack.back(), CurInlinedAt);
+  SmallVector<uint64_t> Expr;
+  DBuilder.insertDeclare(PseudoVar, D, DBuilder.createExpression(Expr), DIL,
+                         Load);
+
+  Builder.restoreIP(SaveInsertionPoint);
+  Builder.SetCurrentDebugLocation(SaveDebugLoc);
+}
+
 void CGDebugInfo::EmitGlobalAlias(const llvm::GlobalValue *GV,
                                   const GlobalDecl GD) {