1//===------ BPFPreserveStaticOffset.cpp -----------------------------------===//
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// TLDR: replaces llvm.preserve.static.offset + GEP + load / store
10// with llvm.bpf.getelementptr.and.load / store
11//
12// This file implements BPFPreserveStaticOffsetPass transformation.
13// This transformation address two BPF verifier specific issues:
14//
15// (a) Access to the fields of some structural types is allowed only
16// using load and store instructions with static immediate offsets.
17//
18// Examples of such types are `struct __sk_buff` and `struct
19// bpf_sock_ops`. This is so because offsets of the fields of
20// these structures do not match real offsets in the running
21// kernel. During BPF program load LDX and STX instructions
22// referring to the fields of these types are rewritten so that
23// offsets match real offsets. For this rewrite to happen field
24// offsets have to be encoded as immediate operands of the
25// instructions.
26//
27// See kernel/bpf/verifier.c:convert_ctx_access function in the
28// Linux kernel source tree for details.
29//
30// (b) Pointers to context parameters of BPF programs must not be
31// modified before access.
32//
33// During BPF program verification a tag PTR_TO_CTX is tracked for
34// register values. In case if register with such tag is modified
35// BPF program is not allowed to read or write memory using this
36// register. See kernel/bpf/verifier.c:check_mem_access function
37// in the Linux kernel source tree for details.
38//
39// The following sequence of the IR instructions:
40//
41// %x = getelementptr %ptr, %constant_offset
42// %y = load %x
43//
44// Is translated as a single machine instruction:
45//
46// LDW %ptr, %constant_offset
47//
48// In order for cases (a) and (b) to work the sequence %x-%y above has
49// to be preserved by the IR passes.
50//
51// However, several optimization passes might sink `load` instruction
52// or hoist `getelementptr` instruction so that the instructions are
53// no longer in sequence. Examples of such passes are:
54// SimplifyCFGPass, InstCombinePass, GVNPass.
55// After such modification the verifier would reject the BPF program.
56//
57// To avoid this issue the patterns like (load/store (getelementptr ...))
58// are replaced by calls to BPF specific intrinsic functions:
59// - llvm.bpf.getelementptr.and.load
60// - llvm.bpf.getelementptr.and.store
61//
62// These calls are lowered back to (load/store (getelementptr ...))
63// by BPFCheckAndAdjustIR pass right before the translation from IR to
64// machine instructions.
65//
66// The transformation is split into the following steps:
67// - When IR is generated from AST the calls to intrinsic function
68// llvm.preserve.static.offset are inserted.
69// - BPFPreserveStaticOffsetPass is executed as early as possible
70// with AllowPatial set to true, this handles marked GEP chains
71// with constant offsets.
72// - BPFPreserveStaticOffsetPass is executed at ScalarOptimizerLateEPCallback
73// with AllowPatial set to false, this handles marked GEP chains
74// with offsets that became constant after loop unrolling, e.g.
75// to handle the following code:
76//
77// struct context { int x[4]; } __attribute__((preserve_static_offset));
78//
79// struct context *ctx = ...;
80// #pragma clang loop unroll(full)
81// for (int i = 0; i < 4; ++i)
82// foo(ctx->x[i]);
83//
84// The early BPFPreserveStaticOffsetPass run is necessary to allow
85// additional GVN / CSE opportunities after functions inlining.
86// The relative order of optimization applied to function:
87// - early stage (1)
88// - ...
89// - function inlining (2)
90// - ...
91// - loop unrolling
92// - ...
93// - ScalarOptimizerLateEPCallback (3)
94//
95// When function A is inlined into function B all optimizations for A
96// are already done, while some passes remain for B. In case if
97// BPFPreserveStaticOffsetPass is done at (3) but not done at (1)
98// the code after (2) would contain a mix of
99// (load (gep %p)) and (get.and.load %p) usages:
100// - the (load (gep %p)) would come from the calling function;
101// - the (get.and.load %p) would come from the callee function.
