[SelectOpti][2/5] Select-to-branch base transformation

[apostolakis]

Summary

This patch implements the actual transformation of selects to branches.
It includes only the base transformation without any sinking.

Depends on D120230

[davidxl]

llvm/lib/CodeGen/SelectOptimize.cpp

178

The MD_prof data needs to be passed from select to the branch.

llvm/test/CodeGen/X86/select-optimize.ll

1

Add more test cases with prof meta data on select?

[apostolakis]

llvm/lib/CodeGen/SelectOptimize.cpp

178

Actually this is already done. IB.CreateCondBr (line 191) will copy the metadata (i.e., the branch weights) from the select instruction to the branch instruction.

llvm/test/CodeGen/X86/select-optimize.ll

1

Added some IR-to-IR tests that check that profile metadata are preserved. More tests with profile metadata are added in the subsequent patches.

[apostolakis]

Add some IR-to-IR tests to check profile metadata.

[apostolakis]

Use only opt tests to avoid redundancy

[davidxl]

llvm/lib/CodeGen/SelectOptimize.cpp

154

Can we have a test case to demonstrate that machine sinking is actually enabled with this transformation?

229

should it skip debug stmts?

[apostolakis]

llvm/lib/CodeGen/SelectOptimize.cpp

154

This patch only includes the base transformation (as depicted in the comments here) without any sinking of eligible instructions to the newly true/false blocks of the new branch. This is why here there isn't even a new true block created and why the tests are simple. The last patch of this series (D120233) optimizes this transformation by aggressively sinking eligible instructions to the newly created true/false blocks and interleaves dependence slices to maximize ILP. In this last patch, you can see the sinking tested with several more complex tests (with the optimized transformation all eligible instructions are sunk). When lowered to assembly these instructions remain sunk. I removed the llc tests that tested assembly code since the opt tests just check the application of this pass without the effects of subsequent codegen passes that could eventually break the tests.

229

Right. If there are debug statements in between select instructions, they should be skipped and not prevent the formation of a select group. Patched the code to address this.

[apostolakis]

Skip debug/pseudo instructions when forming select group

[davidxl]

Perhaps modify the test case to include debug statement for better coverage. Otherwise LGTM.

[apostolakis]

Add debug statement in-between selects in a test case and move such debug insts just after the new phi nodes.

llvm/lib/CodeGen/SelectOptimize.cpp

 //===--- 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/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/BlockFrequencyInfo.h"
+#include "llvm/Analysis/BranchProbabilityInfo.h"
+#include "llvm/Analysis/LoopInfo.h"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/CodeGen/TargetSchedule.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instruction.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
+#include "llvm/Target/TargetMachine.h"
 
 using namespace llvm;
 
+#define DEBUG_TYPE "select-optimize"
+
+STATISTIC(NumSelectsConverted, "Number of selects converted");
+
 namespace {
 
 class SelectOptimize : public FunctionPass {
+  const TargetMachine *TM = nullptr;
+  const TargetSubtargetInfo *TSI;
+  const TargetLowering *TLI = nullptr;
+  const LoopInfo *LI;
+  std::unique_ptr<BlockFrequencyInfo> BFI;
+  std::unique_ptr<BranchProbabilityInfo> BPI;
+
 public:
   static char ID;
   SelectOptimize() : FunctionPass(ID) {
     initializeSelectOptimizePass(*PassRegistry::getPassRegistry());
   }
 
   bool runOnFunction(Function &F) override;
 
-  void getAnalysisUsage(AnalysisUsage &AU) const override {}
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<TargetPassConfig>();
+    AU.addRequired<LoopInfoWrapperPass>();
+  }
+
+private:
+  // Select groups consist of consecutive select instructions with the same
+  // condition.
+  using SelectGroup = SmallVector<SelectInst *, 2>;
+  using SelectGroups = SmallVector<SelectGroup, 2>;
+
+  bool optimizeSelects(Function &F);
+  void convertProfitableSIGroups(SelectGroups &ProfSIGroups);
+  void collectSelectGroups(BasicBlock &BB, SelectGroups &SIGroups);
+  bool isSelectKindSupported(SelectInst *SI);
 };
 } // namespace
 
 char SelectOptimize::ID = 0;
-INITIALIZE_PASS(SelectOptimize, "select-optimize", "Optimize selects", false,
+
+INITIALIZE_PASS_BEGIN(SelectOptimize, DEBUG_TYPE, "Optimize selects", false,
+                      false)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
+INITIALIZE_PASS_END(SelectOptimize, DEBUG_TYPE, "Optimize selects", false,
                     false)
 
