[SelectOpti][1/5] Setup new select-optimize pass

This is the first commit for the cmov-vs-branch optimization pass.
The goal is to develop a new profile-guided and target-independent cost/benefit analysis
for selecting conditional moves over branches when optimizing for performance.

Initially, this new pass is expected to be enabled only for instrumentation-based PGO.

RFC: https://discourse.llvm.org/t/rfc-cmov-vs-branch-optimization/6040

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D120230

llvm/include/llvm/CodeGen/CodeGenPassBuilder.h

@@ -663,6 +663,10 @@ void CodeGenPassBuilder<Derived>::addIRPasses(AddIRPass &addPass) const {
 
   // Expand reduction intrinsics into shuffle sequences if the target wants to.
   addPass(ExpandReductionsPass());
+
+  // Convert conditional moves to conditional jumps when profitable.
+  if (getOptLevel() != CodeGenOpt::None && !Opt.DisableSelectOptimize)
+    addPass(SelectOptimizePass());
 }
 
 /// Turn exception handling constructs into something the code generators can

llvm/include/llvm/CodeGen/MachinePassRegistry.def

@@ -120,6 +120,7 @@ DUMMY_FUNCTION_PASS("indirectbr-expand", IndirectBrExpandPass, ())
 DUMMY_FUNCTION_PASS("cfguard-dispatch", CFGuardDispatchPass, ())
 DUMMY_FUNCTION_PASS("cfguard-check", CFGuardCheckPass, ())
 DUMMY_FUNCTION_PASS("gc-info-printer", GCInfoPrinterPass, ())
+DUMMY_FUNCTION_PASS("select-optimize", SelectOptimizePass, ())
 #undef DUMMY_FUNCTION_PASS
 
 #ifndef DUMMY_MODULE_PASS

llvm/include/llvm/CodeGen/Passes.h

@@ -560,6 +560,9 @@ namespace llvm {
 
   /// JMC instrument pass.
   ModulePass *createJMCInstrumenterPass();
+
+  /// This pass converts conditional moves to conditional jumps when profitable.
+  FunctionPass *createSelectOptimizePass();
 } // End llvm namespace
 
 #endif

llvm/include/llvm/InitializePasses.h

@@ -394,6 +394,7 @@ void initializeSROALegacyPassPass(PassRegistry&);
 void initializeSafeStackLegacyPassPass(PassRegistry&);
 void initializeSafepointIRVerifierPass(PassRegistry&);
 void initializeSampleProfileLoaderLegacyPassPass(PassRegistry&);
+void initializeSelectOptimizePass(PassRegistry &);
 void initializeModuleSanitizerCoverageLegacyPassPass(PassRegistry &);
 void initializeScalarEvolutionWrapperPassPass(PassRegistry&);
 void initializeScalarizeMaskedMemIntrinLegacyPassPass(PassRegistry &);

llvm/include/llvm/LinkAllPasses.h

@@ -232,6 +232,7 @@ namespace {
       (void) llvm::createUnifyLoopExitsPass();
       (void) llvm::createFixIrreduciblePass();
       (void)llvm::createFunctionSpecializationPass();
+      (void)llvm::createSelectOptimizePass();
 
       (void)new llvm::IntervalPartition();
       (void)new llvm::ScalarEvolutionWrapperPass();

llvm/include/llvm/Target/CGPassBuilderOption.h

@@ -42,6 +42,7 @@ struct CGPassBuilderOption {
   bool DisableMergeICmps = false;
   bool DisablePartialLibcallInlining = false;
   bool DisableConstantHoisting = false;
+  bool DisableSelectOptimize = true;
   bool PrintISelInput = false;
   bool PrintGCInfo = false;
   bool RequiresCodeGenSCCOrder = false;

llvm/lib/CodeGen/CMakeLists.txt

@@ -197,6 +197,7 @@ add_llvm_component_library(LLVMCodeGen
   ScheduleDAGInstrs.cpp
   ScheduleDAGPrinter.cpp
   ScoreboardHazardRecognizer.cpp
+  SelectOptimize.cpp
   ShadowStackGCLowering.cpp
   ShrinkWrap.cpp
   SjLjEHPrepare.cpp