102// Thus clobbering CSE / GVN passes done after inlining.
103
104#include "BPF.h"
105#include "BPFCORE.h"
106#include "llvm/ADT/SmallPtrSet.h"
107#include "llvm/ADT/SmallVector.h"
108#include "llvm/IR/Argument.h"
109#include "llvm/IR/Attributes.h"
110#include "llvm/IR/BasicBlock.h"
111#include "llvm/IR/Constants.h"
112#include "llvm/IR/DebugInfoMetadata.h"
113#include "llvm/IR/DiagnosticInfo.h"
114#include "llvm/IR/IRBuilder.h"
115#include "llvm/IR/InstIterator.h"
116#include "llvm/IR/Instructions.h"
117#include "llvm/IR/Intrinsics.h"
118#include "llvm/IR/IntrinsicsBPF.h"
119#include "llvm/Support/Debug.h"
120#include "llvm/Support/ErrorHandling.h"
121
122#define DEBUG_TYPE "bpf-preserve-static-offset"
123
124using namespace llvm;
125
126static const unsigned GepAndLoadFirstIdxArg = 6;
127static const unsigned GepAndStoreFirstIdxArg = 7;
128
129static bool isIntrinsicCall(Value *I, Intrinsic::ID Id) {
130 if (auto *Call = dyn_cast<CallInst>(Val: I))
131 if (Function *Func = Call->getCalledFunction())
132 return Func->getIntrinsicID() == Id;
133 return false;
134}
135
136static bool isPreserveStaticOffsetCall(Value *I) {
137 return isIntrinsicCall(I, Intrinsic::preserve_static_offset);
138}
139
140static CallInst *isGEPAndLoad(Value *I) {
141 if (isIntrinsicCall(I, Intrinsic::bpf_getelementptr_and_load))
142 return cast<CallInst>(Val: I);
143 return nullptr;
144}
145
146static CallInst *isGEPAndStore(Value *I) {
147 if (isIntrinsicCall(I, Intrinsic::bpf_getelementptr_and_store))
148 return cast<CallInst>(Val: I);
149 return nullptr;
150}
151
152template <class T = Instruction>
153static DILocation *mergeDILocations(SmallVector<T *> &Insns) {
154 DILocation *Merged = (*Insns.begin())->getDebugLoc();
155 for (T *I : Insns)
156 Merged = DILocation::getMergedLocation(LocA: Merged, LocB: I->getDebugLoc());
157 return Merged;
158}
159
160static CallInst *makeIntrinsicCall(Module *M,
161 Intrinsic::BPFIntrinsics Intrinsic,
162 ArrayRef<Type *> Types,
163 ArrayRef<Value *> Args) {
164
165 Function *Fn = Intrinsic::getDeclaration(M, id: Intrinsic, Tys: Types);
166 return CallInst::Create(Func: Fn, Args);
167}
168
169static void setParamElementType(CallInst *Call, unsigned ArgNo, Type *Type) {
170 LLVMContext &C = Call->getContext();
171 Call->addParamAttr(ArgNo, Attribute::get(C, Attribute::ElementType, Type));
172}
173
174static void setParamReadNone(CallInst *Call, unsigned ArgNo) {
175 LLVMContext &C = Call->getContext();
176 Call->addParamAttr(ArgNo, Attribute::get(C, Attribute::ReadNone));
177}
178
179static void setParamReadOnly(CallInst *Call, unsigned ArgNo) {
180 LLVMContext &C = Call->getContext();
181 Call->addParamAttr(ArgNo, Attribute::get(C, Attribute::ReadOnly));
182}
183
184static void setParamWriteOnly(CallInst *Call, unsigned ArgNo) {
185 LLVMContext &C = Call->getContext();
186 Call->addParamAttr(ArgNo, Attribute::get(C, Attribute::WriteOnly));
187}
188
189namespace {
190struct GEPChainInfo {
191 bool InBounds;
