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
14#include "llvm/Transforms/Scalar/Float2Int.h"
15#include "llvm/ADT/APInt.h"
16#include "llvm/ADT/APSInt.h"
17#include "llvm/ADT/SmallVector.h"
18#include "llvm/Analysis/GlobalsModRef.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"
23#include "llvm/Support/CommandLine.h"
24#include "llvm/Support/Debug.h"
25#include "llvm/Support/raw_ostream.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.
44static cl::opt<unsigned>
45MaxIntegerBW("float2int-max-integer-bw", cl::init(Val: 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.
51static CmpInst::Predicate mapFCmpPred(CmpInst::Predicate P) {
52 switch (P) {
53 case CmpInst::FCMP_OEQ:
54 case CmpInst::FCMP_UEQ:
55 return CmpInst::ICMP_EQ;
56 case CmpInst::FCMP_OGT:
57 case CmpInst::FCMP_UGT:
58 return CmpInst::ICMP_SGT;
59 case CmpInst::FCMP_OGE:
60 case CmpInst::FCMP_UGE:
61 return CmpInst::ICMP_SGE;
62 case CmpInst::FCMP_OLT:
63 case CmpInst::FCMP_ULT:
64 return CmpInst::ICMP_SLT;
65 case CmpInst::FCMP_OLE:
66 case CmpInst::FCMP_ULE:
67 return CmpInst::ICMP_SLE;
68 case CmpInst::FCMP_ONE:
69 case CmpInst::FCMP_UNE:
70 return CmpInst::ICMP_NE;
71 default:
72 return CmpInst::BAD_ICMP_PREDICATE;
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(A: &BB))
94 continue;
95
96 for (Instruction &I : BB) {
97 if (isa<VectorType>(Val: I.getType()))
98 continue;
99 switch (I.getOpcode()) {
100 default: break;
101 case Instruction::FPToUI:
102 case Instruction::FPToSI:
103 Roots.insert(X: &I);
104 break;
105 case Instruction::FCmp:
106 if (mapFCmpPred(P: cast<CmpInst>(Val: &I)->getPredicate()) !=
107 CmpInst::BAD_ICMP_PREDICATE)
108 Roots.insert(X: &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 auto IT = SeenInsts.find(Key: I);
119 if (IT != SeenInsts.end())
120 IT->second = std::move(R);
121 else
122 SeenInsts.insert(KV: std::make_pair(x&: I, y: std::move(R)));
123}
124
125// Helper - get a range representing a poison value.
126ConstantRange Float2IntPass::badRange() {
127 return ConstantRange::getFull(BitWidth: MaxIntegerBW + 1);
128}
129ConstantRange Float2IntPass::unknownRange() {
130 return ConstantRange::getEmpty(BitWidth: MaxIntegerBW + 1);
131}
132ConstantRange Float2IntPass::validateRange(ConstantRange R) {
133 if (R.getBitWidth() > MaxIntegerBW + 1)
134 return badRange();
135 return R;
136}
137
138// The most obvious way to structure the search is a depth-first, eager
139// search from each root. However, that require direct recursion and so
140// can only handle small instruction sequences. Instead, we split the search
141// up into two phases:
142// - walkBackwards: A breadth-first walk of the use-def graph starting from
143// the roots. Populate "SeenInsts" with interesting
144// instructions and poison values if they're obvious and
145// cheap to compute. Calculate the equivalance set structure
146// while we're here too.
147// - walkForwards: Iterate over SeenInsts in reverse order, so we visit
148// defs before their uses. Calculate the real range info.
149
150// Breadth-first walk of the use-def graph; determine the set of nodes
151// we care about and eagerly determine if some of them are poisonous.
152void Float2IntPass::walkBackwards() {
153 std::deque<Instruction*> Worklist(Roots.begin(), Roots.end());
154 while (!Worklist.empty()) {
155 Instruction *I = Worklist.back();
156 Worklist.pop_back();
157
158 if (SeenInsts.contains(Key: I))
159 // Seen already.
160 continue;
161
162 switch (I->getOpcode()) {
163 // FIXME: Handle select and phi nodes.
164 default:
165 // Path terminated uncleanly.
166 seen(I, R: badRange());
167 break;
168
169 case Instruction::UIToFP:
170 case Instruction::SIToFP: {
171 // Path terminated cleanly - use the type of the integer input to seed
172 // the analysis.
