LLVM 20.0.0git
Float2Int.cpp
Go to the documentation of this file.
1//===- Float2Int.cpp - Demote floating point ops to work on integers ------===//
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 implements the Float2Int pass, which aims to demote floating
10// point operations to work on integers, where that is losslessly possible.
11//
12//===----------------------------------------------------------------------===//
13
15#include "llvm/ADT/APInt.h"
16#include "llvm/ADT/APSInt.h"
19#include "llvm/IR/Constants.h"
20#include "llvm/IR/Dominators.h"
21#include "llvm/IR/IRBuilder.h"
22#include "llvm/IR/Module.h"
24#include "llvm/Support/Debug.h"
26#include <deque>
27
28#define DEBUG_TYPE "float2int"
29
30using namespace llvm;
31
32// The algorithm is simple. Start at instructions that convert from the
33// float to the int domain: fptoui, fptosi and fcmp. Walk up the def-use
34// graph, using an equivalence datastructure to unify graphs that interfere.
35//
36// Mappable instructions are those with an integer corrollary that, given
37// integer domain inputs, produce an integer output; fadd, for example.
38//
39// If a non-mappable instruction is seen, this entire def-use graph is marked
40// as non-transformable. If we see an instruction that converts from the
41// integer domain to FP domain (uitofp,sitofp), we terminate our walk.
42
43/// The largest integer type worth dealing with.
45MaxIntegerBW("float2int-max-integer-bw", cl::init(64), cl::Hidden,
46 cl::desc("Max integer bitwidth to consider in float2int"
47 "(default=64)"));
48
49// Given a FCmp predicate, return a matching ICmp predicate if one
50// exists, otherwise return BAD_ICMP_PREDICATE.
52 switch (P) {
55 return CmpInst::ICMP_EQ;
58 return CmpInst::ICMP_SGT;
61 return CmpInst::ICMP_SGE;
64 return CmpInst::ICMP_SLT;
67 return CmpInst::ICMP_SLE;
70 return CmpInst::ICMP_NE;
71 default:
73 }
74}
75
76// Given a floating point binary operator, return the matching
77// integer version.
78static Instruction::BinaryOps mapBinOpcode(unsigned Opcode) {
79 switch (Opcode) {
80 default: llvm_unreachable("Unhandled opcode!");
81 case Instruction::FAdd: return Instruction::Add;
82 case Instruction::FSub: return Instruction::Sub;
83 case Instruction::FMul: return Instruction::Mul;
84 }
85}
86
87// Find the roots - instructions that convert from the FP domain to
88// integer domain.
89void Float2IntPass::findRoots(Function &F, const DominatorTree &DT) {
90 for (BasicBlock &BB : F) {
91 // Unreachable code can take on strange forms that we are not prepared to
92 // handle. For example, an instruction may have itself as an operand.
93 if (!DT.isReachableFromEntry(&BB))
94 continue;
95
96 for (Instruction &I : BB) {
97 if (isa<VectorType>(I.getType()))
98 continue;
99 switch (I.getOpcode()) {
100 default: break;
101 case Instruction::FPToUI:
102 case Instruction::FPToSI:
103 Roots.insert(&I);
104 break;
105 case Instruction::FCmp:
106 if (mapFCmpPred(cast<CmpInst>(&I)->getPredicate()) !=
108 Roots.insert(&I);
109 break;
110 }
111 }
112 }
113}
114
115// Helper - mark I as having been traversed, having range R.
116void Float2IntPass::seen(Instruction *I, ConstantRange R) {
117 LLVM_DEBUG(dbgs() << "F2I: " << *I << ":" << R << "\n");
118 SeenInsts.insert_or_assign(I, std::move(R));
119}
120
121// Helper - get a range representing a poison value.
