1//===------------ BPFIRPeephole.cpp - IR Peephole Transformation ----------===//
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// IR level peephole optimization, specifically removing @llvm.stacksave() and
10// @llvm.stackrestore().
11//
12//===----------------------------------------------------------------------===//
13
14#include "BPF.h"
15#include "llvm/IR/Instruction.h"
16#include "llvm/IR/Instructions.h"
17#include "llvm/IR/IntrinsicInst.h"
18#include "llvm/IR/Module.h"
19#include "llvm/IR/PassManager.h"
20#include "llvm/IR/Type.h"
21#include "llvm/IR/User.h"
22#include "llvm/IR/Value.h"
23#include "llvm/Pass.h"
24
25#define DEBUG_TYPE "bpf-ir-peephole"
26
27using namespace llvm;
28
29namespace {
30
31static bool BPFIRPeepholeImpl(Function &F) {
32  LLVM_DEBUG(dbgs() << "******** BPF IR Peephole ********\n");
33
34  bool Changed = false;
35  Instruction *ToErase = nullptr;
36  for (auto &BB : F) {
37    for (auto &I : BB) {
38      // The following code pattern is handled:
39      //     %3 = call i8* @llvm.stacksave()
40      //     store i8* %3, i8** %saved_stack, align 8
41      //     ...
42      //     %4 = load i8*, i8** %saved_stack, align 8
43      //     call void @llvm.stackrestore(i8* %4)
44      //     ...
45      // The goal is to remove the above four instructions,
46      // so we won't have instructions with r11 (stack pointer)
47      // if eventually there is no variable length stack allocation.
48      // InstrCombine also tries to remove the above instructions,
49      // if it is proven safe (constant alloca etc.), but depending
50      // on code pattern, it may still miss some.
51      //
52      // With unconditionally removing these instructions, if alloca is
53      // constant, we are okay then. Otherwise, SelectionDag will complain
54      // since BPF does not support dynamic allocation yet.
55      if (ToErase) {
56        ToErase->eraseFromParent();
57        ToErase = nullptr;
58      }
59
60      if (auto *II = dyn_cast<IntrinsicInst>(&I)) {
61        if (II->getIntrinsicID() != Intrinsic::stacksave)
62          continue;
63        if (!II->hasOneUser())
64          continue;
65        auto *Inst = cast<Instruction>(*II->user_begin());
66        LLVM_DEBUG(dbgs() << "Remove:"; I.dump());
67        LLVM_DEBUG(dbgs() << "Remove:"; Inst->dump(); dbgs() << '\n');
68        Changed = true;
69        Inst->eraseFromParent();
70        ToErase = &I;
71        continue;
72      }
73
74      if (auto *LD = dyn_cast<LoadInst>(&I)) {
75        if (!LD->hasOneUser())
76          continue;
77        auto *II = dyn_cast<IntrinsicInst>(*LD->user_begin());
78        if (!II)
79          continue;
80        if (II->getIntrinsicID() != Intrinsic::stackrestore)
81          continue;
82        LLVM_DEBUG(dbgs() << "Remove:"; I.dump());
83        LLVM_DEBUG(dbgs() << "Remove:"; II->dump(); dbgs() << '\n');
84        Changed = true;
85        II->eraseFromParent();
86        ToErase = &I;
87      }
88    }
89  }
90
91  return Changed;
92}
93} // End anonymous namespace
94
95PreservedAnalyses BPFIRPeepholePass::run(Function &F,
96                                         FunctionAnalysisManager &AM) {
97  return BPFIRPeepholeImpl(F) ? PreservedAnalyses::none()
98                              : PreservedAnalyses::all();
99}
100