[Modules] Fix rebuilding an updated module for each of its consumers.

Marking a module for a rebuild when its signature differs from the
expected one causes redundant module rebuilds for incremental builds.
When a module is updated, its signature changes. But its consumers still
have the old signature and loading them will result in signature
mismatches. It will correctly cause the rebuilds for the consumers but
we don't need to rebuild the common module for each of them as it is
already up to date.

In practice this bug causes longer build times. We are doing more work
than required and only a single process can build a module, so parallel
builds degrade to a single-process mode where extra processes are just
waiting on a file lock.

Fix by not marking a module dependency for a rebuild on signature
mismatch. We'll check if it is up to date when we load it.

rdar://problem/50212358

Reviewers: dexonsmith, bruno, rsmith

Reviewed By: dexonsmith, bruno

Subscribers: jkorous, ributzka, cfe-commits, aprantl

Differential Revision: https://reviews.llvm.org/D66907

llvm-svn: 370274

clang/lib/Serialization/ModuleManager.cpp

@@ -204,13 +204,8 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
   // Read the signature eagerly now so that we can check it.  Avoid calling
   // ReadSignature unless there's something to check though.
   if (ExpectedSignature && checkSignature(ReadSignature(NewModule->Data),
-                                          ExpectedSignature, ErrorStr)) {
-    // Try to remove the buffer.  If it can't be removed, then it was already
-    // validated by this process.
-    if (!getModuleCache().tryToDropPCM(NewModule->FileName))
-      FileMgr.invalidateCache(NewModule->File);
+                                          ExpectedSignature, ErrorStr))
     return OutOfDate;
-  }
 
   // We're keeping this module.  Store it everywhere.
   Module = Modules[Entry] = NewModule.get();

clang/test/Modules/Inputs/implicit-invalidate-common/A.h

@@ -0,0 +1,2 @@
+// A
+#include "Common.h"

clang/test/Modules/Inputs/implicit-invalidate-common/B.h

@@ -0,0 +1,2 @@
+// B
+#include "Common.h"

clang/test/Modules/Inputs/implicit-invalidate-common/Common.h

@@ -0,0 +1 @@
+// Common

clang/test/Modules/Inputs/implicit-invalidate-common/module.modulemap

@@ -0,0 +1,3 @@
+module A { header "A.h" }
+module B { header "B.h" }
+module Common { header "Common.h" }

clang/test/Modules/implicit-invalidate-common.c

@@ -0,0 +1,36 @@
+// REQUIRES: shell
+// RUN: rm -rf %t
+// RUN: mkdir -p %t/implicit-invalidate-common
+// RUN: cp -r %S/Inputs/implicit-invalidate-common %t/
+// RUN: echo '#include "A.h"' > %t/A.c
+// RUN: echo '#include "B.h"' > %t/B.c
+
+// Build with an empty module cache. Module 'Common' should be built only once.
+//
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/Cache \
+// RUN:     -fsyntax-only -I %t/implicit-invalidate-common -Rmodule-build \
+// RUN:     %t/A.c 2> %t/initial_build.txt
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/Cache \
+// RUN:     -fsyntax-only -I %t/implicit-invalidate-common -Rmodule-build \
+// RUN:     %t/B.c 2>> %t/initial_build.txt
+// RUN: FileCheck %s --implicit-check-not "remark:" --input-file %t/initial_build.txt
+
+// Update module 'Common' and build with the populated module cache. Module
+// 'Common' still should be built only once. Note that we are using the same
+// flags for A.c and B.c to avoid building Common.pcm at different paths.
+//
+// RUN: echo ' // ' >> %t/implicit-invalidate-common/Common.h
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/Cache \
+// RUN:     -fsyntax-only -I %t/implicit-invalidate-common -Rmodule-build \
+// RUN:     %t/A.c 2> %t/incremental_build.txt
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/Cache \
+// RUN:     -fsyntax-only -I %t/implicit-invalidate-common -Rmodule-build \
+// RUN:     %t/B.c 2>> %t/incremental_build.txt
+// RUN: FileCheck %s --implicit-check-not "remark:" --input-file %t/incremental_build.txt
+
+// CHECK: remark: building module 'A'
+// CHECK: remark: building module 'Common'
+// CHECK: remark: finished building module 'Common'
+// CHECK: remark: finished building module 'A'
+// CHECK: remark: building module 'B'
+// CHECK: remark: finished building module 'B'