Tweak the ThinLTO pass pipeline

Summary:
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.

Reviewers: tejohnson

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19773

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268769

1 files changed, 7 insertions, 17 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 8ce4f032dac..fd10c2bd06e 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -242,11 +242,6 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   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
@@ -431,26 +426,21 @@ void PassManagerBuilder::populateModulePassManager(
     return;
   }
 
+  if (PerformThinLTO)
+    // Optimize globals now when performing ThinLTO, this enables more
+    // optimizations later.
+    MPM.add(createGlobalOptimizerPass());
+
   // 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
-  // size. By placing it just after inlining other optimizations which runs 
-  // later might get benefit of no-alias assumption in clone loop. 
+  // size. By placing it just after inlining other optimizations which runs
+  // later might get benefit of no-alias assumption in clone loop.
   if (UseLoopVersioningLICM) {
     MPM.add(createLoopVersioningLICMPass());    // Do LoopVersioningLICM
     MPM.add(createLICMPass());                  // Hoist loop invariants
   }
 
-  if (PerformThinLTO) {
-    // Remove dead fns and globals. Removing unreferenced functions could lead
-    // to more opportunities for globalopt.
-    MPM.add(createGlobalDCEPass());
-    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
     // useful as the above will have inlined, DCE'ed, and function-attr