From 8f4ea36bfe4caf7d08f9778ee2a347b78f02bc0f Mon Sep 17 00:00:00 2001
From: Adam Czachorowski <adamcz@google.com>
Date: Fri, 25 Feb 2022 17:21:32 +0100
Subject: [clang] Improve laziness of resolving module map headers.

clang has support for lazy headers in module maps - if size and/or
modtime and provided in the cppmap file, headers are only resolved when
an include directive for a file with that size/modtime is encoutered.

Before this change, the lazy resolution was all-or-nothing per module.
That means as soon as even one file in that module potentially matched
an include, all lazy files in that module were resolved. With this
change, only files with matching size/modtime will be resolved.

The goal is to avoid unnecessary stat() calls on non-included files,
which is especially valuable on networked file systems, with higher
latency.

Differential Revision: https://reviews.llvm.org/D120569
---
 clang/lib/Lex/ModuleMap.cpp | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

(limited to 'clang/lib/Lex/ModuleMap.cpp')

diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
index 0b136ae..824b2bb 100644
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -482,7 +482,7 @@ void ModuleMap::diagnoseHeaderInclusion(Module *RequestingModule,
 
   if (RequestingModule) {
     resolveUses(RequestingModule, /*Complain=*/false);
-    resolveHeaderDirectives(RequestingModule);
+    resolveHeaderDirectives(RequestingModule, /*File=*/llvm::None);
   }
 
   bool Excluded = false;
@@ -1191,25 +1191,35 @@ void ModuleMap::resolveHeaderDirectives(const FileEntry *File) const {
   auto BySize = LazyHeadersBySize.find(File->getSize());
   if (BySize != LazyHeadersBySize.end()) {
     for (auto *M : BySize->second)
-      resolveHeaderDirectives(M);
+      resolveHeaderDirectives(M, File);
     LazyHeadersBySize.erase(BySize);
   }
 
   auto ByModTime = LazyHeadersByModTime.find(File->getModificationTime());
   if (ByModTime != LazyHeadersByModTime.end()) {
     for (auto *M : ByModTime->second)
-      resolveHeaderDirectives(M);
+      resolveHeaderDirectives(M, File);
     LazyHeadersByModTime.erase(ByModTime);
   }
 }
 
-void ModuleMap::resolveHeaderDirectives(Module *Mod) const {
+void ModuleMap::resolveHeaderDirectives(
+    Module *Mod, llvm::Optional<const FileEntry *> File) const {
   bool NeedsFramework = false;
-  for (auto &Header : Mod->UnresolvedHeaders)
-    // This operation is logically const; we're just changing how we represent
-    // the header information for this file.
-    const_cast<ModuleMap*>(this)->resolveHeader(Mod, Header, NeedsFramework);
-  Mod->UnresolvedHeaders.clear();
+  SmallVector<Module::UnresolvedHeaderDirective, 1> NewHeaders;
+  const auto Size = File ? File.getValue()->getSize() : 0;
+  const auto ModTime = File ? File.getValue()->getModificationTime() : 0;
+
+  for (auto &Header : Mod->UnresolvedHeaders) {
+    if (File && ((Header.ModTime && Header.ModTime != ModTime) ||
+                 (Header.Size && Header.Size != Size)))
+      NewHeaders.push_back(Header);
+    else
+      // This operation is logically const; we're just changing how we represent
+      // the header information for this file.
+      const_cast<ModuleMap *>(this)->resolveHeader(Mod, Header, NeedsFramework);
+  }
+  Mod->UnresolvedHeaders.swap(NewHeaders);
 }
 
 void ModuleMap::addHeader(Module *Mod, Module::Header Header,
-- 
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