[Windows SEH] Fix the frame-ptr of a nested-filter within a _finally

This change fixed a SEH bug (exposed by test58 & test61 in MSVC test xcpt4u.c);
when an Except-filter is located inside a finally, the frame-pointer generated today
via intrinsic @llvm.eh.recoverfp is the frame-pointer of the immediate
parent _finally, not the frame-ptr of outermost host function.

The fix is to retrieve the Establisher's frame-pointer that was previously saved in
parent's frame.
The prolog of a filter inside a _finally should be like code below:

%0 = call i8* @llvm.eh.recoverfp(i8* bitcast (@"?fin$0@0@main@@"), i8*%frame_pointer)
%1 = call i8* @llvm.localrecover(i8* bitcast (@"?fin$0@0@main@@"), i8*%0, i32 0)
%2 = bitcast i8* %1 to i8**
%3 = load i8*, i8** %2, align 8

Differential Revision: https://reviews.llvm.org/D77982

1 files changed, 43 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGException.cpp b/clang/lib/CodeGen/CGException.cpp
index 2494f38..bdf7025 100644
--- a/clang/lib/CodeGen/CGException.cpp
+++ b/clang/lib/CodeGen/CGException.cpp
@@ -1815,6 +1815,48 @@ void CodeGenFunction::EmitCapturedLocals(CodeGenFunction &ParentCGF,
     llvm::Constant *ParentI8Fn =
         llvm::ConstantExpr::getBitCast(ParentCGF.CurFn, Int8PtrTy);
     ParentFP = Builder.CreateCall(RecoverFPIntrin, {ParentI8Fn, EntryFP});
+
+    // if the parent is a _finally, the passed-in ParentFP is the FP
+    // of parent _finally, not Establisher's FP (FP of outermost function).
+    // Establkisher FP is 2nd paramenter passed into parent _finally.
+    // Fortunately, it's always saved in parent's frame. The following
+    // code retrieves it, and escapes it so that spill instruction won't be
+    // optimized away.
+    if (ParentCGF.ParentCGF != nullptr) {
+      // Locate and escape Parent's frame_pointer.addr alloca
+      // Depending on target, should be 1st/2nd one in LocalDeclMap.
+      // Let's just scan for ImplicitParamDecl with VoidPtrTy.
+      llvm::AllocaInst *FramePtrAddrAlloca = nullptr;
+      for (auto &I : ParentCGF.LocalDeclMap) {
+        const VarDecl *D = cast<VarDecl>(I.first);
+        if (isa<ImplicitParamDecl>(D) &&
+            D->getType() == getContext().VoidPtrTy) {
+          assert(D->getName().startswith("frame_pointer"));
+          FramePtrAddrAlloca = cast<llvm::AllocaInst>(I.second.getPointer());
+          break;
+        }
+      }
+      assert(FramePtrAddrAlloca);
+      auto InsertPair = ParentCGF.EscapedLocals.insert(
+          std::make_pair(FramePtrAddrAlloca, ParentCGF.EscapedLocals.size()));
+      int FrameEscapeIdx = InsertPair.first->second;
+
+      // an example of a filter's prolog::
+      // %0 = call i8* @llvm.eh.recoverfp(bitcast(@"?fin$0@0@main@@"),..)
+      // %1 = call i8* @llvm.localrecover(bitcast(@"?fin$0@0@main@@"),..)
+      // %2 = bitcast i8* %1 to i8**
+      // %3 = load i8*, i8* *%2, align 8
+      //   ==> %3 is the frame-pointer of outermost host function
+      llvm::Function *FrameRecoverFn = llvm::Intrinsic::getDeclaration(
+          &CGM.getModule(), llvm::Intrinsic::localrecover);
+      llvm::Constant *ParentI8Fn =
+          llvm::ConstantExpr::getBitCast(ParentCGF.CurFn, Int8PtrTy);
+      ParentFP = Builder.CreateCall(
+          FrameRecoverFn, {ParentI8Fn, ParentFP,
+                           llvm::ConstantInt::get(Int32Ty, FrameEscapeIdx)});
+      ParentFP = Builder.CreateBitCast(ParentFP, CGM.VoidPtrPtrTy);
+      ParentFP = Builder.CreateLoad(Address(ParentFP, getPointerAlign()));
+    }
   }
 
   // Create llvm.localrecover calls for all captures.
@@ -2013,6 +2055,7 @@ void CodeGenFunction::pushSEHCleanup(CleanupKind Kind,
 
 void CodeGenFunction::EnterSEHTryStmt(const SEHTryStmt &S) {
   CodeGenFunction HelperCGF(CGM, /*suppressNewContext=*/true);
+  HelperCGF.ParentCGF = this;
   if (const SEHFinallyStmt *Finally = S.getFinallyHandler()) {
     // Outline the finally block.
     llvm::Function *FinallyFunc =