192 Type *SourceElementType;
193 SmallVector<Value *> Indices;
194 SmallVector<GetElementPtrInst *> Members;
195
196 GEPChainInfo() { reset(); }
197
198 void reset() {
199 InBounds = true;
200 SourceElementType = nullptr;
201 Indices.clear();
202 Members.clear();
203 }
204};
205} // Anonymous namespace
206
207template <class T = std::disjunction<LoadInst, StoreInst>>
208static void fillCommonArgs(LLVMContext &C, SmallVector<Value *> &Args,
209 GEPChainInfo &GEP, T *Insn) {
210 Type *Int8Ty = Type::getInt8Ty(C);
211 Type *Int1Ty = Type::getInt1Ty(C);
212 // Implementation of Align guarantees that ShiftValue < 64
213 unsigned AlignShiftValue = Log2_64(Insn->getAlign().value());
214 Args.push_back(Elt: GEP.Members[0]->getPointerOperand());
215 Args.push_back(Elt: ConstantInt::get(Int1Ty, Insn->isVolatile()));
216 Args.push_back(Elt: ConstantInt::get(Ty: Int8Ty, V: (unsigned)Insn->getOrdering()));
217 Args.push_back(Elt: ConstantInt::get(Ty: Int8Ty, V: (unsigned)Insn->getSyncScopeID()));
218 Args.push_back(Elt: ConstantInt::get(Ty: Int8Ty, V: AlignShiftValue));
219 Args.push_back(Elt: ConstantInt::get(Ty: Int1Ty, V: GEP.InBounds));
220 Args.append(in_start: GEP.Indices.begin(), in_end: GEP.Indices.end());
221}
222
223static Instruction *makeGEPAndLoad(Module *M, GEPChainInfo &GEP,
224 LoadInst *Load) {
225 SmallVector<Value *> Args;
226 fillCommonArgs(C&: M->getContext(), Args, GEP, Insn: Load);
227 CallInst *Call = makeIntrinsicCall(M, Intrinsic::bpf_getelementptr_and_load,
228 {Load->getType()}, Args);
229 setParamElementType(Call, ArgNo: 0, Type: GEP.SourceElementType);
230 Call->applyMergedLocation(LocA: mergeDILocations(Insns&: GEP.Members), LocB: Load->getDebugLoc());
231 Call->setName((*GEP.Members.rbegin())->getName());
232 if (Load->isUnordered()) {
233 Call->setOnlyReadsMemory();
234 Call->setOnlyAccessesArgMemory();
235 setParamReadOnly(Call, ArgNo: 0);
236 }
237 for (unsigned I = GepAndLoadFirstIdxArg; I < Args.size(); ++I)
238 Call->addParamAttr(I, Attribute::ImmArg);
239 Call->setAAMetadata(Load->getAAMetadata());
240 return Call;
241}
242
243static Instruction *makeGEPAndStore(Module *M, GEPChainInfo &GEP,
244 StoreInst *Store) {
245 SmallVector<Value *> Args;
246 Args.push_back(Elt: Store->getValueOperand());
247 fillCommonArgs(C&: M->getContext(), Args, GEP, Insn: Store);
248 CallInst *Call =
249 makeIntrinsicCall(M, Intrinsic::bpf_getelementptr_and_store,
250 {Store->getValueOperand()->getType()}, Args);
251 setParamElementType(Call, ArgNo: 1, Type: GEP.SourceElementType);
252 if (Store->getValueOperand()->getType()->isPointerTy())
253 setParamReadNone(Call, ArgNo: 0);
254 Call->applyMergedLocation(LocA: mergeDILocations(Insns&: GEP.Members),
255 LocB: Store->getDebugLoc());
256 if (Store->isUnordered()) {
257 Call->setOnlyWritesMemory();
258 Call->setOnlyAccessesArgMemory();
259 setParamWriteOnly(Call, ArgNo: 1);
260 }
261 for (unsigned I = GepAndStoreFirstIdxArg; I < Args.size(); ++I)
