[llvm] r295260 - PMB: Add an importing WPD pass to the start of the ThinLTO backend pipeline.

[Peter Collingbourne via llvm-commits]

Author: pcc
Date: Wed Feb 15 17:48:38 2017
New Revision: 295260

URL: http://llvm.org/viewvc/llvm-project?rev=295260&view=rev
Log:
PMB: Add an importing WPD pass to the start of the ThinLTO backend pipeline.

Differential Revision: https://reviews.llvm.org/D30008

Modified:
    llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp

Modified: llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp?rev=295260&r1=295259&r2=295260&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp Wed Feb 15 17:48:38 2017
@@ -830,8 +830,22 @@ void PassManagerBuilder::populateThinLTO
   if (VerifyInput)
     PM.add(createVerifierPass());
 
-  if (Summary)
+  if (Summary) {
+    // These passes import type identifier resolutions for whole-program
+    // devirtualization and CFI. They must run early because other passes may
+    // disturb the specific instruction patterns that these passes look for,
+    // creating dependencies on resolutions that may not appear in the summary.
+    //
+    // For example, GVN may transform the pattern assume(type.test) appearing in
+    // two basic blocks into assume(phi(type.test, type.test)), which would
+    // transform a dependency on a WPD resolution into a dependency on a type
+    // identifier resolution for CFI.
+    //
+    // Also, WPD has access to more precise information than ICP and can
+    // devirtualize more effectively, so it should operate on the IR first.
+    PM.add(createWholeProgramDevirtPass(PassSummaryAction::Import, Summary));
     PM.add(createLowerTypeTestsPass(PassSummaryAction::Import, Summary));
+  }
 
   populateModulePassManager(PM);