122ConstantRange Float2IntPass::badRange() {
123 return ConstantRange::getFull(MaxIntegerBW + 1);
124}
125ConstantRange Float2IntPass::unknownRange() {
126 return ConstantRange::getEmpty(MaxIntegerBW + 1);
127}
128ConstantRange Float2IntPass::validateRange(ConstantRange R) {
129 if (R.getBitWidth() > MaxIntegerBW + 1)
130 return badRange();
131 return R;
132}
133
134// The most obvious way to structure the search is a depth-first, eager
135// search from each root. However, that require direct recursion and so
136// can only handle small instruction sequences. Instead, we split the search
137// up into two phases:
138// - walkBackwards: A breadth-first walk of the use-def graph starting from
139// the roots. Populate "SeenInsts" with interesting
140// instructions and poison values if they're obvious and
141// cheap to compute. Calculate the equivalance set structure
142// while we're here too.
143// - walkForwards: Iterate over SeenInsts in reverse order, so we visit
144// defs before their uses. Calculate the real range info.
145
146// Breadth-first walk of the use-def graph; determine the set of nodes
147// we care about and eagerly determine if some of them are poisonous.
148void Float2IntPass::walkBackwards() {
149 std::deque<Instruction*> Worklist(Roots.begin(), Roots.end());
150 while (!Worklist.empty()) {
151 Instruction *I = Worklist.back();
152 Worklist.pop_back();
153
154 if (SeenInsts.contains(I))
155 // Seen already.
156 continue;
157
158 switch (I->getOpcode()) {
159 // FIXME: Handle select and phi nodes.
160 default:
161 // Path terminated uncleanly.
162 seen(I, badRange());
163 break;
164
165 case Instruction::UIToFP:
166 case Instruction::SIToFP: {
167 // Path terminated cleanly - use the type of the integer input to seed
168 // the analysis.
169 unsigned BW = I->getOperand(0)->getType()->getPrimitiveSizeInBits();
170 auto Input = ConstantRange::getFull(BW);
171 auto CastOp = (Instruction::CastOps)I->getOpcode();
172 seen(I, validateRange(Input.castOp(CastOp, MaxIntegerBW+1)));
173 continue;
174 }
175
176 case Instruction::FNeg:
177 case Instruction::FAdd:
178 case Instruction::FSub:
179 case Instruction::FMul:
180 case Instruction::FPToUI:
181 case Instruction::FPToSI:
182 case Instruction::FCmp:
183 seen(I, unknownRange());
184 break;
185 }
186
187 for (Value *O : I->operands()) {
188 if (Instruction *OI = dyn_cast<Instruction>(O)) {
189 // Unify def-use chains if they interfere.
190 ECs.unionSets(I, OI);
191 if (SeenInsts.find(I)->second != badRange())
192 Worklist.push_back(OI);
193 } else if (!isa<ConstantFP>(O)) {
194 // Not an instruction or ConstantFP? we can't do anything.
195 seen(I, badRange());
196 }
197 }
198 }
199}
200
201// Calculate result range from operand ranges.
202// Return std::nullopt if the range cannot be calculated yet.
203std::optional<ConstantRange> Float2IntPass::calcRange(Instruction *I) {
205 for (Value *O : I->operands()) {
206 if (Instruction *OI = dyn_cast<Instruction>(O)) {
207 auto OpIt = SeenInsts.find(OI);
208 assert(OpIt != SeenInsts.end() && "def not seen before use!");
209 if (OpIt->second == unknownRange())
210 return std::nullopt; // Wait until operand range has been calculated.
211 OpRanges.push_back(OpIt->second);
212 } else if (ConstantFP *CF = dyn_cast<ConstantFP>(O)) {
213 // Work out if the floating point number can be losslessly represented
214 // as an integer.
215 // APFloat::convertToInteger(&Exact) purports to do what we want, but
216 // the exactness can be too precise. For example, negative zero can
217 // never be exactly converted to an integer.
218 //
219 // Instead, we ask APFloat to round itself to an integral value - this
220 // preserves sign-of-zero - then compare the result with the original.
221 //
222 const APFloat &F = CF->getValueAPF();
223
224 // First, weed out obviously incorrect values. Non-finite numbers
225 // can't be represented and neither can negative zero, unless
226 // we're in fast math mode.