 FunctionPass *llvm::createSelectOptimizePass() { return new SelectOptimize(); }
 
 bool SelectOptimize::runOnFunction(Function &F) {
-  llvm_unreachable("Unimplemented");
+  TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
+  TSI = TM->getSubtargetImpl(F);
+  TLI = TSI->getTargetLowering();
+  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+  BPI.reset(new BranchProbabilityInfo(F, *LI));
+  BFI.reset(new BlockFrequencyInfo(F, *BPI, *LI));
+
+  return optimizeSelects(F);
+}
+
+bool SelectOptimize::optimizeSelects(Function &F) {
+  // Collect all the select groups.
+  SelectGroups SIGroups;
+  for (BasicBlock &BB : F) {
+    collectSelectGroups(BB, SIGroups);
+  }
+
+  // Determine for which select groups it is profitable converting to branches.
+  SelectGroups ProfSIGroups;
+  // For now assume that all select groups can be profitably converted to
+  // branches.
+  for (SelectGroup &ASI : SIGroups) {
+    ProfSIGroups.push_back(ASI);
+  }
+
+  // Convert to branches the select groups that were deemed
+  // profitable-to-convert.
+  convertProfitableSIGroups(ProfSIGroups);
+
+  // Code modified if at least one select group was converted.
+  return !ProfSIGroups.empty();
+}
+
+/// If \p isTrue is true, return the true value of \p SI, otherwise return
+/// false value of \p SI. If the true/false value of \p SI is defined by any
+/// select instructions in \p Selects, look through the defining select
+/// instruction until the true/false value is not defined in \p Selects.
+static Value *
+getTrueOrFalseValue(SelectInst *SI, bool isTrue,
+                    const SmallPtrSet<const Instruction *, 2> &Selects) {
+  Value *V = nullptr;
+  for (SelectInst *DefSI = SI; DefSI != nullptr && Selects.count(DefSI);
+       DefSI = dyn_cast<SelectInst>(V)) {
+    assert(DefSI->getCondition() == SI->getCondition() &&
+           "The condition of DefSI does not match with SI");
+    V = (isTrue ? DefSI->getTrueValue() : DefSI->getFalseValue());
+  }
+  assert(V && "Failed to get select true/false value");
+  return V;
+}
+
+void SelectOptimize::convertProfitableSIGroups(SelectGroups &ProfSIGroups) {
+  for (SelectGroup &ASI : ProfSIGroups) {
+    // TODO: eliminate the redundancy of logic transforming selects to branches
+    // by removing CodeGenPrepare::optimizeSelectInst and optimizing here
+    // selects for all cases (with and without profile information).
+
+    // Transform a sequence like this:
+    //    start:
+    //       %cmp = cmp uge i32 %a, %b
+    //       %sel = select i1 %cmp, i32 %c, i32 %d
+    //
+    // Into:
+    //    start:
+    //       %cmp = cmp uge i32 %a, %b
+    //       %cmp.frozen = freeze %cmp
+    //       br i1 %cmp.frozen, label %select.end, label %select.false
+    //    select.false:
+    //       br label %select.end
+    //    select.end:
+    //       %sel = phi i32 [ %c, %start ], [ %d, %select.false ]
+    //
+    // %cmp should be frozen, otherwise it may introduce undefined behavior.
+
+    // We split the block containing the select(s) into two blocks.
+    SelectInst *SI = ASI.front();
+    SelectInst *LastSI = ASI.back();
+    BasicBlock *StartBlock = SI->getParent();
+    BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(LastSI));
+    BasicBlock *EndBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");
+    BFI->setBlockFreq(EndBlock, BFI->getBlockFreq(StartBlock).getFrequency());
+    // Delete the unconditional branch that was just created by the split.
+    StartBlock->getTerminator()->eraseFromParent();
+
+    // These are the new basic blocks for the conditional branch.
+    // For now, no instruction sinking to the true/false blocks.
+    // Thus both True and False blocks will be empty.
+    BasicBlock *TrueBlock = nullptr, *FalseBlock = nullptr;
+
+    // Use the 'false' side for a new input value to the PHI.
+    FalseBlock = BasicBlock::Create(SI->getContext(), "select.false",
+                                    EndBlock->getParent(), EndBlock);
+    auto *FalseBranch = BranchInst::Create(EndBlock, FalseBlock);
+    FalseBranch->setDebugLoc(SI->getDebugLoc());
+
+    // For the 'true' side the path originates from the start block from the
+    // point view of the new PHI.
+    TrueBlock = StartBlock;
+
+    // Insert the real conditional branch based on the original condition.
+    BasicBlock *TT, *FT;
+    TT = EndBlock;
+    FT = FalseBlock;
+    IRBuilder<> IB(SI);
+    auto *CondFr =
+        IB.CreateFreeze(SI->getCondition(), SI->getName() + ".frozen");
+    IB.CreateCondBr(CondFr, TT, FT, SI);
+
+    SmallPtrSet<const Instruction *, 2> INS;
+    INS.insert(ASI.begin(), ASI.end());
+    // Use reverse iterator because later select may use the value of the
+    // earlier select, and we need to propagate value through earlier select
+    // to get the PHI operand.
+    for (auto It = ASI.rbegin(); It != ASI.rend(); ++It) {
+      SelectInst *SI = *It;
+      // The select itself is replaced with a PHI Node.
+      PHINode *PN = PHINode::Create(SI->getType(), 2, "", &EndBlock->front());
+      PN->takeName(SI);
+      PN->addIncoming(getTrueOrFalseValue(SI, true, INS), TrueBlock);
+      PN->addIncoming(getTrueOrFalseValue(SI, false, INS), FalseBlock);
+      PN->setDebugLoc(SI->getDebugLoc());
+
+      SI->replaceAllUsesWith(PN);
+      SI->eraseFromParent();
+      INS.erase(SI);
+      ++NumSelectsConverted;
+    }
+  }
+}
+
+void SelectOptimize::collectSelectGroups(BasicBlock &BB,
+                                         SelectGroups &SIGroups) {
+  BasicBlock::iterator BBIt = BB.begin();
+  while (BBIt != BB.end()) {
+    Instruction *I = &*BBIt++;
+    if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
+      SelectGroup SIGroup;
+      SIGroup.push_back(SI);
+      while (BBIt != BB.end()) {
+        SelectInst *NI = dyn_cast<SelectInst>(&*BBIt);
+        if (NI && SI->getCondition() == NI->getCondition()) {
+          SIGroup.push_back(NI);
+          ++BBIt;
+        } else {
+          break;
+        }
+      }
+
+      // If the select type is not supported, no point optimizing it.
+      // Instruction selection will take care of it.
+      if (!isSelectKindSupported(SI))
+        continue;
+
+      SIGroups.push_back(SIGroup);
+    }
+  }
+}
+
+bool SelectOptimize::isSelectKindSupported(SelectInst *SI) {
+  bool VectorCond = !SI->getCondition()->getType()->isIntegerTy(1);
+  if (VectorCond)
+    return false;
+  TargetLowering::SelectSupportKind SelectKind;
+  if (SI->getType()->isVectorTy())
+    SelectKind = TargetLowering::ScalarCondVectorVal;
+  else
+    SelectKind = TargetLowering::ScalarValSelect;
+  return TLI->isSelectSupported(SelectKind);
 }

