1 //===- FunctionComparator.h - Function Comparator ---------------*- 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 // This file defines the FunctionComparator and GlobalNumberState classes which 10 // are used by the MergeFunctions pass for comparing functions. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #ifndef LLVM_TRANSFORMS_UTILS_FUNCTIONCOMPARATOR_H 15 #define LLVM_TRANSFORMS_UTILS_FUNCTIONCOMPARATOR_H 16 17 #include "llvm/ADT/DenseMap.h" 18 #include "llvm/ADT/StringRef.h" 19 #include "llvm/IR/Attributes.h" 20 #include "llvm/IR/Instructions.h" 21 #include "llvm/IR/Operator.h" 22 #include "llvm/IR/ValueMap.h" 23 #include "llvm/Support/AtomicOrdering.h" 24 #include "llvm/Support/Casting.h" 25 #include <cstdint> 26 #include <tuple> 27 28 namespace llvm { 29 30 class APFloat; 31 class APInt; 32 class BasicBlock; 33 class Constant; 34 class Function; 35 class GlobalValue; 36 class InlineAsm; 37 class Instruction; 38 class MDNode; 39 class Type; 40 class Value; 41 42 /// GlobalNumberState assigns an integer to each global value in the program, 43 /// which is used by the comparison routine to order references to globals. This 44 /// state must be preserved throughout the pass, because Functions and other 45 /// globals need to maintain their relative order. Globals are assigned a number 46 /// when they are first visited. This order is deterministic, and so the 47 /// assigned numbers are as well. When two functions are merged, neither number 48 /// is updated. If the symbols are weak, this would be incorrect. If they are 49 /// strong, then one will be replaced at all references to the other, and so 50 /// direct callsites will now see one or the other symbol, and no update is 51 /// necessary. Note that if we were guaranteed unique names, we could just 52 /// compare those, but this would not work for stripped bitcodes or for those 53 /// few symbols without a name. 54 class GlobalNumberState { 55 struct Config : ValueMapConfig<GlobalValue *> { 56 enum { FollowRAUW = false }; 57 }; 58 59 // Each GlobalValue is mapped to an identifier. The Config ensures when RAUW 60 // occurs, the mapping does not change. Tracking changes is unnecessary, and 61 // also problematic for weak symbols (which may be overwritten). 62 using ValueNumberMap = ValueMap<GlobalValue *, uint64_t, Config>; 63 ValueNumberMap GlobalNumbers; 64 65 // The next unused serial number to assign to a global. 66 uint64_t NextNumber = 0; 67 68 public: 69 GlobalNumberState() = default; 70 getNumber(GlobalValue * Global)71 uint64_t getNumber(GlobalValue* Global) { 72 ValueNumberMap::iterator MapIter; 73 bool Inserted; 74 std::tie(MapIter, Inserted) = GlobalNumbers.insert({Global, NextNumber}); 75 if (Inserted) 76 NextNumber++; 77 return MapIter->second; 78 } 79 erase(GlobalValue * Global)80 void erase(GlobalValue *Global) { 81 GlobalNumbers.erase(Global); 82 } 83 clear()84 void clear() { 85 GlobalNumbers.clear(); 86 } 87 }; 88 89 /// FunctionComparator - Compares two functions to determine whether or not 90 /// they will generate machine code with the same behaviour. DataLayout is 91 /// used if available. The comparator always fails conservatively (erring on the 92 /// side of claiming that two functions are different). 93 class FunctionComparator { 94 public: FunctionComparator(const Function * F1,const Function * F2,GlobalNumberState * GN)95 FunctionComparator(const Function *F1, const Function *F2, 96 GlobalNumberState* GN) 97 : FnL(F1), FnR(F2), GlobalNumbers(GN) {} 98 99 /// Test whether the two functions have equivalent behaviour. 100 int compare(); 101 102 /// Hash a function. Equivalent functions will have the same hash, and unequal 103 /// functions will have different hashes with high probability. 104 using FunctionHash = uint64_t; 105 static FunctionHash functionHash(Function &); 106 107 protected: 108 /// Start the comparison. beginCompare()109 void beginCompare() { 110 sn_mapL.clear(); 111 sn_mapR.clear(); 112 } 113 114 /// Compares the signature and other general attributes of the two functions. 115 int compareSignature() const; 116 117 /// Test whether two basic blocks have equivalent behaviour. 118 int cmpBasicBlocks(const BasicBlock *BBL, const BasicBlock *BBR) const; 119 120 /// Constants comparison. 121 /// Its analog to lexicographical comparison between hypothetical numbers 122 /// of next format: 123 /// <bitcastability-trait><raw-bit-contents> 124 /// 125 /// 1. Bitcastability. 