[Serialization] Don't pack bits for the function scope index of ParmVarDecl

Close llvm#76443

Previously we assume the bits of the function scope index of ParmVarDecl
won't exceed 8. But this is a misreading. See the implementation of
`ParmVarDecl::getParameterIndex()`, which may
exceed the size of the normal bitfield. So it may be better to not
pack these bits.

Author: ChuanqiXu9

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -1706,10 +1706,10 @@ void ASTDeclReader::VisitImplicitParamDecl(ImplicitParamDecl *PD) {
 void ASTDeclReader::VisitParmVarDecl(ParmVarDecl *PD) {
   VisitVarDecl(PD);
 
+  unsigned scopeIndex = Record.readInt();
   BitsUnpacker ParmVarDeclBits(Record.readInt());
   unsigned isObjCMethodParam = ParmVarDeclBits.getNextBit();
   unsigned scopeDepth = ParmVarDeclBits.getNextBits(/*Width=*/7);
-  unsigned scopeIndex = ParmVarDeclBits.getNextBits(/*Width=*/8);
   unsigned declQualifier = ParmVarDeclBits.getNextBits(/*Width=*/7);
   if (isObjCMethodParam) {
     assert(scopeDepth == 0);

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1163,10 +1163,14 @@ void ASTDeclWriter::VisitImplicitParamDecl(ImplicitParamDecl *D) {
 void ASTDeclWriter::VisitParmVarDecl(ParmVarDecl *D) {
   VisitVarDecl(D);
 
+  // See the implementation of `ParmVarDecl::getParameterIndex()`, which may
+  // exceed the size of the normal bitfield. So it may be better to not pack
+  // these bits.
+  Record.push_back(D->getFunctionScopeIndex());
+
   BitsPacker ParmVarDeclBits;
   ParmVarDeclBits.addBit(D->isObjCMethodParameter());
   ParmVarDeclBits.addBits(D->getFunctionScopeDepth(), /*BitsWidth=*/7);
-  ParmVarDeclBits.addBits(D->getFunctionScopeIndex(), /*BitsWidth=*/8);
   // FIXME: stable encoding
   ParmVarDeclBits.addBits(D->getObjCDeclQualifier(), /*BitsWidth=*/7);
   ParmVarDeclBits.addBit(D->isKNRPromoted());
@@ -2350,10 +2354,11 @@ void ASTWriter::WriteDeclAbbrevs() {
                             // isARCPseudoStrong, Linkage, ModulesCodegen
   Abv->Add(BitCodeAbbrevOp(0));                          // VarKind (local enum)
   // ParmVarDecl
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // ScopeIndex
   Abv->Add(BitCodeAbbrevOp(
       BitCodeAbbrevOp::Fixed,
-      27)); // Packed Parm Var Decl bits: IsObjCMethodParameter, ScopeDepth,
-            // ScopeIndex, ObjCDeclQualifier, KNRPromoted,
+      19)); // Packed Parm Var Decl bits: IsObjCMethodParameter, ScopeDepth,
+            // ObjCDeclQualifier, KNRPromoted,
             // HasInheritedDefaultArg, HasUninstantiatedDefaultArg
   // Type Source Info
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));

clang/test/PCH/pr76443.cpp

@@ -0,0 +1,24 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+//
+// RUN: %clang_cc1 -std=c++17 -emit-pch %s -o %t/h.pcm
+
+//--- header.h
+template <int... Nx> int stringData(const char (&...x)[Nx]) {
+  return 0;
+}
+int qt_meta_stringdata_CLASSQStyleENDCLASS = stringData(
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "",
+    "", "", "", "", "", "", "", "", "", "");