[PATCH] D87322: [Passes] Move ADCE before DSE & LICM.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
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The adjustment seems to have very little impact on optimizations.
The only binary change with -O3 MultiSource/SPEC2000/SPEC2006 on X86 is
in consumer-typeset and the size there actually decreases by -0.1%, with
not significant changes in the stats.

On its own, it is mildly positive in terms of compile-time, most likely
due to LICM & DSE having to process slightly less instructions. It
should also be unlikely that DSE/LICM make new code dead.

http://llvm-compile-time-tracker.com/compare.php?from=df63eedef64d715ce1f31843f7de9c11fe1e597f&to=e3bdfcf94a9eeae6e006d010464f0c1b3550577d&stat=instructions

With DSE & MemorySSA, it gives some nice compile-time improvements, due
to the fact that DSE can re-use the PDT from ADCE, if it does not make
any changes:

http://llvm-compile-time-tracker.com/compare.php?from=15fdd6cd7c24c745df1bb419e72ff66fd138aa7e&to=481f494515fc89cb7caea8d862e40f2c910dc994&stat=instructions


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D87322

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


Index: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -463,6 +463,11 @@
   if (OptLevel > 1) {
     MPM.add(createJumpThreadingPass());         // Thread jumps
     MPM.add(createCorrelatedValuePropagationPass());
+  }
+  MPM.add(createAggressiveDCEPass()); // Delete dead instructions
+
+  // TODO: Investigate if this is too expensive at O1.
+  if (OptLevel > 1) {
     MPM.add(createDeadStoreEliminationPass());  // Delete dead stores
     MPM.add(createLICMPass(LicmMssaOptCap, LicmMssaNoAccForPromotionCap));
   }
@@ -472,8 +477,6 @@
   if (RerollLoops)
     MPM.add(createLoopRerollPass());
 
-  // TODO: Investigate if this is too expensive at O1.
-  MPM.add(createAggressiveDCEPass());         // Delete dead instructions
   MPM.add(createCFGSimplificationPass()); // Merge & remove BBs
   // Clean up after everything.
   MPM.add(createInstructionCombiningPass());


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