262 Call->addParamAttr(I, Attribute::ImmArg);
263 Call->setAAMetadata(Store->getAAMetadata());
264 return Call;
265}
266
267static unsigned getOperandAsUnsigned(CallInst *Call, unsigned ArgNo) {
268 if (auto *Int = dyn_cast<ConstantInt>(Val: Call->getOperand(i_nocapture: ArgNo)))
269 return Int->getValue().getZExtValue();
270 std::string Report;
271 raw_string_ostream ReportS(Report);
272 ReportS << "Expecting ConstantInt as argument #" << ArgNo << " of " << *Call
273 << "\n";
274 report_fatal_error(reason: StringRef(Report));
275}
276
277static GetElementPtrInst *reconstructGEP(CallInst *Call, int Delta) {
278 SmallVector<Value *> Indices;
279 Indices.append(in_start: Call->data_operands_begin() + 6 + Delta,
280 in_end: Call->data_operands_end());
281 Type *GEPPointeeType = Call->getParamElementType(ArgNo: Delta);
282 auto *GEP =
283 GetElementPtrInst::Create(PointeeType: GEPPointeeType, Ptr: Call->getOperand(i_nocapture: Delta),
284 IdxList: ArrayRef<Value *>(Indices), NameStr: Call->getName());
285 GEP->setIsInBounds(getOperandAsUnsigned(Call, ArgNo: 5 + Delta));
286 return GEP;
287}
288
289template <class T = std::disjunction<LoadInst, StoreInst>>
290static void reconstructCommon(CallInst *Call, GetElementPtrInst *GEP, T *Insn,
291 int Delta) {
292 Insn->setVolatile(getOperandAsUnsigned(Call, ArgNo: 1 + Delta));
293 Insn->setOrdering((AtomicOrdering)getOperandAsUnsigned(Call, ArgNo: 2 + Delta));
294 Insn->setSyncScopeID(getOperandAsUnsigned(Call, ArgNo: 3 + Delta));
295 unsigned AlignShiftValue = getOperandAsUnsigned(Call, ArgNo: 4 + Delta);
296 Insn->setAlignment(Align(1ULL << AlignShiftValue));
297 GEP->setDebugLoc(Call->getDebugLoc());
298 Insn->setDebugLoc(Call->getDebugLoc());
299 Insn->setAAMetadata(Call->getAAMetadata());
300}
301
302std::pair<GetElementPtrInst *, LoadInst *>
303BPFPreserveStaticOffsetPass::reconstructLoad(CallInst *Call) {
304 GetElementPtrInst *GEP = reconstructGEP(Call, Delta: 0);
305 Type *ReturnType = Call->getFunctionType()->getReturnType();
306 auto *Load = new LoadInst(ReturnType, GEP, "",
307 /* These would be set in reconstructCommon */
308 false, Align(1));
309 reconstructCommon(Call, GEP, Insn: Load, Delta: 0);
310 return std::pair{GEP, Load};
311}
312
313std::pair<GetElementPtrInst *, StoreInst *>
314BPFPreserveStaticOffsetPass::reconstructStore(CallInst *Call) {
315 GetElementPtrInst *GEP = reconstructGEP(Call, Delta: 1);
316 auto *Store = new StoreInst(Call->getOperand(i_nocapture: 0), GEP,
317 /* These would be set in reconstructCommon */
318 false, Align(1));
319 reconstructCommon(Call, GEP, Insn: Store, Delta: 1);
320 return std::pair{GEP, Store};
321}
322
323static bool isZero(Value *V) {
324 auto *CI = dyn_cast<ConstantInt>(Val: V);
325 return CI && CI->isZero();
326}
327
328// Given a chain of GEP instructions collect information necessary to
329// merge this chain as a single GEP instruction of form:
330// getelementptr %<type>, ptr %p, i32 0, <field_idx1>, <field_idx2>, ...
