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