1 //===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass lowers the 'expect' intrinsic to LLVM metadata.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
15 #include "llvm/ADT/SmallVector.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/IR/BasicBlock.h"
18 #include "llvm/IR/Constants.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/Intrinsics.h"
22 #include "llvm/IR/LLVMContext.h"
23 #include "llvm/IR/MDBuilder.h"
24 #include "llvm/IR/Metadata.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Transforms/Scalar.h"
29 
30 using namespace llvm;
31 
32 #define DEBUG_TYPE "lower-expect-intrinsic"
33 
34 STATISTIC(ExpectIntrinsicsHandled,
35           "Number of 'expect' intrinsic instructions handled");
36 
37 // These default values are chosen to represent an extremely skewed outcome for
38 // a condition, but they leave some room for interpretation by later passes.
39 //
40 // If the documentation for __builtin_expect() was made explicit that it should
41 // only be used in extreme cases, we could make this ratio higher. As it stands,
42 // programmers may be using __builtin_expect() / llvm.expect to annotate that a
43 // branch is likely or unlikely to be taken.
44 //
45 // There is a known dependency on this ratio in CodeGenPrepare when transforming
46 // 'select' instructions. It may be worthwhile to hoist these values to some
47 // shared space, so they can be used directly by other passes.
48 
49 static cl::opt<uint32_t> LikelyBranchWeight(
50     "likely-branch-weight", cl::Hidden, cl::init(2000),
51     cl::desc("Weight of the branch likely to be taken (default = 2000)"));
52 static cl::opt<uint32_t> UnlikelyBranchWeight(
53     "unlikely-branch-weight", cl::Hidden, cl::init(1),
54     cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
55 
56 static bool handleSwitchExpect(SwitchInst &SI) {
57   CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
58   if (!CI)
59     return false;
60 
61   Function *Fn = CI->getCalledFunction();
62   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
63     return false;
64 
65   Value *ArgValue = CI->getArgOperand(0);
66   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
67   if (!ExpectedValue)
68     return false;
69 
70   SwitchInst::CaseIt Case = SI.findCaseValue(ExpectedValue);
71   unsigned n = SI.getNumCases(); // +1 for default case.
72   SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
73 
74   if (Case == SI.case_default())
75     Weights[0] = LikelyBranchWeight;
76   else
77     Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
78 
79   SI.setMetadata(LLVMContext::MD_prof,
80                  MDBuilder(CI->getContext()).createBranchWeights(Weights));
81 
82   SI.setCondition(ArgValue);
83   return true;
84 }
85 
86 // Handle both BranchInst and SelectInst.
87 template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) {
88 
89   // Handle non-optimized IR code like:
90   //   %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
91   //   %tobool = icmp ne i64 %expval, 0
92   //   br i1 %tobool, label %if.then, label %if.end
93   //
94   // Or the following simpler case:
95   //   %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
96   //   br i1 %expval, label %if.then, label %if.end
97 
98   CallInst *CI;
99 
100   ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition());
101   if (!CmpI) {
102     CI = dyn_cast<CallInst>(BSI.getCondition());
103   } else {
104     if (CmpI->getPredicate() != CmpInst::ICMP_NE)
105       return false;
106     CI = dyn_cast<CallInst>(CmpI->getOperand(0));
107   }
108 
109   if (!CI)
110     return false;
111 
112   Function *Fn = CI->getCalledFunction();
113   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
114     return false;
115 
116   Value *ArgValue = CI->getArgOperand(0);
117   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
118   if (!ExpectedValue)
119     return false;
120 
121   MDBuilder MDB(CI->getContext());
122   MDNode *Node;
123 
124   // If expect value is equal to 1 it means that we are more likely to take
125   // branch 0, in other case more likely is branch 1.
126   if (ExpectedValue->isOne())
127     Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
128   else
129     Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
130 
131   BSI.setMetadata(LLVMContext::MD_prof, Node);
132 
133   if (CmpI)
134     CmpI->setOperand(0, ArgValue);
135   else
136     BSI.setCondition(ArgValue);
137   return true;
138 }
139 
140 static bool handleBranchExpect(BranchInst &BI) {
141   if (BI.isUnconditional())
142     return false;
143 
144   return handleBrSelExpect<BranchInst>(BI);
145 }
146 
147 static bool lowerExpectIntrinsic(Function &F) {
148   bool Changed = false;
149 
150   for (BasicBlock &BB : F) {
151     // Create "block_weights" metadata.
152     if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
153       if (handleBranchExpect(*BI))
154         ExpectIntrinsicsHandled++;
155     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
156       if (handleSwitchExpect(*SI))
157         ExpectIntrinsicsHandled++;
158     }
159 
160     // Remove llvm.expect intrinsics. Iterate backwards in order
161     // to process select instructions before the intrinsic gets
162     // removed.
163     for (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) {
164       Instruction *Inst = &*BI++;
165       CallInst *CI = dyn_cast<CallInst>(Inst);
166       if (!CI) {
167         if (SelectInst *SI = dyn_cast<SelectInst>(Inst)) {
168           if (handleBrSelExpect(*SI))
169             ExpectIntrinsicsHandled++;
170         }
171         continue;
172       }
173 
174       Function *Fn = CI->getCalledFunction();
175       if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
176         Value *Exp = CI->getArgOperand(0);
177         CI->replaceAllUsesWith(Exp);
178         CI->eraseFromParent();
179         Changed = true;
180       }
181     }
182   }
183 
184   return Changed;
185 }
186 
187 PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
188                                                 FunctionAnalysisManager &) {
189   if (lowerExpectIntrinsic(F))
190     return PreservedAnalyses::none();
191 
192   return PreservedAnalyses::all();
193 }
194 
195 namespace {
196 /// \brief Legacy pass for lowering expect intrinsics out of the IR.
197 ///
198 /// When this pass is run over a function it uses expect intrinsics which feed
199 /// branches and switches to provide branch weight metadata for those
200 /// terminators. It then removes the expect intrinsics from the IR so the rest
201 /// of the optimizer can ignore them.
202 class LowerExpectIntrinsic : public FunctionPass {
203 public:
204   static char ID;
205   LowerExpectIntrinsic() : FunctionPass(ID) {
206     initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
207   }
208 
209   bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
210 };
211 }
212 
213 char LowerExpectIntrinsic::ID = 0;
214 INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
215                 "Lower 'expect' Intrinsics", false, false)
216 
217 FunctionPass *llvm::createLowerExpectIntrinsicPass() {
218   return new LowerExpectIntrinsic();
219 }
220