llvm/test/CodeGen/X86/select-optimize.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=x86_64-unknown-unknown -disable-select-optimize=false -x86-cmov-converter=false -disable-cgp-select2branch=true < %s | FileCheck %s
+
+; Single select converted to branch
+define i32 @single_select(i32 %a, i32 %b, i1 %cmp) {
+; CHECK-LABEL: single_select:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl %edi, %eax
+; CHECK-NEXT:    testb $1, %dl
+; CHECK-NEXT:    jne .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %select.false
+; CHECK-NEXT:    movl %esi, %eax
+; CHECK-NEXT:  .LBB0_2: # %select.end
+; CHECK-NEXT:    retq
+  %sel = select i1 %cmp, i32 %a, i32 %b
+  ret i32 %sel
+}
+
+; Select group converted to branch
+define i32 @select_group(i32 %a, i32 %b, i32 %c, i1 %cmp) {
+; CHECK-LABEL: select_group:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    testb $1, %cl
+; CHECK-NEXT:    je .LBB1_2
+; CHECK-NEXT:  # %bb.1:
+; CHECK-NEXT:    movl %edi, %eax
+; CHECK-NEXT:    addl %edx, %eax
+; CHECK-NEXT:    retq
+; CHECK-NEXT:  .LBB1_2: # %select.false
+; CHECK-NEXT:    movl %esi, %eax
+; CHECK-NEXT:    addl %edi, %eax
+; CHECK-NEXT:    retq
+  %sel1 = select i1 %cmp, i32 %a, i32 %b
+  %sel2 = select i1 %cmp, i32 %c, i32 %a
+  %add = add i32 %sel1, %sel2
+  ret i32 %add
+}
+
+; Select group with intra-group dependence converted to branch
+define i32 @select_group_intra_group(i32 %a, i32 %b, i32 %c, i1 %cmp) {
+; CHECK-LABEL: select_group_intra_group:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl %edi, %eax
+; CHECK-NEXT:    testb $1, %cl
+; CHECK-NEXT:    jne .LBB2_2
+; CHECK-NEXT:  # %bb.1: # %select.false
+; CHECK-NEXT:    movl %esi, %eax
+; CHECK-NEXT:    movl %esi, %edx
+; CHECK-NEXT:  .LBB2_2: # %select.end
+; CHECK-NEXT:    subl %edx, %eax
+; CHECK-NEXT:    retq
+  %sel1 = select i1 %cmp, i32 %a, i32 %b
+  %sel2 = select i1 %cmp, i32 %c, i32 %sel1
+  %sub = sub i32 %sel1, %sel2
+  ret i32 %sub
+}