126 /// Check whether L's type could be losslessly bitcasted to R's type. 127 /// On this stage method, in case when lossless bitcast is not possible 128 /// method returns -1 or 1, thus also defining which type is greater in 129 /// context of bitcastability. 130 /// Stage 0: If types are equal in terms of cmpTypes, then we can go straight 131 /// to the contents comparison. 132 /// If types differ, remember types comparison result and check 133 /// whether we still can bitcast types. 134 /// Stage 1: Types that satisfies isFirstClassType conditions are always 135 /// greater then others. 136 /// Stage 2: Vector is greater then non-vector. 137 /// If both types are vectors, then vector with greater bitwidth is 138 /// greater. 139 /// If both types are vectors with the same bitwidth, then types 140 /// are bitcastable, and we can skip other stages, and go to contents 141 /// comparison. 142 /// Stage 3: Pointer types are greater than non-pointers. If both types are 143 /// pointers of the same address space - go to contents comparison. 144 /// Different address spaces: pointer with greater address space is 145 /// greater. 146 /// Stage 4: Types are neither vectors, nor pointers. And they differ. 147 /// We don't know how to bitcast them. So, we better don't do it, 148 /// and return types comparison result (so it determines the 149 /// relationship among constants we don't know how to bitcast). 150 /// 151 /// Just for clearance, let's see how the set of constants could look 152 /// on single dimension axis: 153 /// 154 /// [NFCT], [FCT, "others"], [FCT, pointers], [FCT, vectors] 155 /// Where: NFCT - Not a FirstClassType 156 /// FCT - FirstClassTyp: 157 /// 158 /// 2. Compare raw contents. 159 /// It ignores types on this stage and only compares bits from L and R. 160 /// Returns 0, if L and R has equivalent contents. 161 /// -1 or 1 if values are different. 162 /// Pretty trivial: 163 /// 2.1. If contents are numbers, compare numbers. 164 /// Ints with greater bitwidth are greater. Ints with same bitwidths 165 /// compared by their contents. 166 /// 2.2. "And so on". Just to avoid discrepancies with comments 167 /// perhaps it would be better to read the implementation itself. 168 /// 3. And again about overall picture. Let's look back at how the ordered set 169 /// of constants will look like: 170 /// [NFCT], [FCT, "others"], [FCT, pointers], [FCT, vectors] 171 /// 172 /// Now look, what could be inside [FCT, "others"], for example: 173 /// [FCT, "others"] = 174 /// [ 175 /// [double 0.1], [double 1.23], 176 /// [i32 1], [i32 2], 177 /// { double 1.0 }, ; StructTyID, NumElements = 1 178 /// { i32 1 }, ; StructTyID, NumElements = 1 179 /// { double 1, i32 1 }, ; StructTyID, NumElements = 2 180 /// { i32 1, double 1 } ; StructTyID, NumElements = 2 181 /// ] 182 /// 183 /// Let's explain the order. Float numbers will be less than integers, just 184 /// because of cmpType terms: FloatTyID < IntegerTyID. 185 /// Floats (with same fltSemantics) are sorted according to their value. 186 /// Then you can see integers, and they are, like a floats, 187 /// could be easy sorted among each others. 188 /// The structures. Structures are grouped at the tail, again because of their 189 /// TypeID: StructTyID > IntegerTyID > FloatTyID. 190 /// Structures with greater number of elements are greater. Structures with 191 /// greater elements going first are greater. 192 /// The same logic with vectors, arrays and other possible complex types. 193 /// 194 /// Bitcastable constants. 195 /// Let's assume, that some constant, belongs to some group of 196 /// "so-called-equal" values with different types, and at the same time 197 /// belongs to another group of constants with equal types 198 /// and "really" equal values. 199 /// 200 /// Now, prove that this is impossible: 201 /// 202 /// If constant A with type TyA is bitcastable to B with type TyB, then: 203 /// 1. All constants with equal types to TyA, are bitcastable to B. Since 204 /// those should be vectors (if TyA is vector), pointers 205 /// (if TyA is pointer), or else (if TyA equal to TyB), those types should 206 /// be equal to TyB. 207 /// 2. All constants with non-equal, but bitcastable types to TyA, are 208 /// bitcastable to B. 209 /// Once again, just because we allow it to vectors and pointers only. 210 /// This statement could be expanded as below: 211 /// 2.1. All vectors with equal bitwidth to vector A, has equal bitwidth to 212 /// vector B, and thus bitcastable to B as well. 213 /// 2.2. All pointers of the same address space, no matter what they point to, 214 /// bitcastable. So if C is pointer, it could be bitcasted to A and to B. 215 /// So any constant equal or bitcastable to A is equal or bitcastable to B. 216 /// QED. 217 /// 218 /// In another words, for pointers and vectors, we ignore top-level type and 219 /// look at their particular properties (bit-width for vectors, and 220 /// address space for pointers). 