227 if (!F.isFinite() ||
228 (F.isZero() && F.isNegative() && isa<FPMathOperator>(I) &&
229 !I->hasNoSignedZeros()))
230 return badRange();
231
232 APFloat NewF = F;
234 if (Res != APFloat::opOK || NewF != F)
235 return badRange();
236
237 // OK, it's representable. Now get it.
238 APSInt Int(MaxIntegerBW+1, false);
239 bool Exact;
240 CF->getValueAPF().convertToInteger(Int,
242 &Exact);
243 OpRanges.push_back(ConstantRange(Int));
244 } else {
245 llvm_unreachable("Should have already marked this as badRange!");
246 }
247 }
248
249 switch (I->getOpcode()) {
250 // FIXME: Handle select and phi nodes.
251 default:
252 case Instruction::UIToFP:
253 case Instruction::SIToFP:
254 llvm_unreachable("Should have been handled in walkForwards!");
255
256 case Instruction::FNeg: {
257 assert(OpRanges.size() == 1 && "FNeg is a unary operator!");
258 unsigned Size = OpRanges[0].getBitWidth();
260 return Zero.sub(OpRanges[0]);
261 }
262
263 case Instruction::FAdd:
264 case Instruction::FSub:
265 case Instruction::FMul: {
266 assert(OpRanges.size() == 2 && "its a binary operator!");
267 auto BinOp = (Instruction::BinaryOps) I->getOpcode();
268 return OpRanges[0].binaryOp(BinOp, OpRanges[1]);
269 }
270
271 //
272 // Root-only instructions - we'll only see these if they're the
273 // first node in a walk.
274 //
275 case Instruction::FPToUI:
276 case Instruction::FPToSI: {
277 assert(OpRanges.size() == 1 && "FPTo[US]I is a unary operator!");
278 // Note: We're ignoring the casts output size here as that's what the
279 // caller expects.
280 auto CastOp = (Instruction::CastOps)I->getOpcode();
281 return OpRanges[0].castOp(CastOp, MaxIntegerBW+1);
282 }
283
284 case Instruction::FCmp:
285 assert(OpRanges.size() == 2 && "FCmp is a binary operator!");
286 return OpRanges[0].unionWith(OpRanges[1]);
287 }
288}
289
290// Walk forwards down the list of seen instructions, so we visit defs before
291// uses.
292void Float2IntPass::walkForwards() {
293 std::deque<Instruction *> Worklist;
294 for (const auto &Pair : SeenInsts)
295 if (Pair.second == unknownRange())
296 Worklist.push_back(Pair.first);
297
298 while (!Worklist.empty()) {
299 Instruction *I = Worklist.back();
300 Worklist.pop_back();
301
302 if (std::optional<ConstantRange> Range = calcRange(I))
303 seen(I, *Range);
304 else
305 Worklist.push_front(I); // Reprocess later.
306 }
307}
308
309// If there is a valid transform to be done, do it.
310bool Float2IntPass::validateAndTransform(const DataLayout &DL) {
311 bool MadeChange = false;
312
313 // Iterate over every disjoint partition of the def-use graph.
314 for (auto It = ECs.begin(), E = ECs.end(); It != E; ++It) {
315 ConstantRange R(MaxIntegerBW + 1, false);
316 bool Fail = false;
317 Type *ConvertedToTy = nullptr;
318
319 // For every member of the partition, union all the ranges together.
320 for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
321 MI != ME; ++MI) {
322 Instruction *I = *MI;
323 auto SeenI = SeenInsts.find(I);
324 if (SeenI == SeenInsts.end())
325 continue;
326
327 R = R.unionWith(SeenI->second);
328 // We need to ensure I has no users that have not been seen.
329 // If it does, transformation would be illegal.
330 //
331 // Don't count the roots, as they terminate the graphs.
332 if (!Roots.contains(I)) {
333 // Set the type of the conversion while we're here.
