Don't assert when redefining a built-in macro in a PCH, PR29119

PCH files store the macro history for a given macro, and the whole history list
for one identifier is given to the Preprocessor at once via
Preprocessor::setLoadedMacroDirective(). This contained an assert that no macro
history exists yet for that identifier. That's usually true, but it's not true
for builtin macros, which are created in Preprocessor() before flags and pchs
are processed. Luckily, ASTWriter stops writing macro history lists at builtins
(see shouldIgnoreMacro() in ASTWriter.cpp), so the head of the history list was
missing for builtin macros. So make the assert weaker, and splice the history
list to the existing single define for builtins.

https://reviews.llvm.org/D27545

llvm-svn: 289228

1 files changed, 26 insertions, 3 deletions

diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
index f3a57e3348d..aebebaac46a 100644
--- a/clang/lib/Lex/PPMacroExpansion.cpp
+++ b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -92,12 +92,35 @@ void Preprocessor::appendMacroDirective(IdentifierInfo *II, MacroDirective *MD){
 }
 
 void Preprocessor::setLoadedMacroDirective(IdentifierInfo *II,
+                                           MacroDirective *ED,
                                            MacroDirective *MD) {
+  // Normally, when a macro is defined, it goes through appendMacroDirective()
+  // above, which chains a macro to previous defines, undefs, etc.
+  // However, in a pch, the whole macro history up to the end of the pch is
+  // stored, so ASTReader goes through this function instead.
+  // However, built-in macros are already registered in the Preprocessor
+  // ctor, and ASTWriter stops writing the macro chain at built-in macros,
+  // so in that case the chain from the pch needs to be spliced to the existing
+  // built-in.
+
   assert(II && MD);
   MacroState &StoredMD = CurSubmoduleState->Macros[II];
-  assert(!StoredMD.getLatest() &&
-         "the macro history was modified before initializing it from a pch");
-  StoredMD = MD;
+
+  if (auto *OldMD = StoredMD.getLatest()) {
+    // shouldIgnoreMacro() in ASTWriter also stops at macros from the
+    // predefines buffer in module builds. However, in module builds, modules
+    // are loaded completely before predefines are processed, so StoredMD
+    // will be nullptr for them when they're loaded. StoredMD should only be
+    // non-nullptr for builtins read from a pch file.
+    assert(OldMD->getMacroInfo()->isBuiltinMacro() &&
+           "only built-ins should have an entry here");
+    assert(!OldMD->getPrevious() && "builtin should only have a single entry");
+    ED->setPrevious(OldMD);
+    StoredMD.setLatest(MD);
+  } else {
+    StoredMD = MD;
+  }
+
   // Setup the identifier as having associated macro history.
   II->setHasMacroDefinition(true);
   if (!MD->isDefined() && LeafModuleMacros.find(II) == LeafModuleMacros.end())