2011-10-17 06:15:07 +08:00
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//===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass lowers the 'expect' intrinsic to LLVM metadata.
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//
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//===----------------------------------------------------------------------===//
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2015-01-24 19:13:02 +08:00
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#include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
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2015-01-24 18:47:13 +08:00
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#include "llvm/ADT/SmallVector.h"
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2012-12-04 00:50:05 +08:00
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#include "llvm/ADT/Statistic.h"
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2013-01-02 19:36:10 +08:00
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/IR/MDBuilder.h"
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#include "llvm/IR/Metadata.h"
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2011-07-07 02:22:43 +08:00
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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2015-01-24 19:13:02 +08:00
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#include "llvm/Transforms/Scalar.h"
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2011-07-07 02:22:43 +08:00
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using namespace llvm;
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2014-04-22 10:55:47 +08:00
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#define DEBUG_TYPE "lower-expect-intrinsic"
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2015-01-24 18:57:25 +08:00
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STATISTIC(ExpectIntrinsicsHandled,
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"Number of 'expect' intrinsic instructions handled");
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2011-07-07 02:22:43 +08:00
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2016-04-27 06:23:38 +08:00
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// These default values are chosen to represent an extremely skewed outcome for
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// a condition, but they leave some room for interpretation by later passes.
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//
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// If the documentation for __builtin_expect() was made explicit that it should
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// only be used in extreme cases, we could make this ratio higher. As it stands,
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// programmers may be using __builtin_expect() / llvm.expect to annotate that a
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// branch is likely or unlikely to be taken.
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//
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// There is a known dependency on this ratio in CodeGenPrepare when transforming
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// 'select' instructions. It may be worthwhile to hoist these values to some
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// shared space, so they can be used directly by other passes.
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static cl::opt<uint32_t> LikelyBranchWeight(
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"likely-branch-weight", cl::Hidden, cl::init(2000),
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cl::desc("Weight of the branch likely to be taken (default = 2000)"));
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static cl::opt<uint32_t> UnlikelyBranchWeight(
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"unlikely-branch-weight", cl::Hidden, cl::init(1),
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cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
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2011-07-07 02:22:43 +08:00
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2015-01-24 18:39:24 +08:00
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static bool handleSwitchExpect(SwitchInst &SI) {
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CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
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2011-07-07 02:22:43 +08:00
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if (!CI)
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return false;
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Function *Fn = CI->getCalledFunction();
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if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
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return false;
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Value *ArgValue = CI->getArgOperand(0);
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ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
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if (!ExpectedValue)
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return false;
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2015-01-24 18:39:24 +08:00
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SwitchInst::CaseIt Case = SI.findCaseValue(ExpectedValue);
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unsigned n = SI.getNumCases(); // +1 for default case.
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SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
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if (Case == SI.case_default())
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Weights[0] = LikelyBranchWeight;
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else
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Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
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SI.setMetadata(LLVMContext::MD_prof,
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MDBuilder(CI->getContext()).createBranchWeights(Weights));
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SI.setCondition(ArgValue);
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return true;
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}
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2015-01-24 18:39:24 +08:00
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static bool handleBranchExpect(BranchInst &BI) {
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if (BI.isUnconditional())
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2011-07-07 02:22:43 +08:00
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return false;
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// Handle non-optimized IR code like:
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2014-02-03 06:43:55 +08:00
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// %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
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2011-07-07 02:22:43 +08:00
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// %tobool = icmp ne i64 %expval, 0
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// br i1 %tobool, label %if.then, label %if.end
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//
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// Or the following simpler case:
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// %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
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// br i1 %expval, label %if.then, label %if.end
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CallInst *CI;
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ICmpInst *CmpI = dyn_cast<ICmpInst>(BI.getCondition());
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if (!CmpI) {
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CI = dyn_cast<CallInst>(BI.getCondition());
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} else {
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if (CmpI->getPredicate() != CmpInst::ICMP_NE)
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return false;
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CI = dyn_cast<CallInst>(CmpI->getOperand(0));
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}
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2011-07-07 02:22:43 +08:00
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if (!CI)
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return false;
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Function *Fn = CI->getCalledFunction();
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if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
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return false;
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Value *ArgValue = CI->getArgOperand(0);
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ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
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if (!ExpectedValue)
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return false;
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2012-05-26 21:59:43 +08:00
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MDBuilder MDB(CI->getContext());
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MDNode *Node;
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2011-07-07 02:22:43 +08:00
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// If expect value is equal to 1 it means that we are more likely to take
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// branch 0, in other case more likely is branch 1.