221 /// If these properties are equal - compare their contents. 222 int cmpConstants(const Constant *L, const Constant *R) const; 223 224 /// Compares two global values by number. Uses the GlobalNumbersState to 225 /// identify the same gobals across function calls. 226 int cmpGlobalValues(GlobalValue *L, GlobalValue *R) const; 227 228 /// Assign or look up previously assigned numbers for the two values, and 229 /// return whether the numbers are equal. Numbers are assigned in the order 230 /// visited. 231 /// Comparison order: 232 /// Stage 0: Value that is function itself is always greater then others. 233 /// If left and right values are references to their functions, then 234 /// they are equal. 235 /// Stage 1: Constants are greater than non-constants. 236 /// If both left and right are constants, then the result of 237 /// cmpConstants is used as cmpValues result. 238 /// Stage 2: InlineAsm instances are greater than others. If both left and 239 /// right are InlineAsm instances, InlineAsm* pointers casted to 240 /// integers and compared as numbers. 241 /// Stage 3: For all other cases we compare order we meet these values in 242 /// their functions. If right value was met first during scanning, 243 /// then left value is greater. 244 /// In another words, we compare serial numbers, for more details 245 /// see comments for sn_mapL and sn_mapR. 246 int cmpValues(const Value *L, const Value *R) const; 247 248 /// Compare two Instructions for equivalence, similar to 249 /// Instruction::isSameOperationAs. 250 /// 251 /// Stages are listed in "most significant stage first" order: 252 /// On each stage below, we do comparison between some left and right 253 /// operation parts. If parts are non-equal, we assign parts comparison 254 /// result to the operation comparison result and exit from method. 255 /// Otherwise we proceed to the next stage. 256 /// Stages: 257 /// 1. Operations opcodes. Compared as numbers. 258 /// 2. Number of operands. 259 /// 3. Operation types. Compared with cmpType method. 260 /// 4. Compare operation subclass optional data as stream of bytes: 261 /// just convert it to integers and call cmpNumbers. 262 /// 5. Compare in operation operand types with cmpType in 263 /// most significant operand first order. 264 /// 6. Last stage. Check operations for some specific attributes. 265 /// For example, for Load it would be: 266 /// 6.1.Load: volatile (as boolean flag) 267 /// 6.2.Load: alignment (as integer numbers) 268 /// 6.3.Load: ordering (as underlying enum class value) 269 /// 6.4.Load: synch-scope (as integer numbers) 270 /// 6.5.Load: range metadata (as integer ranges) 271 /// On this stage its better to see the code, since its not more than 10-15 272 /// strings for particular instruction, and could change sometimes. 273 /// 274 /// Sets \p needToCmpOperands to true if the operands of the instructions 275 /// still must be compared afterwards. In this case it's already guaranteed 276 /// that both instructions have the same number of operands. 277 int cmpOperations(const Instruction *L, const Instruction *R, 278 bool &needToCmpOperands) const; 279 280 /// cmpType - compares two types, 281 /// defines total ordering among the types set. 282 /// 283 /// Return values: 284 /// 0 if types are equal, 285 /// -1 if Left is less than Right, 286 /// +1 if Left is greater than Right. 287 /// 288 /// Description: 289 /// Comparison is broken onto stages. Like in lexicographical comparison 290 /// stage coming first has higher priority. 291 /// On each explanation stage keep in mind total ordering properties. 292 /// 293 /// 0. Before comparison we coerce pointer types of 0 address space to 294 /// integer. 295 /// We also don't bother with same type at left and right, so 296 /// just return 0 in this case. 297 /// 298 /// 1. If types are of different kind (different type IDs). 299 /// Return result of type IDs comparison, treating them as numbers. 