173 unsigned BW = I->getOperand(i: 0)->getType()->getPrimitiveSizeInBits();
174 auto Input = ConstantRange::getFull(BitWidth: BW);
175 auto CastOp = (Instruction::CastOps)I->getOpcode();
176 seen(I, R: validateRange(R: Input.castOp(CastOp, BitWidth: MaxIntegerBW+1)));
177 continue;
178 }
179
180 case Instruction::FNeg:
181 case Instruction::FAdd:
182 case Instruction::FSub:
183 case Instruction::FMul:
184 case Instruction::FPToUI:
185 case Instruction::FPToSI:
186 case Instruction::FCmp:
187 seen(I, R: unknownRange());
188 break;
189 }
190
191 for (Value *O : I->operands()) {
192 if (Instruction *OI = dyn_cast<Instruction>(Val: O)) {
193 // Unify def-use chains if they interfere.
194 ECs.unionSets(V1: I, V2: OI);
195 if (SeenInsts.find(Key: I)->second != badRange())
196 Worklist.push_back(x: OI);
197 } else if (!isa<ConstantFP>(Val: O)) {
198 // Not an instruction or ConstantFP? we can't do anything.
199 seen(I, R: badRange());
200 }
201 }
202 }
203}
204
205// Calculate result range from operand ranges.
206// Return std::nullopt if the range cannot be calculated yet.
207std::optional<ConstantRange> Float2IntPass::calcRange(Instruction *I) {
208 SmallVector<ConstantRange, 4> OpRanges;
209 for (Value *O : I->operands()) {
210 if (Instruction *OI = dyn_cast<Instruction>(Val: O)) {
211 auto OpIt = SeenInsts.find(Key: OI);
212 assert(OpIt != SeenInsts.end() && "def not seen before use!");
213 if (OpIt->second == unknownRange())
214 return std::nullopt; // Wait until operand range has been calculated.
215 OpRanges.push_back(Elt: OpIt->second);
216 } else if (ConstantFP *CF = dyn_cast<ConstantFP>(Val: O)) {
217 // Work out if the floating point number can be losslessly represented
218 // as an integer.
219 // APFloat::convertToInteger(&Exact) purports to do what we want, but
220 // the exactness can be too precise. For example, negative zero can
221 // never be exactly converted to an integer.
222 //
223 // Instead, we ask APFloat to round itself to an integral value - this
224 // preserves sign-of-zero - then compare the result with the original.
225 //
226 const APFloat &F = CF->getValueAPF();
227
228 // First, weed out obviously incorrect values. Non-finite numbers
229 // can't be represented and neither can negative zero, unless
230 // we're in fast math mode.
231 if (!F.isFinite() ||
232 (F.isZero() && F.isNegative() && isa<FPMathOperator>(Val: I) &&
233 !I->hasNoSignedZeros()))
234 return badRange();
235
236 APFloat NewF = F;
237 auto Res = NewF.roundToIntegral(RM: APFloat::rmNearestTiesToEven);
238 if (Res != APFloat::opOK || NewF != F)
239 return badRange();
240
241 // OK, it's representable. Now get it.
242 APSInt Int(MaxIntegerBW+1, false);
243 bool Exact;
244 CF->getValueAPF().convertToInteger(Result&: Int,
245 RM: APFloat::rmNearestTiesToEven,
246 IsExact: &Exact);
247 OpRanges.push_back(Elt: ConstantRange(Int));
248 } else {
249 llvm_unreachable("Should have already marked this as badRange!");
250 }
251 }
252
253 switch (I->getOpcode()) {
254 // FIXME: Handle select and phi nodes.
255 default:
256 case Instruction::UIToFP:
257 case Instruction::SIToFP:
258 llvm_unreachable("Should have been handled in walkForwards!");
259
260 case Instruction::FNeg: {
261 assert(OpRanges.size() == 1 && "FNeg is a unary operator!");
262 unsigned Size = OpRanges[0].getBitWidth();
263 auto Zero = ConstantRange(APInt::getZero(numBits: Size));
264 return Zero.sub(Other: OpRanges[0]);
265 }
266
267 case Instruction::FAdd:
268 case Instruction::FSub:
269 case Instruction::FMul: {
270 assert(OpRanges.size() == 2 && "its a binary operator!");
271 auto BinOp = (Instruction::BinaryOps) I->getOpcode();
272 return OpRanges[0].binaryOp(BinOp, Other: OpRanges[1]);
273 }
274
275 //
276 // Root-only instructions - we'll only see these if they're the
277 // first node in a walk.
278 //
279 case Instruction::FPToUI:
280 case Instruction::FPToSI: {
281 assert(OpRanges.size() == 1 && "FPTo[US]I is a unary operator!");
282 // Note: We're ignoring the casts output size here as that's what the
283 // caller expects.