334 if (!ConvertedToTy)
335 ConvertedToTy = I->getType();
336 for (User *U : I->users()) {
337 Instruction *UI = dyn_cast<Instruction>(U);
338 if (!UI || !SeenInsts.contains(UI)) {
339 LLVM_DEBUG(dbgs() << "F2I: Failing because of " << *U << "\n");
340 Fail = true;
341 break;
342 }
343 }
344 }
345 if (Fail)
346 break;
347 }
348
349 // If the set was empty, or we failed, or the range is poisonous,
350 // bail out.
351 if (ECs.member_begin(It) == ECs.member_end() || Fail ||
352 R.isFullSet() || R.isSignWrappedSet())
353 continue;
354 assert(ConvertedToTy && "Must have set the convertedtoty by this point!");
355
356 // The number of bits required is the maximum of the upper and
357 // lower limits, plus one so it can be signed.
358 unsigned MinBW = R.getMinSignedBits() + 1;
359 LLVM_DEBUG(dbgs() << "F2I: MinBitwidth=" << MinBW << ", R: " << R << "\n");
360
361 // If we've run off the realms of the exactly representable integers,
362 // the floating point result will differ from an integer approximation.
363
364 // Do we need more bits than are in the mantissa of the type we converted
365 // to? semanticsPrecision returns the number of mantissa bits plus one
366 // for the sign bit.
367 unsigned MaxRepresentableBits
368 = APFloat::semanticsPrecision(ConvertedToTy->getFltSemantics()) - 1;
369 if (MinBW > MaxRepresentableBits) {
370 LLVM_DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
371 continue;
372 }
373
374 // OK, R is known to be representable.
375 // Pick the smallest legal type that will fit.
376 Type *Ty = DL.getSmallestLegalIntType(*Ctx, MinBW);
377 if (!Ty) {
378 // Every supported target supports 64-bit and 32-bit integers,
379 // so fallback to a 32 or 64-bit integer if the value fits.
380 if (MinBW <= 32) {
381 Ty = Type::getInt32Ty(*Ctx);
382 } else if (MinBW <= 64) {
383 Ty = Type::getInt64Ty(*Ctx);
384 } else {
385 LLVM_DEBUG(dbgs() << "F2I: Value requires more bits to represent than "
386 "the target supports!\n");
387 continue;
388 }
389 }
390
391 for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
392 MI != ME; ++MI)
393 convert(*MI, Ty);
394 MadeChange = true;
395 }
396
397 return MadeChange;
398}
399
400Value *Float2IntPass::convert(Instruction *I, Type *ToTy) {
401 if (auto It = ConvertedInsts.find(I); It != ConvertedInsts.end())
402 // Already converted this instruction.
403 return It->second;
404
405 SmallVector<Value*,4> NewOperands;
406 for (Value *V : I->operands()) {
407 // Don't recurse if we're an instruction that terminates the path.
408 if (I->getOpcode() == Instruction::UIToFP ||
409 I->getOpcode() == Instruction::SIToFP) {
410 NewOperands.push_back(V);
411 } else if (Instruction *VI = dyn_cast<Instruction>(V)) {
412 NewOperands.push_back(convert(VI, ToTy));
413 } else if (ConstantFP *CF = dyn_cast<ConstantFP>(V)) {
414 APSInt Val(ToTy->getPrimitiveSizeInBits(), /*isUnsigned=*/false);
415 bool Exact;
416 CF->getValueAPF().convertToInteger(Val,
418 &Exact);
419 NewOperands.push_back(ConstantInt::get(ToTy, Val));
420 } else {
421 llvm_unreachable("Unhandled operand type?");
422 }
423 }
424
425 // Now create a new instruction.
