[PGO] Control Height Reduction

Summary:
Control height reduction merges conditional blocks of code and reduces the
number of conditional branches in the hot path based on profiles.

if (hot_cond1) { // Likely true.
  do_stg_hot1();
}
if (hot_cond2) { // Likely true.
  do_stg_hot2();
}

->

if (hot_cond1 && hot_cond2) { // Hot path.
  do_stg_hot1();
  do_stg_hot2();
} else { // Cold path.
  if (hot_cond1) {
    do_stg_hot1();
  }
  if (hot_cond2) {
    do_stg_hot2();
  }
}

This speeds up some internal benchmarks up to ~30%.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: xbolva00, dmgreen, mehdi_amini, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D50591

llvm-svn: 341386

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Passes/PassBuilder.cpp b/llvm/lib/Passes/PassBuilder.cpp
index eb04dcc8b6e..d7b9dfca34a 100644
--- a/llvm/lib/Passes/PassBuilder.cpp
+++ b/llvm/lib/Passes/PassBuilder.cpp
@@ -87,6 +87,7 @@
 #include "llvm/Transforms/IPO/WholeProgramDevirt.h"
 #include "llvm/Transforms/InstCombine/InstCombine.h"
 #include "llvm/Transforms/Instrumentation/BoundsChecking.h"
+#include "llvm/Transforms/Instrumentation/ControlHeightReduction.h"
 #include "llvm/Transforms/Instrumentation/GCOVProfiler.h"
 #include "llvm/Transforms/Instrumentation/InstrProfiling.h"
 #include "llvm/Transforms/Instrumentation/PGOInstrumentation.h"
@@ -193,6 +194,10 @@ static cl::opt<bool> EnableSyntheticCounts(
 static Regex DefaultAliasRegex(
     "^(default|thinlto-pre-link|thinlto|lto-pre-link|lto)<(O[0123sz])>$");
 
+static cl::opt<bool>
+    EnableCHR("enable-chr-npm", cl::init(true), cl::Hidden,
+              cl::desc("Enable control height reduction optimization (CHR)"));
+
 static bool isOptimizingForSize(PassBuilder::OptimizationLevel Level) {
   switch (Level) {
   case PassBuilder::O0:
@@ -486,6 +491,10 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
   FPM.addPass(InstCombinePass());
   invokePeepholeEPCallbacks(FPM, Level);
 
+  if (EnableCHR && Level == O3 && PGOOpt &&
+      (!PGOOpt->ProfileUseFile.empty() || !PGOOpt->SampleProfileFile.empty()))
+    FPM.addPass(ControlHeightReductionPass());
+
   return FPM;
 }