1 //===- CmpInstAnalysis.cpp - Utils to help fold compares ---------------===//
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 file holds routines to help analyse compare instructions
11 // and fold them into constants or other compare instructions
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Transforms/Utils/CmpInstAnalysis.h"
16 #include "llvm/IR/Constants.h"
17 #include "llvm/IR/Instructions.h"
18
19 using namespace llvm;
20
21 /// getICmpCode - Encode a icmp predicate into a three bit mask. These bits
22 /// are carefully arranged to allow folding of expressions such as:
23 ///
24 /// (A < B) | (A > B) --> (A != B)
25 ///
26 /// Note that this is only valid if the first and second predicates have the
27 /// same sign. Is illegal to do: (A u< B) | (A s> B)
28 ///
29 /// Three bits are used to represent the condition, as follows:
30 /// 0 A > B
31 /// 1 A == B
32 /// 2 A < B
33 ///
34 /// <=> Value Definition
35 /// 000 0 Always false
36 /// 001 1 A > B
37 /// 010 2 A == B
38 /// 011 3 A >= B
39 /// 100 4 A < B
40 /// 101 5 A != B
41 /// 110 6 A <= B
42 /// 111 7 Always true
43 ///
getICmpCode(const ICmpInst * ICI,bool InvertPred)44 unsigned llvm::getICmpCode(const ICmpInst *ICI, bool InvertPred) {
45 ICmpInst::Predicate Pred = InvertPred ? ICI->getInversePredicate()
46 : ICI->getPredicate();
47 switch (Pred) {
48 // False -> 0
49 case ICmpInst::ICMP_UGT: return 1; // 001
50 case ICmpInst::ICMP_SGT: return 1; // 001
51 case ICmpInst::ICMP_EQ: return 2; // 010
52 case ICmpInst::ICMP_UGE: return 3; // 011
53 case ICmpInst::ICMP_SGE: return 3; // 011
54 case ICmpInst::ICMP_ULT: return 4; // 100
55 case ICmpInst::ICMP_SLT: return 4; // 100
56 case ICmpInst::ICMP_NE: return 5; // 101
57 case ICmpInst::ICMP_ULE: return 6; // 110
58 case ICmpInst::ICMP_SLE: return 6; // 110
59 // True -> 7
60 default:
61 llvm_unreachable("Invalid ICmp predicate!");
62 }
63 }
64
65 /// getICmpValue - This is the complement of getICmpCode, which turns an
66 /// opcode and two operands into either a constant true or false, or the
67 /// predicate for a new ICmp instruction. The sign is passed in to determine
68 /// which kind of predicate to use in the new icmp instruction.
69 /// Non-NULL return value will be a true or false constant.
70 /// NULL return means a new ICmp is needed. The predicate for which is
71 /// output in NewICmpPred.
getICmpValue(bool Sign,unsigned Code,Value * LHS,Value * RHS,CmpInst::Predicate & NewICmpPred)72 Value *llvm::getICmpValue(bool Sign, unsigned Code, Value *LHS, Value *RHS,
73 CmpInst::Predicate &NewICmpPred) {
74 switch (Code) {
75 default: llvm_unreachable("Illegal ICmp code!");
76 case 0: // False.
77 return ConstantInt::get(CmpInst::makeCmpResultType(LHS->getType()), 0);
78 case 1: NewICmpPred = Sign ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT; break;
79 case 2: NewICmpPred = ICmpInst::ICMP_EQ; break;
80 case 3: NewICmpPred = Sign ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE; break;
81 case 4: NewICmpPred = Sign ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT; break;
82 case 5: NewICmpPred = ICmpInst::ICMP_NE; break;
83 case 6: NewICmpPred = Sign ? ICmpInst::ICMP_SLE : ICmpInst::ICMP_ULE; break;
84 case 7: // True.
85 return ConstantInt::get(CmpInst::makeCmpResultType(LHS->getType()), 1);
86 }
87 return nullptr;
88 }
89
90 /// PredicatesFoldable - Return true if both predicates match sign or if at
91 /// least one of them is an equality comparison (which is signless).
PredicatesFoldable(ICmpInst::Predicate p1,ICmpInst::Predicate p2)92 bool llvm::PredicatesFoldable(ICmpInst::Predicate p1, ICmpInst::Predicate p2) {
93 return (CmpInst::isSigned(p1) == CmpInst::isSigned(p2)) ||
94 (CmpInst::isSigned(p1) && ICmpInst::isEquality(p2)) ||
95 (CmpInst::isSigned(p2) && ICmpInst::isEquality(p1));
96 }
97