426 IRBuilder<> IRB(I);
427 Value *NewV = nullptr;
428 switch (I->getOpcode()) {
429 default: llvm_unreachable("Unhandled instruction!");
430
431 case Instruction::FPToUI:
432 NewV = IRB.CreateZExtOrTrunc(NewOperands[0], I->getType());
433 break;
434
435 case Instruction::FPToSI:
436 NewV = IRB.CreateSExtOrTrunc(NewOperands[0], I->getType());
437 break;
438
439 case Instruction::FCmp: {
440 CmpInst::Predicate P = mapFCmpPred(cast<CmpInst>(I)->getPredicate());
441 assert(P != CmpInst::BAD_ICMP_PREDICATE && "Unhandled predicate!");
442 NewV = IRB.CreateICmp(P, NewOperands[0], NewOperands[1], I->getName());
443 break;
444 }
445
446 case Instruction::UIToFP:
447 NewV = IRB.CreateZExtOrTrunc(NewOperands[0], ToTy);
448 break;
449
450 case Instruction::SIToFP:
451 NewV = IRB.CreateSExtOrTrunc(NewOperands[0], ToTy);
452 break;
453
454 case Instruction::FNeg:
455 NewV = IRB.CreateNeg(NewOperands[0], I->getName());
456 break;
457
458 case Instruction::FAdd:
459 case Instruction::FSub:
460 case Instruction::FMul:
461 NewV = IRB.CreateBinOp(mapBinOpcode(I->getOpcode()),
462 NewOperands[0], NewOperands[1],
463 I->getName());
464 break;
465 }
466
467 // If we're a root instruction, RAUW.
468 if (Roots.count(I))
469 I->replaceAllUsesWith(NewV);
470
471 ConvertedInsts[I] = NewV;
472 return NewV;
473}
474
475// Perform dead code elimination on the instructions we just modified.
476void Float2IntPass::cleanup() {
477 for (auto &I : reverse(ConvertedInsts))
478 I.first->eraseFromParent();
479}
480
482 LLVM_DEBUG(dbgs() << "F2I: Looking at function " << F.getName() << "\n");
483 // Clear out all state.
485 SeenInsts.clear();
486 ConvertedInsts.clear();
487 Roots.clear();
488
489 Ctx = &F.getParent()->getContext();
490
491 findRoots(F, DT);
492
493 walkBackwards();
494 walkForwards();
495
496 const DataLayout &DL = F.getDataLayout();
497 bool Modified = validateAndTransform(DL);
498 if (Modified)
499 cleanup();
500 return Modified;
501}
502
505 if (!runImpl(F, DT))
506 return PreservedAnalyses::all();
507
510 return PA;
511}
#define Fail
This file implements a class to represent arbitrary precision integral constant values and operations...
This file implements the APSInt class, which is a simple class that represents an arbitrary sized int...
MachineBasicBlock MachineBasicBlock::iterator DebugLoc DL
This file contains the declarations for the subclasses of Constant, which represent the different fla...
#define LLVM_DEBUG(...)
Definition: Debug.h:106
uint64_t Size
expand large fp convert
static CmpInst::Predicate mapFCmpPred(CmpInst::Predicate P)
Definition: Float2Int.cpp:51
static Instruction::BinaryOps mapBinOpcode(unsigned Opcode)
Definition: Float2Int.cpp:78
static cl::opt< unsigned > MaxIntegerBW("float2int-max-integer-bw", cl::init(64), cl::Hidden, cl::desc("Max integer bitwidth to consider in float2int" "(default=64)"))
The largest integer type worth dealing with.
This is the interface for a simple mod/ref and alias analysis over globals.
IRTranslator LLVM IR MI
Module.h This file contains the declarations for the Module class.
#define F(x, y, z)
Definition: MD5.cpp:55
#define I(x, y, z)
Definition: MD5.cpp:58
ConstantRange Range(APInt(BitWidth, Low), APInt(BitWidth, High))
#define P(N)
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
This file defines the SmallVector class.
opStatus roundToIntegral(roundingMode RM)
Definition: APFloat.h:1245
static APInt getZero(unsigned numBits)
Get the '0' value for the specified bit-width.
Definition: APInt.h:200
An arbitrary precision integer that knows its signedness.
Definition: APSInt.h:23
A container for analyses that lazily runs them and caches their results.
Definition: PassManager.h:253
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:410
LLVM Basic Block Representation.
