[Comment] Add a comment to CompilationCacheTable::Lookup/PutScript.

Change-Id: Ia5fa003e8a3bc205ea0384bea9ebb3f6532a1771
Reviewed-on: https://chromium-review.googlesource.com/1163619Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54938}

src/objects.cc

@@ -17670,6 +17670,10 @@ FeedbackCell* SearchLiteralsMap(CompilationCacheTable* cache, int cache_entry,
 MaybeHandle<SharedFunctionInfo> CompilationCacheTable::LookupScript(
     Handle<String> src, Handle<Context> native_context,
     LanguageMode language_mode) {
+  // We use the empty function SFI as part of the key. Although the
+  // empty_function is native context dependent, the SFI is de-duped on
+  // snapshot builds by the PartialSnapshotCache, and so this does not prevent
+  // reuse of scripts in the compilation cache across native contexts.
   Handle<SharedFunctionInfo> shared(native_context->empty_function()->shared(),
                                     native_context->GetIsolate());
   StringSharedKey key(src, shared, language_mode, kNoSourcePosition);
@@ -17732,6 +17736,10 @@ Handle<CompilationCacheTable> CompilationCacheTable::PutScript(
     Handle<Context> native_context, LanguageMode language_mode,
     Handle<SharedFunctionInfo> value) {
   Isolate* isolate = native_context->GetIsolate();
+  // We use the empty function SFI as part of the key. Although the
+  // empty_function is native context dependent, the SFI is de-duped on
+  // snapshot builds by the PartialSnapshotCache, and so this does not prevent
+  // reuse of scripts in the compilation cache across native contexts.
   Handle<SharedFunctionInfo> shared(native_context->empty_function()->shared(),
                                     isolate);
   StringSharedKey key(src, shared, language_mode, kNoSourcePosition);