300 /// 2. If types are integers, check that they have the same width. If they 301 /// are vectors, check that they have the same count and subtype. 302 /// 3. Types have the same ID, so check whether they are one of: 303 /// * Void 304 /// * Float 305 /// * Double 306 /// * X86_FP80 307 /// * FP128 308 /// * PPC_FP128 309 /// * Label 310 /// * Metadata 311 /// We can treat these types as equal whenever their IDs are same. 312 /// 4. If Left and Right are pointers, return result of address space 313 /// comparison (numbers comparison). We can treat pointer types of same 314 /// address space as equal. 315 /// 5. If types are complex. 316 /// Then both Left and Right are to be expanded and their element types will 317 /// be checked with the same way. If we get Res != 0 on some stage, return it. 318 /// Otherwise return 0. 319 /// 6. For all other cases put llvm_unreachable. 320 int cmpTypes(Type *TyL, Type *TyR) const; 321 322 int cmpNumbers(uint64_t L, uint64_t R) const; 323 int cmpAPInts(const APInt &L, const APInt &R) const; 324 int cmpAPFloats(const APFloat &L, const APFloat &R) const; 325 int cmpMem(StringRef L, StringRef R) const; 326 327 // The two functions undergoing comparison. 328 const Function *FnL, *FnR; 329 330 private: 331 int cmpOrderings(AtomicOrdering L, AtomicOrdering R) const; 332 int cmpInlineAsm(const InlineAsm *L, const InlineAsm *R) const; 333 int cmpAttrs(const AttributeList L, const AttributeList R) const; 334 int cmpRangeMetadata(const MDNode *L, const MDNode *R) const; 335 int cmpOperandBundlesSchema(const Instruction *L, const Instruction *R) const; 336 337 /// Compare two GEPs for equivalent pointer arithmetic. 338 /// Parts to be compared for each comparison stage, 339 /// most significant stage first: 340 /// 1. Address space. As numbers. 341 /// 2. Constant offset, (using GEPOperator::accumulateConstantOffset method). 342 /// 3. Pointer operand type (using cmpType method). 343 /// 4. Number of operands. 344 /// 5. Compare operands, using cmpValues method. 345 int cmpGEPs(const GEPOperator *GEPL, const GEPOperator *GEPR) const; cmpGEPs(const GetElementPtrInst * GEPL,const GetElementPtrInst * GEPR)346 int cmpGEPs(const GetElementPtrInst *GEPL, 347 const GetElementPtrInst *GEPR) const { 348 return cmpGEPs(cast<GEPOperator>(GEPL), cast<GEPOperator>(GEPR)); 349 } 350 351 /// Assign serial numbers to values from left function, and values from 352 /// right function. 353 /// Explanation: 354 /// Being comparing functions we need to compare values we meet at left and 355 /// right sides. 356 /// Its easy to sort things out for external values. It just should be 357 /// the same value at left and right. 358 /// But for local values (those were introduced inside function body) 359 /// we have to ensure they were introduced at exactly the same place, 360 /// and plays the same role. 361 /// Let's assign serial number to each value when we meet it first time. 362 /// Values that were met at same place will be with same serial numbers. 363 /// In this case it would be good to explain few points about values assigned 364 /// to BBs and other ways of implementation (see below). 365 /// 366 /// 1. Safety of BB reordering. 367 /// It's safe to change the order of BasicBlocks in function. 368 /// Relationship with other functions and serial numbering will not be 369 /// changed in this case. 370 /// As follows from FunctionComparator::compare(), we do CFG walk: we start 371 /// from the entry, and then take each terminator. So it doesn't matter how in 372 /// fact BBs are ordered in function. And since cmpValues are called during 373 /// this walk, the numbering depends only on how BBs located inside the CFG. 374 /// So the answer is - yes. We will get the same numbering. 375 /// 376 /// 2. Impossibility to use dominance properties of values. 377 /// If we compare two instruction operands: first is usage of local 378 /// variable AL from function FL, and second is usage of local variable AR 379 /// from FR, we could compare their origins and check whether they are 380 /// defined at the same place. 381 /// But, we are still not able to compare operands of PHI nodes, since those 382 /// could be operands from further BBs we didn't scan yet. 383 /// So it's impossible to use dominance properties in general. 384 mutable DenseMap<const Value*, int> sn_mapL, sn_mapR; 385 386 // The global state we will use 387 GlobalNumberState* GlobalNumbers; 388 }; 389 390 } // end namespace llvm 391 392 #endif // LLVM_TRANSFORMS_UTILS_FUNCTIONCOMPARATOR_H 393