Definition: BasicBlock.h:61
Represents analyses that only rely on functions' control flow.
Definition: Analysis.h:72
Predicate
This enumeration lists the possible predicates for CmpInst subclasses.
Definition: InstrTypes.h:673
@ FCMP_OEQ
0 0 0 1 True if ordered and equal
Definition: InstrTypes.h:676
@ ICMP_SLT
signed less than
Definition: InstrTypes.h:702
@ ICMP_SLE
signed less or equal
Definition: InstrTypes.h:703
@ FCMP_OLT
0 1 0 0 True if ordered and less than
Definition: InstrTypes.h:679
@ FCMP_ULE
1 1 0 1 True if unordered, less than, or equal
Definition: InstrTypes.h:688
@ FCMP_OGT
0 0 1 0 True if ordered and greater than
Definition: InstrTypes.h:677
@ FCMP_OGE
0 0 1 1 True if ordered and greater than or equal
Definition: InstrTypes.h:678
@ ICMP_SGT
signed greater than
Definition: InstrTypes.h:700
@ FCMP_ULT
1 1 0 0 True if unordered or less than
Definition: InstrTypes.h:687
@ FCMP_ONE
0 1 1 0 True if ordered and operands are unequal
Definition: InstrTypes.h:681
@ FCMP_UEQ
1 0 0 1 True if unordered or equal
Definition: InstrTypes.h:684
@ FCMP_UGT
1 0 1 0 True if unordered or greater than
Definition: InstrTypes.h:685
@ FCMP_OLE
0 1 0 1 True if ordered and less than or equal
Definition: InstrTypes.h:680
@ ICMP_EQ
equal
Definition: InstrTypes.h:694
@ ICMP_NE
not equal
Definition: InstrTypes.h:695
@ ICMP_SGE
signed greater or equal
Definition: InstrTypes.h:701
@ FCMP_UNE
1 1 1 0 True if unordered or not equal
Definition: InstrTypes.h:689
@ FCMP_UGE
1 0 1 1 True if unordered, greater than, or equal
Definition: InstrTypes.h:686
ConstantFP - Floating Point Values [float, double].
Definition: Constants.h:271
This class represents a range of values.
Definition: ConstantRange.h:47
A parsed version of the target data layout string in and methods for querying it.
Definition: DataLayout.h:63
Analysis pass which computes a DominatorTree.
Definition: Dominators.h:279
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree.
Definition: Dominators.h:162
bool isReachableFromEntry(const Use &U) const
Provide an overload for a Use.
Definition: Dominators.cpp:321
EquivalenceClasses - This represents a collection of equivalence classes and supports three efficient...
bool runImpl(Function &F, const DominatorTree &DT)
Definition: Float2Int.cpp:481
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
Definition: Float2Int.cpp:503
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2697
A set of analyses that are preserved following a run of a transformation pass.
Definition: Analysis.h:111
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: Analysis.h:117
void preserveSet()
Mark an analysis set as preserved.
Definition: Analysis.h:146
size_t size() const
Definition: SmallVector.h:78
void push_back(const T &Elt)
Definition: SmallVector.h:413
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1196
The instances of the Type class are immutable: once they are created, they are never changed.
Definition: Type.h:45
const fltSemantics & getFltSemantics() const
static IntegerType * getInt32Ty(LLVMContext &C)
static IntegerType * getInt64Ty(LLVMContext &C)
TypeSize getPrimitiveSizeInBits() const LLVM_READONLY
Return the basic size of this type if it is a primitive type.
LLVM Value Representation.
Definition: Value.h:74
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
Predicate getPredicate(unsigned Condition, unsigned Hint)
Return predicate consisting of specified condition and hint bits.
Definition: PPCPredicates.h:87
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:443
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
auto reverse(ContainerTy &&C)
Definition: STLExtras.h:420
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
static constexpr roundingMode rmNearestTiesToEven
Definition: APFloat.h:297
static unsigned int semanticsPrecision(const fltSemantics &)
Definition: APFloat.cpp:317