src/llvm/lib/Transforms/Utils/CmpInstAnalysis.cpp

   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 ///
  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 }