331static bool foldGEPChainAsStructAccess(SmallVector<GetElementPtrInst *> &GEPs,
332 GEPChainInfo &Info) {
333 if (GEPs.empty())
334 return false;
335
336 if (!all_of(Range&: GEPs, P: [=](GetElementPtrInst *GEP) {
337 return GEP->hasAllConstantIndices();
338 }))
339 return false;
340
341 GetElementPtrInst *First = GEPs[0];
342 Info.InBounds = First->isInBounds();
343 Info.SourceElementType = First->getSourceElementType();
344 Type *ResultElementType = First->getResultElementType();
345 Info.Indices.append(in_start: First->idx_begin(), in_end: First->idx_end());
346 Info.Members.push_back(Elt: First);
347
348 for (auto *Iter = GEPs.begin() + 1; Iter != GEPs.end(); ++Iter) {
349 GetElementPtrInst *GEP = *Iter;
350 if (!isZero(V: *GEP->idx_begin())) {
351 Info.reset();
352 return false;
353 }
354 if (!GEP->getSourceElementType() ||
355 GEP->getSourceElementType() != ResultElementType) {
356 Info.reset();
357 return false;
358 }
359 Info.InBounds &= GEP->isInBounds();
360 Info.Indices.append(in_start: GEP->idx_begin() + 1, in_end: GEP->idx_end());
361 Info.Members.push_back(Elt: GEP);
362 ResultElementType = GEP->getResultElementType();
363 }
364
365 return true;
366}
367
368// Given a chain of GEP instructions collect information necessary to
369// merge this chain as a single GEP instruction of form:
370// getelementptr i8, ptr %p, i64 %offset
371static bool foldGEPChainAsU8Access(SmallVector<GetElementPtrInst *> &GEPs,
372 GEPChainInfo &Info) {
373 if (GEPs.empty())
374 return false;
375
376 GetElementPtrInst *First = GEPs[0];
377 const DataLayout &DL = First->getModule()->getDataLayout();
378 LLVMContext &C = First->getContext();
379 Type *PtrTy = First->getType()->getScalarType();
380 APInt Offset(DL.getIndexTypeSizeInBits(Ty: PtrTy), 0);
381 for (GetElementPtrInst *GEP : GEPs) {
382 if (!GEP->accumulateConstantOffset(DL, Offset)) {
383 Info.reset();
384 return false;
385 }
386 Info.InBounds &= GEP->isInBounds();
387 Info.Members.push_back(Elt: GEP);
388 }
389 Info.SourceElementType = Type::getInt8Ty(C);
390 Info.Indices.push_back(Elt: ConstantInt::get(Context&: C, V: Offset));
391
392 return true;
393}
394
395static void reportNonStaticGEPChain(Instruction *Insn) {
396 auto Msg = DiagnosticInfoUnsupported(
397 *Insn->getFunction(),
398 Twine("Non-constant offset in access to a field of a type marked "
399 "with preserve_static_offset might be rejected by BPF verifier")
400 .concat(Suffix: Insn->getDebugLoc()
401 ? ""
402 : " (pass -g option to get exact location)"),
403 Insn->getDebugLoc(), DS_Warning);
404 Insn->getContext().diagnose(DI: Msg);
405}
406
407static bool allZeroIndices(SmallVector<GetElementPtrInst *> &GEPs) {
408 return GEPs.empty() || all_of(Range&: GEPs, P: [=](GetElementPtrInst *GEP) {
409 return GEP->hasAllZeroIndices();
410 });
411}
412
413static bool tryToReplaceWithGEPBuiltin(Instruction *LoadOrStoreTemplate,
414 SmallVector<GetElementPtrInst *> &GEPs,
415 Instruction *InsnToReplace) {
416 GEPChainInfo GEPChain;
417 if (!foldGEPChainAsStructAccess(GEPs, Info&: GEPChain) &&
418 !foldGEPChainAsU8Access(GEPs, Info&: GEPChain)) {
419 return false;
420 }
421 Module *M = InsnToReplace->getModule();
422 if (auto *Load = dyn_cast<LoadInst>(Val: LoadOrStoreTemplate)) {