llvm/lib/CodeGen/CodeGen.cpp

@@ -107,6 +107,7 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeRemoveRedundantDebugValuesPass(Registry);
   initializeRenameIndependentSubregsPass(Registry);
   initializeSafeStackLegacyPassPass(Registry);
+  initializeSelectOptimizePass(Registry);
   initializeShadowStackGCLoweringPass(Registry);
   initializeShrinkWrapPass(Registry);
   initializeSjLjEHPreparePass(Registry);

llvm/lib/CodeGen/SelectOptimize.cpp

@@ -0,0 +1,43 @@
+//===--- SelectOptimize.cpp - Convert select to branches if profitable ---===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass converts selects to conditional jumps when profitable.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+
+using namespace llvm;
+
+namespace {
+
+class SelectOptimize : public FunctionPass {
+public:
+  static char ID;
+  SelectOptimize() : FunctionPass(ID) {
+    initializeSelectOptimizePass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnFunction(Function &F) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {}
+};
+} // namespace
+
+char SelectOptimize::ID = 0;
+INITIALIZE_PASS(SelectOptimize, "select-optimize", "Optimize selects", false,
+                false)
+
+FunctionPass *llvm::createSelectOptimizePass() { return new SelectOptimize(); }
+
+bool SelectOptimize::runOnFunction(Function &F) {
+  llvm_unreachable("Unimplemented");
+}

llvm/lib/CodeGen/TargetPassConfig.cpp

@@ -260,6 +260,11 @@ static cl::opt<bool> DisableExpandReductions(
     "disable-expand-reductions", cl::init(false), cl::Hidden,
     cl::desc("Disable the expand reduction intrinsics pass from running"));
 
+/// Disable the select optimization pass.
+static cl::opt<bool> DisableSelectOptimize(
+    "disable-select-optimize", cl::init(true), cl::Hidden,
+    cl::desc("Disable the select-optimization pass from running"));
+
 /// Allow standard passes to be disabled by command line options. This supports
 /// simple binary flags that either suppress the pass or do nothing.
 /// i.e. -disable-mypass=false has no effect.
@@ -494,6 +499,7 @@ CGPassBuilderOption llvm::getCGPassBuilderOption() {
   SET_BOOLEAN_OPTION(DisableConstantHoisting)
   SET_BOOLEAN_OPTION(DisableCGP)
   SET_BOOLEAN_OPTION(DisablePartialLibcallInlining)
+  SET_BOOLEAN_OPTION(DisableSelectOptimize)
   SET_BOOLEAN_OPTION(PrintLSR)
   SET_BOOLEAN_OPTION(PrintISelInput)
   SET_BOOLEAN_OPTION(PrintGCInfo)
@@ -935,6 +941,10 @@ void TargetPassConfig::addIRPasses() {
 
   if (getOptLevel() != CodeGenOpt::None)
     addPass(createTLSVariableHoistPass());
+
+  // Convert conditional moves to conditional jumps when profitable.
+  if (getOptLevel() != CodeGenOpt::None && !DisableSelectOptimize)
+    addPass(createSelectOptimizePass());
 }
 
 /// Turn exception handling constructs into something the code generators can

llvm/tools/opt/opt.cpp

@@ -493,7 +493,8 @@ static bool shouldPinPassToLegacyPM(StringRef Pass) {
       "polyhedral-info",      "print-polyhedral-info",
       "replace-with-veclib",  "jmc-instrument",
       "dot-regions",          "dot-regions-only",
-      "view-regions",         "view-regions-only"};
+      "view-regions",         "view-regions-only",
+      "select-optimize"};
   for (const auto &P : PassNamePrefix)
     if (Pass.startswith(P))
       return true;
@@ -544,6 +545,7 @@ int main(int argc, char **argv) {
   // supported.
   initializeExpandMemCmpPassPass(Registry);
   initializeScalarizeMaskedMemIntrinLegacyPassPass(Registry);
+  initializeSelectOptimizePass(Registry);
   initializeCodeGenPreparePass(Registry);
   initializeAtomicExpandPass(Registry);
   initializeRewriteSymbolsLegacyPassPass(Registry);