[PassManager] Improve the interaction between -O2 and ThinLTO.

Run GDCE slightly later so that we don't have to repeat it
twice when preparing for Thin. Thanks to Mehdi for the suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314999 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dcci

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -494,17 +494,6 @@ void PassManagerBuilder::populateModulePassManager(
   // we must insert a no-op module pass to reset the pass manager.
   MPM.add(createBarrierNoopPass());
 
-  // The inliner performs some kind of dead code elimination as it goes,
-  // but there are cases that are not really caught by it. We might
-  // at some point consider teaching the inliner about them, but it
-  // is OK for now to run GlobalOpt + GlobalDCE in tandem as their
-  // benefits generally outweight the cost, making the whole pipeline
-  // faster.
-  if (RunInliner) {
-    MPM.add(createGlobalOptimizerPass());
-    MPM.add(createGlobalDCEPass());
-  }
-
   if (!DisableUnitAtATime && OptLevel > 1 && !PrepareForLTO &&
       !PrepareForThinLTO)
     // Remove avail extern fns and globals definitions if we aren't
@@ -521,12 +510,21 @@ void PassManagerBuilder::populateModulePassManager(
   if (!DisableUnitAtATime)
     MPM.add(createReversePostOrderFunctionAttrsPass());
 
+  // The inliner performs some kind of dead code elimination as it goes,
+  // but there are cases that are not really caught by it. We might
+  // at some point consider teaching the inliner about them, but it
+  // is OK for now to run GlobalOpt + GlobalDCE in tandem as their
+  // benefits generally outweight the cost, making the whole pipeline
+  // faster.
+  if (RunInliner) {
+    MPM.add(createGlobalOptimizerPass());
+    MPM.add(createGlobalDCEPass());
+  }
+
   // If we are planning to perform ThinLTO later, let's not bloat the code with
   // unrolling/vectorization/... now. We'll first run the inliner + CGSCC passes
   // during ThinLTO and perform the rest of the optimizations afterward.
   if (PrepareForThinLTO) {
-    // Reduce the size of the IR as much as possible.
-    MPM.add(createGlobalOptimizerPass());
     // Rename anon globals to be able to export them in the summary.
     MPM.add(createNameAnonGlobalPass());
     return;

test/Other/pass-pipelines.ll

@@ -55,15 +55,15 @@
 ; Next we break out of the main Function passes inside the CGSCC pipeline with
 ; a barrier pass.
 ; CHECK-O2: A No-Op Barrier Pass
-; Reduce the size of the IR ASAP after the inliner.
-; CHECK-O2-NEXT: Global Variable Optimizer
-; CHECK-O2: Dead Global Elimination
 ; CHECK-O2-NEXT: Eliminate Available Externally
 ; Inferring function attribute should be right after the CGSCC pipeline, before
 ; any other optimizations/analyses.
 ; CHECK-O2-NEXT: CallGraph
 ; CHECK-O2-NEXT: Deduce function attributes in RPO
 ; CHECK-O2-NOT: Manager
+; Reduce the size of the IR ASAP after the inliner.
+; CHECK-O2-NEXT: Global Variable Optimizer
+; CHECK-O2: Dead Global Elimination
 ; Next is the late function pass pipeline.
 ; CHECK-O2: FunctionPass Manager
 ; CHECK-O2-NOT: Manager