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
28namespace llvm {
29
30class APFloat;
31class APInt;
32class BasicBlock;
33class Constant;
34class Function;
35class GlobalValue;
36class InlineAsm;
37class Instruction;
38class MDNode;
39class Type;
40class 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.
54class 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
68public:
69 GlobalNumberState() = default;
70
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
80 void erase(GlobalValue *Global) {
81 GlobalNumbers.erase(Global);
82 }
83
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).
93class FunctionComparator {
94public:
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
107protected:
108 /// Start the comparison.
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
330private:
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 CallBase &LCS, const CallBase &RCS) 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;
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