[PATCH] D19773: Tweak the ThinLTO pass pipeline

[Mehdi AMINI via llvm-commits]

joker.eph created this revision.
joker.eph added a reviewer: tejohnson.
joker.eph added a subscriber: llvm-commits.
Herald added a subscriber: joker.eph.

The original ThinLTO pipeline was derived from some
work I did tuning FullLTO on the test suite and SPEC. This 
patch reduces the amount of work done in the "linker phase" of
the build, and extend the function simplifications passes 
performed during the "compile phase". This helps the build time
by reducing the IR as much as possible during the compile phase
and limiting the work to be performed during the "link phase",
while keeping the performance "on par" with the existing pipeline.

http://reviews.llvm.org/D19773

Files:
  lib/Transforms/IPO/FunctionAttrs.cpp
  lib/Transforms/IPO/PassManagerBuilder.cpp

Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -242,11 +242,6 @@
   MPM.add(createTailCallEliminationPass()); // Eliminate tail calls
   MPM.add(createCFGSimplificationPass());     // Merge & remove BBs
   MPM.add(createReassociatePass());           // Reassociate expressions
-  if (PrepareForThinLTO) {
-    MPM.add(createAggressiveDCEPass());        // Delete dead instructions
-    addInstructionCombiningPass(MPM);          // Combine silly seq's
-    return;
-  }
   // Rotate Loop - disable header duplication at -Oz
   MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1));
   MPM.add(createLICMPass());                  // Hoist loop invariants
@@ -399,6 +394,14 @@
 
   addFunctionSimplificationPasses(MPM);
 
+  // FIXME: This is a HACK! The inliner pass above implicitly creates a CGSCC
+  // pass manager that we are specifically trying to avoid. To prevent this
+  // we must insert a no-op module pass to reset the pass manager.
+  MPM.add(createBarrierNoopPass());
+
+  if (!DisableUnitAtATime)
+    MPM.add(createReversePostOrderFunctionAttrsPass());
+
   // 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.
@@ -410,11 +413,6 @@
     return;
   }
 
-  // FIXME: This is a HACK! The inliner pass above implicitly creates a CGSCC
-  // pass manager that we are specifically trying to avoid. To prevent this
-  // we must insert a no-op module pass to reset the pass manager.
-  MPM.add(createBarrierNoopPass());
-
   // Scheduling LoopVersioningLICM when inlining is over, because after that
   // we may see more accurate aliasing. Reason to run this late is that too
   // early versioning may prevent further inlining due to increase of code
@@ -425,10 +423,7 @@
     MPM.add(createLICMPass());                  // Hoist loop invariants
   }
 
-  if (!DisableUnitAtATime)
-    MPM.add(createReversePostOrderFunctionAttrsPass());
-
-  if (!DisableUnitAtATime && OptLevel > 1 && !PrepareForLTO)
+  if (!DisableUnitAtATime && OptLevel > 1)
     // Remove avail extern fns and globals definitions if we aren't
     // compiling an object file for later LTO. For LTO we want to preserve
     // these so they are eligible for inlining at link-time. Note if they
@@ -440,15 +435,9 @@
     // and saves running remaining passes on the eliminated functions.
     MPM.add(createEliminateAvailableExternallyPass());
 
-  if (PerformThinLTO) {
-    // Remove dead fns and globals. Removing unreferenced functions could lead
-    // to more opportunities for globalopt.
-    MPM.add(createGlobalDCEPass());
+  if (PerformThinLTO)
+    // Remove dead fns and globals.
     MPM.add(createGlobalOptimizerPass());
-    // Remove dead fns and globals after globalopt.
-    MPM.add(createGlobalDCEPass());
-    addFunctionSimplificationPasses(MPM);
-  }
 
   if (EnableNonLTOGlobalsModRef)
     // We add a fresh GlobalsModRef run at this point. This is particularly
Index: lib/Transforms/IPO/FunctionAttrs.cpp
===================================================================
--- lib/Transforms/IPO/FunctionAttrs.cpp
+++ lib/Transforms/IPO/FunctionAttrs.cpp
@@ -1153,6 +1153,7 @@
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<CallGraphWrapperPass>();
+    AU.addPreserved<CallGraphWrapperPass>();
   }
 };
 }


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