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      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/Constants.h"
     17 #include "llvm/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 ///
     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.
     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 NULL;
     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).
     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