[PATCH] D131817: [InstrProf] Add option to disable loop opt after PGO

[Ellis Hoag via Phabricator via llvm-commits]

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Add the `-enable-post-pgo-loop-rotation` option to enable or disable the loop rotation transformation [1]. With some instrumentations, e.g., function entry coverage [2], loop rotation is not necessary and can lead to some surprise differences in codegen, even for functions where instrumentation is blocked with `noprofile` or `skipprofile`.

[1] https://www.llvm.org/docs/LoopTerminology.html#loop-terminology-loop-rotate
[2] https://reviews.llvm.org/D116180


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131817

Files:
  llvm/lib/Passes/PassBuilderPipelines.cpp


Index: llvm/lib/Passes/PassBuilderPipelines.cpp
===================================================================
--- llvm/lib/Passes/PassBuilderPipelines.cpp
+++ llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -182,6 +182,10 @@
     "enable-merge-functions", cl::init(false), cl::Hidden,
     cl::desc("Enable function merging as part of the optimization pipeline"));
 
+static cl::opt<bool> EnablePostPGOLoopRotation(
+    "enable-post-pgo-loop-rotation", cl::init(true), cl::Hidden,
+    cl::desc("Run the loop rotation transformation after PGO instrumentation"));
+
 PipelineTuningOptions::PipelineTuningOptions() {
   LoopInterleaving = true;
   LoopVectorization = true;
@@ -664,13 +668,15 @@
   // Perform PGO instrumentation.
   MPM.addPass(PGOInstrumentationGen(IsCS));
 
-  // Disable header duplication in loop rotation at -Oz.
-  MPM.addPass(createModuleToFunctionPassAdaptor(
-      createFunctionToLoopPassAdaptor(
-          LoopRotatePass(Level != OptimizationLevel::Oz),
-          /*UseMemorySSA=*/false,
-          /*UseBlockFrequencyInfo=*/false),
-      PTO.EagerlyInvalidateAnalyses));
+  if (EnablePostPGOLoopRotation) {
+    // Disable header duplication in loop rotation at -Oz.
+    MPM.addPass(createModuleToFunctionPassAdaptor(
+        createFunctionToLoopPassAdaptor(
+            LoopRotatePass(Level != OptimizationLevel::Oz),
+            /*UseMemorySSA=*/false,
+            /*UseBlockFrequencyInfo=*/false),
+        PTO.EagerlyInvalidateAnalyses));
+  }
 
   // Add the profile lowering pass.
   InstrProfOptions Options;


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