423 Instruction *Replacement = makeGEPAndLoad(M, GEP&: GEPChain, Load);
424 Replacement->insertBefore(InsertPos: InsnToReplace);
425 InsnToReplace->replaceAllUsesWith(V: Replacement);
426 }
427 if (auto *Store = dyn_cast<StoreInst>(Val: LoadOrStoreTemplate)) {
428 Instruction *Replacement = makeGEPAndStore(M, GEP&: GEPChain, Store);
429 Replacement->insertBefore(InsertPos: InsnToReplace);
430 }
431 return true;
432}
433
434// Check if U->getPointerOperand() == I
435static bool isPointerOperand(Value *I, User *U) {
436 if (auto *L = dyn_cast<LoadInst>(Val: U))
437 return L->getPointerOperand() == I;
438 if (auto *S = dyn_cast<StoreInst>(Val: U))
439 return S->getPointerOperand() == I;
440 if (auto *GEP = dyn_cast<GetElementPtrInst>(Val: U))
441 return GEP->getPointerOperand() == I;
442 if (auto *Call = isGEPAndLoad(I: U))
443 return Call->getArgOperand(i: 0) == I;
444 if (auto *Call = isGEPAndStore(I: U))
445 return Call->getArgOperand(i: 1) == I;
446 return false;
447}
448
449static bool isInlineableCall(User *U) {
450 if (auto *Call = dyn_cast<CallInst>(Val: U))
451 return Call->hasFnAttr(Attribute::InlineHint);
452 return false;
453}
454
455static void rewriteAccessChain(Instruction *Insn,
456 SmallVector<GetElementPtrInst *> &GEPs,
457 SmallVector<Instruction *> &Visited,
458 bool AllowPatial, bool &StillUsed);
459
460static void rewriteUses(Instruction *Insn,
461 SmallVector<GetElementPtrInst *> &GEPs,
462 SmallVector<Instruction *> &Visited, bool AllowPatial,
463 bool &StillUsed) {
464 for (User *U : Insn->users()) {
465 auto *UI = dyn_cast<Instruction>(Val: U);
466 if (UI && (isPointerOperand(I: Insn, U: UI) || isPreserveStaticOffsetCall(I: UI) ||
467 isInlineableCall(U: UI)))
468 rewriteAccessChain(Insn: UI, GEPs, Visited, AllowPatial, StillUsed);
469 else
470 LLVM_DEBUG({
471 llvm::dbgs() << "unsupported usage in BPFPreserveStaticOffsetPass:\n";
472 llvm::dbgs() << " Insn: " << *Insn << "\n";
473 llvm::dbgs() << " User: " << *U << "\n";
474 });
475 }
476}
477
478// A DFS traversal of GEP chain trees starting from Root.
479//
480// Recursion descends through GEP instructions and
481// llvm.preserve.static.offset calls. Recursion stops at any other
482// instruction. If load or store instruction is reached it is replaced
483// by a call to `llvm.bpf.getelementptr.and.load` or
484// `llvm.bpf.getelementptr.and.store` intrinsic.
485// If `llvm.bpf.getelementptr.and.load/store` is reached the accumulated
486// GEPs are merged into the intrinsic call.
487// If nested calls to `llvm.preserve.static.offset` are encountered these
488// calls are marked for deletion.
489//
490// Parameters description:
491// - Insn - current position in the tree
492// - GEPs - GEP instructions for the current branch
493// - Visited - a list of visited instructions in DFS order,
494// order is important for unused instruction deletion.
495// - AllowPartial - when true GEP chains that can't be folded are
496// not reported, otherwise diagnostic message is show for such chains.
497// - StillUsed - set to true if one of the GEP chains could not be
498// folded, makes sense when AllowPartial is false, means that root
499// preserve.static.offset call is still in use and should remain
500// until the next run of this pass.
