llvm/lib/IR/StructuralHash.cpp

   1 //===-- StructuralHash.cpp - IR Hashing -------------------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "llvm/IR/StructuralHash.h"
  10 #include "llvm/ADT/Hashing.h"
  11 #include "llvm/IR/Function.h"
  12 #include "llvm/IR/GlobalVariable.h"
  13 #include "llvm/IR/InstrTypes.h"
  14 #include "llvm/IR/Instructions.h"
  15 #include "llvm/IR/IntrinsicInst.h"
  16 #include "llvm/IR/Module.h"
  17
  18 using namespace llvm;
  19
  20 namespace {
  21
  22 // Basic hashing mechanism to detect structural change to the IR, used to verify
  23 // pass return status consistency with actual change. In addition to being used
  24 // by the MergeFunctions pass.
  25
  26 class StructuralHashImpl {
  27   uint64_t Hash;
  28
  29   void hash(uint64_t V) { Hash = hashing::detail::hash_16_bytes(Hash, V); }
  30
  31   // This will produce different values on 32-bit and 64-bit systens as
  32   // hash_combine returns a size_t. However, this is only used for
  33   // detailed hashing which, in-tree, only needs to distinguish between
  34   // differences in functions.
  35   template <typename T> void hashArbitaryType(const T &V) {
  36     hash(hash_combine(V));
  37   }
  38
  39   void hashType(Type *ValueType) {
  40     hash(ValueType->getTypeID());
  41     if (ValueType->isIntegerTy())
  42       hash(ValueType->getIntegerBitWidth());
  43   }
  44
  45 public:
  46   StructuralHashImpl() : Hash(4) {}
  47
  48   void updateOperand(Value *Operand) {
  49     hashType(Operand->getType());
  50
  51     // The cases enumerated below are not exhaustive and are only aimed to
  52     // get decent coverage over the function.
  53     if (ConstantInt *ConstInt = dyn_cast<ConstantInt>(Operand)) {
  54       hashArbitaryType(ConstInt->getValue());
  55     } else if (ConstantFP *ConstFP = dyn_cast<ConstantFP>(Operand)) {
  56       hashArbitaryType(ConstFP->getValue());
  57     } else if (Argument *Arg = dyn_cast<Argument>(Operand)) {
  58       hash(Arg->getArgNo());
  59     } else if (Function *Func = dyn_cast<Function>(Operand)) {
  60       // Hashing the name will be deterministic as LLVM's hashing infrastructure
  61       // has explicit support for hashing strings and will not simply hash
  62       // the pointer.
  63       hashArbitaryType(Func->getName());
  64     }
  65   }
  66
  67   void updateInstruction(const Instruction &Inst, bool DetailedHash) {
  68     hash(Inst.getOpcode());
  69
  70     if (!DetailedHash)
  71       return;
  72
  73     hashType(Inst.getType());
  74
  75     // Handle additional properties of specific instructions that cause
  76     // semantic differences in the IR.
  77     if (const auto *ComparisonInstruction = dyn_cast<CmpInst>(&Inst))
  78       hash(ComparisonInstruction->getPredicate());
  79
  80     for (const auto &Op : Inst.operands())
  81       updateOperand(Op);
  82   }
  83
  84   // A function hash is calculated by considering only the number of arguments
  85   // and whether a function is varargs, the order of basic blocks (given by the
  86   // successors of each basic block in depth first order), and the order of
  87   // opcodes of each instruction within each of these basic blocks. This mirrors
  88   // the strategy FunctionComparator::compare() uses to compare functions by
  89   // walking the BBs in depth first order and comparing each instruction in
  90   // sequence. Because this hash currently does not look at the operands, it is
  91   // insensitive to things such as the target of calls and the constants used in
  92   // the function, which makes it useful when possibly merging functions which
  93   // are the same modulo constants and call targets.
  94   //
  95   // Note that different users of StructuralHash will want different behavior
  96   // out of it (i.e., MergeFunctions will want something different from PM
  97   // expensive checks for pass modification status). When modifying this
  98   // function, most changes should be gated behind an option and enabled
  99   // selectively.
 100   void update(const Function &F, bool DetailedHash) {
 101     // Declarations don't affect analyses.
 102     if (F.isDeclaration())
 103       return;
 104
 105     hash(0x62642d6b6b2d6b72); // Function header
 106
 107     hash(F.isVarArg());
 108     hash(F.arg_size());
 109
 110     SmallVector<const BasicBlock *, 8> BBs;
 111     SmallPtrSet<const BasicBlock *, 16> VisitedBBs;
 112
 113     // Walk the blocks in the same order as
 114     // FunctionComparator::cmpBasicBlocks(), accumulating the hash of the
 115     // function "structure." (BB and opcode sequence)
 116     BBs.push_back(&F.getEntryBlock());
 117     VisitedBBs.insert(BBs[0]);
 118     while (!BBs.empty()) {
 119       const BasicBlock *BB = BBs.pop_back_val();
 120
 121       // This random value acts as a block header, as otherwise the partition of
 122       // opcodes into BBs wouldn't affect the hash, only the order of the
 123       // opcodes
 124       hash(45798);
 125       for (auto &Inst : *BB)
 126         updateInstruction(Inst, DetailedHash);
 127
 128       for (const BasicBlock *Succ : successors(BB))
 129         if (VisitedBBs.insert(Succ).second)
 130           BBs.push_back(Succ);
 131     }
 132   }
 133
 134   void update(const GlobalVariable &GV) {
 135     // Declarations and used/compiler.used don't affect analyses.
 136     // Since there are several `llvm.*` metadata, like `llvm.embedded.object`,
 137     // we ignore anything with the `.llvm` prefix
 138     if (GV.isDeclaration() || GV.getName().starts_with("llvm."))
 139       return;
 140     hash(23456); // Global header
 141     hash(GV.getValueType()->getTypeID());
 142   }
 143
 144   void update(const Module &M, bool DetailedHash) {
 145     for (const GlobalVariable &GV : M.globals())
 146       update(GV);
 147     for (const Function &F : M)
 148       update(F, DetailedHash);
 149   }
 150
 151   uint64_t getHash() const { return Hash; }
 152 };
 153
 154 } // namespace
 155
 156 IRHash llvm::StructuralHash(const Function &F, bool DetailedHash) {
 157   StructuralHashImpl H;
 158   H.update(F, DetailedHash);
 159   return H.getHash();
 160 }
 161
 162 IRHash llvm::StructuralHash(const Module &M, bool DetailedHash) {
 163   StructuralHashImpl H;
 164   H.update(M, DetailedHash);
 165   return H.getHash();
 166 }