[clang] Support -clear-ast-before-backend without -disable-free

Previously without -disable-free, -clear-ast-before-backend would crash in ~ASTContext() due to various reasons.
This works around that by doing a lot of the cleanup ahead of the destructor so that the destructor doesn't actually do any manual cleanup if we've already cleaned up beforehand.

This actually does save a measurable amount of memory with -clear-ast-before-backend, although at an almost unnoticeable runtime cost:
https://llvm-compile-time-tracker.com/compare.php?from=5d755b32f2775b9219f6d6e2feda5e1417dc993b&to=58ef1c7ad7e2ad45f9c97597905a8cf05a26258c&stat=max-rss

Previously we weren't doing any cleanup with -disable-free, so I tried measuring the impact of always doing the cleanup and didn't measure anything noticeable on llvm-compile-time-tracker.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D111767

1 files changed, 9 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CodeGenAction.cpp b/clang/lib/CodeGen/CodeGenAction.cpp
index 4ca34db..881e30a 100644
--- a/clang/lib/CodeGen/CodeGenAction.cpp
+++ b/clang/lib/CodeGen/CodeGenAction.cpp
@@ -351,10 +351,16 @@ namespace clang {
         }
       }
 
-      // FIXME: Fix cleanup issues with clearing the AST when we properly free
-      // things.
-      if (CodeGenOpts.DisableFree && CodeGenOpts.ClearASTBeforeBackend)
+      if (CodeGenOpts.ClearASTBeforeBackend) {
+        // Access to the AST is no longer available after this.
+        // Other things that the ASTContext manages are still available, e.g.
+        // the SourceManager. It'd be nice if we could separate out all the
+        // things in ASTContext used after this point and null out the
+        // ASTContext, but too many various parts of the ASTContext are still
+        // used in various parts.
+        C.cleanup();
         C.getAllocator().Reset();
+      }
 
       EmbedBitcode(getModule(), CodeGenOpts, llvm::MemoryBufferRef());