501static void rewriteAccessChain(Instruction *Insn,
502 SmallVector<GetElementPtrInst *> &GEPs,
503 SmallVector<Instruction *> &Visited,
504 bool AllowPatial, bool &StillUsed) {
505 auto MarkAndTraverseUses = [&]() {
506 Visited.push_back(Elt: Insn);
507 rewriteUses(Insn, GEPs, Visited, AllowPatial, StillUsed);
508 };
509 auto TryToReplace = [&](Instruction *LoadOrStore) {
510 // Do nothing for (preserve.static.offset (load/store ..)) or for
511 // GEPs with zero indices. Such constructs lead to zero offset and
512 // are simplified by other passes.
513 if (allZeroIndices(GEPs))
514 return;
515 if (tryToReplaceWithGEPBuiltin(LoadOrStoreTemplate: LoadOrStore, GEPs, InsnToReplace: Insn)) {
516 Visited.push_back(Elt: Insn);
517 return;
518 }
519 if (!AllowPatial)
520 reportNonStaticGEPChain(Insn);
521 StillUsed = true;
522 };
523 if (isa<LoadInst>(Val: Insn) || isa<StoreInst>(Val: Insn)) {
524 TryToReplace(Insn);
525 } else if (isGEPAndLoad(I: Insn)) {
526 auto [GEP, Load] =
527 BPFPreserveStaticOffsetPass::reconstructLoad(Call: cast<CallInst>(Val: Insn));
528 GEPs.push_back(Elt: GEP);
529 TryToReplace(Load);
530 GEPs.pop_back();
531 delete Load;
532 delete GEP;
533 } else if (isGEPAndStore(I: Insn)) {
534 // This case can't be merged with the above because
535 // `delete Load` / `delete Store` wants a concrete type,
536 // destructor of Instruction is protected.
537 auto [GEP, Store] =
538 BPFPreserveStaticOffsetPass::reconstructStore(Call: cast<CallInst>(Val: Insn));
539 GEPs.push_back(Elt: GEP);
540 TryToReplace(Store);
541 GEPs.pop_back();
542 delete Store;
543 delete GEP;
544 } else if (auto *GEP = dyn_cast<GetElementPtrInst>(Val: Insn)) {
545 GEPs.push_back(Elt: GEP);
546 MarkAndTraverseUses();
547 GEPs.pop_back();
548 } else if (isPreserveStaticOffsetCall(I: Insn)) {
549 MarkAndTraverseUses();
550 } else if (isInlineableCall(U: Insn)) {
551 // Preserve preserve.static.offset call for parameters of
552 // functions that might be inlined. These would be removed on a
553 // second pass after inlining.
554 // Might happen when a pointer to a preserve_static_offset
555 // structure is passed as parameter of a function that would be
556 // inlined inside a loop that would be unrolled.
557 if (AllowPatial)
558 StillUsed = true;
559 } else {
560 SmallString<128> Buf;
561 raw_svector_ostream BufStream(Buf);
562 BufStream << *Insn;
563 report_fatal_error(
564 reason: Twine("Unexpected rewriteAccessChain Insn = ").concat(Suffix: Buf));
565 }
566}
567
568static void removeMarkerCall(Instruction *Marker) {
569 Marker->replaceAllUsesWith(V: Marker->getOperand(i: 0));
570 Marker->eraseFromParent();
571}
572
573static bool rewriteAccessChain(Instruction *Marker, bool AllowPatial,
574 SmallPtrSetImpl<Instruction *> &RemovedMarkers) {
575 SmallVector<GetElementPtrInst *> GEPs;
576 SmallVector<Instruction *> Visited;
577 bool StillUsed = false;
578 rewriteUses(Insn: Marker, GEPs, Visited, AllowPatial, StillUsed);
579 // Check if Visited instructions could be removed, iterate in
580 // reverse to unblock instructions higher in the chain.
