[PATCH] D20643: Move whole-program virtual call optimization pass after function attribute inference in LTO pipeline.

[Peter Collingbourne via llvm-commits]

pcc created this revision.
pcc added reviewers: pete, mehdi_amini, chandlerc.
pcc added subscribers: llvm-commits, krasin.
Herald added a subscriber: mehdi_amini.

As a result of D18634 we no longer infer certain attributes on linkonce_odr
functions at compile time, and may only infer them at LTO time. The readnone
attribute in particular is required for virtual constant propagation (part
of whole-program virtual call optimization) to work correctly.

This change moves the whole-program virtual call optimization pass after
the function attribute inference passes, and enables the attribute inference
passes at opt level 1, so that virtual constant propagation has a chance to
work correctly for linkonce_odr functions.

http://reviews.llvm.org/D20643

Files:
  lib/Transforms/IPO/PassManagerBuilder.cpp

Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -567,6 +567,10 @@
 }
 
 void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
+  // Remove unused virtual tables to improve the quality of code generated by
+  // whole-program devirtualization and bitset lowering.
+  PM.add(createGlobalDCEPass());
+
   // Provide AliasAnalysis services for optimizations.
   addInitialAliasAnalysisPasses(PM);
 
@@ -585,14 +589,26 @@
   // produce the same result as if we only do promotion here.
   PM.add(createPGOIndirectCallPromotionLegacyPass(true));
 
-  // Propagate constants at call sites into the functions they call.  This
-  // opens opportunities for globalopt (and inlining) by substituting function
-  // pointers passed as arguments to direct uses of functions.
-  PM.add(createIPSCCPPass());
+  if (OptLevel > 1) {
+    // Propagate constants at call sites into the functions they call.  This
+    // opens opportunities for globalopt (and inlining) by substituting function
+    // pointers passed as arguments to direct uses of functions.
+    PM.add(createIPSCCPPass());
+  }
 
-  // Now that we internalized some globals, see if we can hack on them!
+  // Infer attributes about definitions. The readnone attribute in particular is
+  // required for virtual constant propagation.
   PM.add(createPostOrderFunctionAttrsLegacyPass());
   PM.add(createReversePostOrderFunctionAttrsPass());
+
+  // Apply whole-program devirtualization and virtual constant propagation.
+  PM.add(createWholeProgramDevirtPass());
+
+  // That's all we need at opt level 1.
+  if (OptLevel == 1)
+    return;
+
+  // Now that we internalized some globals, see if we can hack on them!
   PM.add(createGlobalOptimizerPass());
   // Promote any localized global vars.
   PM.add(createPromoteMemoryToRegisterPass());
@@ -694,16 +710,6 @@
   PM.add(createJumpThreadingPass());
 }
 
-void PassManagerBuilder::addEarlyLTOOptimizationPasses(
-    legacy::PassManagerBase &PM) {
-  // Remove unused virtual tables to improve the quality of code generated by
-  // whole-program devirtualization and bitset lowering.
-  PM.add(createGlobalDCEPass());
-
-  // Apply whole-program devirtualization and virtual constant propagation.
-  PM.add(createWholeProgramDevirtPass());
-}
-
 void PassManagerBuilder::addLateLTOOptimizationPasses(
     legacy::PassManagerBase &PM) {
   // Delete basic blocks, which optimization passes may have killed.
@@ -746,9 +752,6 @@
     PM.add(createVerifierPass());
 
   if (OptLevel != 0)
-    addEarlyLTOOptimizationPasses(PM);
-
-  if (OptLevel > 1)
     addLTOOptimizationPasses(PM);
 
   // Create a function that performs CFI checks for cross-DSO calls with targets


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