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2012-05-26 21:59:43 +08:00
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if (ExpectedValue->isOne())
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Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
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else
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Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
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2011-07-07 02:22:43 +08:00
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2015-01-24 18:39:24 +08:00
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BI.setMetadata(LLVMContext::MD_prof, Node);
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2011-07-07 02:22:43 +08:00
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2014-02-03 06:43:55 +08:00
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if (CmpI)
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CmpI->setOperand(0, ArgValue);
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else
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BI.setCondition(ArgValue);
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2011-07-07 02:22:43 +08:00
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return true;
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}
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2015-01-24 19:13:02 +08:00
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static bool lowerExpectIntrinsic(Function &F) {
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2015-01-24 19:12:57 +08:00
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bool Changed = false;
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2015-01-24 18:57:19 +08:00
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for (BasicBlock &BB : F) {
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2011-07-07 02:22:43 +08:00
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// Create "block_weights" metadata.
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if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
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2015-01-24 18:39:24 +08:00
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if (handleBranchExpect(*BI))
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2015-01-24 18:57:25 +08:00
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ExpectIntrinsicsHandled++;
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2015-01-24 18:57:19 +08:00
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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
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if (handleSwitchExpect(*SI))
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2015-01-24 18:57:25 +08:00
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ExpectIntrinsicsHandled++;
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2011-07-07 02:22:43 +08:00
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}
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2015-08-25 04:11:14 +08:00
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// Remove llvm.expect intrinsics.
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2015-01-24 18:57:19 +08:00
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for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
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2011-07-07 02:22:43 +08:00
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CallInst *CI = dyn_cast<CallInst>(BI++);
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if (!CI)
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continue;
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Function *Fn = CI->getCalledFunction();
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2011-07-07 07:50:16 +08:00
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if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
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Value *Exp = CI->getArgOperand(0);
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CI->replaceAllUsesWith(Exp);
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2011-07-07 02:22:43 +08:00
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CI->eraseFromParent();
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2015-01-24 19:12:57 +08:00
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Changed = true;
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2011-07-07 07:50:16 +08:00
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}
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2011-07-07 02:22:43 +08:00
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}
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}
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2015-01-24 19:12:57 +08:00
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return Changed;
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2011-07-07 02:22:43 +08:00
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}
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2015-01-24 19:13:02 +08:00
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PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F) {
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if (lowerExpectIntrinsic(F))
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return PreservedAnalyses::none();
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return PreservedAnalyses::all();
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}
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namespace {
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/// \brief Legacy pass for lowering expect intrinsics out of the IR.
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///
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/// When this pass is run over a function it uses expect intrinsics which feed
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/// branches and switches to provide branch weight metadata for those
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/// terminators. It then removes the expect intrinsics from the IR so the rest
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/// of the optimizer can ignore them.
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class LowerExpectIntrinsic : public FunctionPass {
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public:
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static char ID;
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LowerExpectIntrinsic() : FunctionPass(ID) {
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initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
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};
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2015-06-23 17:49:53 +08:00
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}
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2015-01-24 19:13:02 +08:00
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2011-07-07 02:22:43 +08:00
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char LowerExpectIntrinsic::ID = 0;
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2015-01-24 18:30:14 +08:00
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INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
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"Lower 'expect' Intrinsics", false, false)
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2011-07-07 02:22:43 +08:00
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FunctionPass *llvm::createLowerExpectIntrinsicPass() {
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return new LowerExpectIntrinsic();
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}
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