581 for (auto V = Visited.rbegin(); V != Visited.rend(); ++V) {
582 if (isPreserveStaticOffsetCall(I: *V)) {
583 removeMarkerCall(Marker: *V);
584 RemovedMarkers.insert(Ptr: *V);
585 } else if ((*V)->use_empty()) {
586 (*V)->eraseFromParent();
587 }
588 }
589 return StillUsed;
590}
591
592static std::vector<Instruction *>
593collectPreserveStaticOffsetCalls(Function &F) {
594 std::vector<Instruction *> Calls;
595 for (Instruction &Insn : instructions(F))
596 if (isPreserveStaticOffsetCall(I: &Insn))
597 Calls.push_back(x: &Insn);
598 return Calls;
599}
600
601bool isPreserveArrayIndex(Value *V) {
602 return isIntrinsicCall(V, Intrinsic::preserve_array_access_index);
603}
604
605bool isPreserveStructIndex(Value *V) {
606 return isIntrinsicCall(V, Intrinsic::preserve_struct_access_index);
607}
608
609bool isPreserveUnionIndex(Value *V) {
610 return isIntrinsicCall(V, Intrinsic::preserve_union_access_index);
611}
612
613static void removePAICalls(Instruction *Marker) {
614 auto IsPointerOperand = [](Value *Op, User *U) {
615 if (auto *GEP = dyn_cast<GetElementPtrInst>(Val: U))
616 return GEP->getPointerOperand() == Op;
617 if (isPreserveStaticOffsetCall(I: U) || isPreserveArrayIndex(V: U) ||
618 isPreserveStructIndex(V: U) || isPreserveUnionIndex(V: U))
619 return cast<CallInst>(Val: U)->getArgOperand(i: 0) == Op;
620 return false;
621 };
622
623 SmallVector<Value *, 32> WorkList;
624 WorkList.push_back(Elt: Marker);
625 do {
626 Value *V = WorkList.pop_back_val();
627 for (User *U : V->users())
628 if (IsPointerOperand(V, U))
629 WorkList.push_back(Elt: U);
630 auto *Call = dyn_cast<CallInst>(Val: V);
631 if (!Call)
632 continue;
633 if (isPreserveArrayIndex(V))
634 BPFCoreSharedInfo::removeArrayAccessCall(Call);
635 else if (isPreserveStructIndex(V))
636 BPFCoreSharedInfo::removeStructAccessCall(Call);
637 else if (isPreserveUnionIndex(V))
638 BPFCoreSharedInfo::removeUnionAccessCall(Call);
639 } while (!WorkList.empty());
640}
641
642// Look for sequences:
643// - llvm.preserve.static.offset -> getelementptr... -> load
644// - llvm.preserve.static.offset -> getelementptr... -> store
645// And replace those with calls to intrinsics:
646// - llvm.bpf.getelementptr.and.load
647// - llvm.bpf.getelementptr.and.store
648static bool rewriteFunction(Function &F, bool AllowPartial) {
649 LLVM_DEBUG(dbgs() << "********** BPFPreserveStaticOffsetPass (AllowPartial="
650 << AllowPartial << ") ************\n");
651
652 auto MarkerCalls = collectPreserveStaticOffsetCalls(F);
653 SmallPtrSet<Instruction *, 16> RemovedMarkers;
654
655 LLVM_DEBUG(dbgs() << "There are " << MarkerCalls.size()
656 << " preserve.static.offset calls\n");
657
658 if (MarkerCalls.empty())
659 return false;
660
661 for (auto *Call : MarkerCalls)
662 removePAICalls(Marker: Call);
663
664 for (auto *Call : MarkerCalls) {
665 if (RemovedMarkers.contains(Ptr: Call))
666 continue;
667 bool StillUsed = rewriteAccessChain(Marker: Call, AllowPatial: AllowPartial, RemovedMarkers);
668 if (!StillUsed || !AllowPartial)
669 removeMarkerCall(Marker: Call);
670 }
671
672 return true;
673}
674
675PreservedAnalyses
676llvm::BPFPreserveStaticOffsetPass::run(Function &F,
677 FunctionAnalysisManager &AM) {
678 return rewriteFunction(F, AllowPartial) ? PreservedAnalyses::none()
679 : PreservedAnalyses::all();
680}
681

source code of llvm/lib/Target/BPF/BPFPreserveStaticOffset.cpp