284 auto CastOp = (Instruction::CastOps)I->getOpcode();
285 return OpRanges[0].castOp(CastOp, BitWidth: MaxIntegerBW+1);
286 }
287
288 case Instruction::FCmp:
289 assert(OpRanges.size() == 2 && "FCmp is a binary operator!");
290 return OpRanges[0].unionWith(CR: OpRanges[1]);
291 }
292}
293
294// Walk forwards down the list of seen instructions, so we visit defs before
295// uses.
296void Float2IntPass::walkForwards() {
297 std::deque<Instruction *> Worklist;
298 for (const auto &Pair : SeenInsts)
299 if (Pair.second == unknownRange())
300 Worklist.push_back(x: Pair.first);
301
302 while (!Worklist.empty()) {
303 Instruction *I = Worklist.back();
304 Worklist.pop_back();
305
306 if (std::optional<ConstantRange> Range = calcRange(I))
307 seen(I, R: *Range);
308 else
309 Worklist.push_front(x: I); // Reprocess later.
310 }
311}
312
313// If there is a valid transform to be done, do it.
314bool Float2IntPass::validateAndTransform(const DataLayout &DL) {
315 bool MadeChange = false;
316
317 // Iterate over every disjoint partition of the def-use graph.
318 for (auto It = ECs.begin(), E = ECs.end(); It != E; ++It) {
319 ConstantRange R(MaxIntegerBW + 1, false);
320 bool Fail = false;
321 Type *ConvertedToTy = nullptr;
322
323 // For every member of the partition, union all the ranges together.
324 for (auto MI = ECs.member_begin(I: It), ME = ECs.member_end();
325 MI != ME; ++MI) {
326 Instruction *I = *MI;
327 auto SeenI = SeenInsts.find(Key: I);
328 if (SeenI == SeenInsts.end())
329 continue;
330
331 R = R.unionWith(CR: SeenI->second);
332 // We need to ensure I has no users that have not been seen.
333 // If it does, transformation would be illegal.
334 //
335 // Don't count the roots, as they terminate the graphs.
336 if (!Roots.contains(key: I)) {
337 // Set the type of the conversion while we're here.
338 if (!ConvertedToTy)
339 ConvertedToTy = I->getType();
340 for (User *U : I->users()) {
341 Instruction *UI = dyn_cast<Instruction>(Val: U);
342 if (!UI || !SeenInsts.contains(Key: UI)) {
343 LLVM_DEBUG(dbgs() << "F2I: Failing because of " << *U << "\n");
344 Fail = true;
345 break;
346 }
347 }
348 }
349 if (Fail)
350 break;
351 }
352
353 // If the set was empty, or we failed, or the range is poisonous,
354 // bail out.
355 if (ECs.member_begin(I: It) == ECs.member_end() || Fail ||
356 R.isFullSet() || R.isSignWrappedSet())
357 continue;
358 assert(ConvertedToTy && "Must have set the convertedtoty by this point!");
359
360 // The number of bits required is the maximum of the upper and
361 // lower limits, plus one so it can be signed.
362 unsigned MinBW = R.getMinSignedBits() + 1;
363 LLVM_DEBUG(dbgs() << "F2I: MinBitwidth=" << MinBW << ", R: " << R << "\n");
364
365 // If we've run off the realms of the exactly representable integers,
366 // the floating point result will differ from an integer approximation.
367
368 // Do we need more bits than are in the mantissa of the type we converted
369 // to? semanticsPrecision returns the number of mantissa bits plus one
370 // for the sign bit.
371 unsigned MaxRepresentableBits
372 = APFloat::semanticsPrecision(ConvertedToTy->getFltSemantics()) - 1;
373 if (MinBW > MaxRepresentableBits) {
374 LLVM_DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
375 continue;
376 }
377
378 // OK, R is known to be representable.
379 // Pick the smallest legal type that will fit.
380 Type *Ty = DL.getSmallestLegalIntType(C&: *Ctx, Width: MinBW);
381 if (!Ty) {
382 // Every supported target supports 64-bit and 32-bit integers,
383 // so fallback to a 32 or 64-bit integer if the value fits.
384 if (MinBW <= 32) {
385 Ty = Type::getInt32Ty(C&: *Ctx);
386 } else if (MinBW <= 64) {
387 Ty = Type::getInt64Ty(C&: *Ctx);
388 } else {
389 LLVM_DEBUG(dbgs() << "F2I: Value requires more bits to represent than "
390 "the target supports!\n");
391 continue;
392 }
393 }
394
395 for (auto MI = ECs.member_begin(I: It), ME = ECs.member_end();
396 MI != ME; ++MI)
397 convert(I: *MI, ToTy: Ty);
398 MadeChange = true;
399 }
400
401 return MadeChange;
402}
403
404Value *Float2IntPass::convert(Instruction *I, Type *ToTy) {
405 if (ConvertedInsts.contains(Key: I))
406 // Already converted this instruction.
