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