407 return ConvertedInsts[I];
408
409 SmallVector<Value*,4> NewOperands;
410 for (Value *V : I->operands()) {
411 // Don't recurse if we're an instruction that terminates the path.
412 if (I->getOpcode() == Instruction::UIToFP ||
413 I->getOpcode() == Instruction::SIToFP) {
414 NewOperands.push_back(Elt: V);
415 } else if (Instruction *VI = dyn_cast<Instruction>(Val: V)) {
416 NewOperands.push_back(Elt: convert(I: VI, ToTy));
417 } else if (ConstantFP *CF = dyn_cast<ConstantFP>(Val: V)) {
418 APSInt Val(ToTy->getPrimitiveSizeInBits(), /*isUnsigned=*/false);
419 bool Exact;
420 CF->getValueAPF().convertToInteger(Result&: Val,
421 RM: APFloat::rmNearestTiesToEven,
422 IsExact: &Exact);
423 NewOperands.push_back(Elt: ConstantInt::get(Ty: ToTy, V: Val));
424 } else {
425 llvm_unreachable("Unhandled operand type?");
426 }
427 }
428
429 // Now create a new instruction.
430 IRBuilder<> IRB(I);
431 Value *NewV = nullptr;
432 switch (I->getOpcode()) {
433 default: llvm_unreachable("Unhandled instruction!");
434
435 case Instruction::FPToUI:
436 NewV = IRB.CreateZExtOrTrunc(V: NewOperands[0], DestTy: I->getType());
437 break;
438
439 case Instruction::FPToSI:
440 NewV = IRB.CreateSExtOrTrunc(V: NewOperands[0], DestTy: I->getType());
441 break;
442
443 case Instruction::FCmp: {
444 CmpInst::Predicate P = mapFCmpPred(P: cast<CmpInst>(Val: I)->getPredicate());
445 assert(P != CmpInst::BAD_ICMP_PREDICATE && "Unhandled predicate!");
446 NewV = IRB.CreateICmp(P, LHS: NewOperands[0], RHS: NewOperands[1], Name: I->getName());
447 break;
448 }
449
450 case Instruction::UIToFP:
451 NewV = IRB.CreateZExtOrTrunc(V: NewOperands[0], DestTy: ToTy);
452 break;
453
454 case Instruction::SIToFP:
455 NewV = IRB.CreateSExtOrTrunc(V: NewOperands[0], DestTy: ToTy);
456 break;
457
458 case Instruction::FNeg:
459 NewV = IRB.CreateNeg(V: NewOperands[0], Name: I->getName());
460 break;
461
462 case Instruction::FAdd:
463 case Instruction::FSub:
464 case Instruction::FMul:
465 NewV = IRB.CreateBinOp(Opc: mapBinOpcode(Opcode: I->getOpcode()),
466 LHS: NewOperands[0], RHS: NewOperands[1],
467 Name: I->getName());
468 break;
469 }
470
471 // If we're a root instruction, RAUW.
472 if (Roots.count(key: I))
473 I->replaceAllUsesWith(V: NewV);
474
475 ConvertedInsts[I] = NewV;
476 return NewV;
477}
478
479// Perform dead code elimination on the instructions we just modified.
480void Float2IntPass::cleanup() {
481 for (auto &I : reverse(C&: ConvertedInsts))
482 I.first->eraseFromParent();
483}
484
485bool Float2IntPass::runImpl(Function &F, const DominatorTree &DT) {
486 LLVM_DEBUG(dbgs() << "F2I: Looking at function " << F.getName() << "\n");
487 // Clear out all state.
488 ECs = EquivalenceClasses<Instruction*>();
489 SeenInsts.clear();
490 ConvertedInsts.clear();
491 Roots.clear();
492
493 Ctx = &F.getParent()->getContext();
494
495 findRoots(F, DT);
496
497 walkBackwards();
498 walkForwards();
499
500 const DataLayout &DL = F.getDataLayout();
501 bool Modified = validateAndTransform(DL);
502 if (Modified)
503 cleanup();
504 return Modified;
505}
506
507PreservedAnalyses Float2IntPass::run(Function &F, FunctionAnalysisManager &AM) {
508 const DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
509 if (!runImpl(F, DT))
510 return PreservedAnalyses::all();
511
512 PreservedAnalyses PA;
513 PA.preserveSet<CFGAnalyses>();
514 return PA;
515}
516