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
27 using namespace llvm;
28
29 namespace {
30
BPFIRPeepholeImpl(Function & F)31 static 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
run(Function & F,FunctionAnalysisManager & AM)95 PreservedAnalyses BPFIRPeepholePass::run(Function &F,
96 FunctionAnalysisManager &AM) {
97 return BPFIRPeepholeImpl(F) ? PreservedAnalyses::none()
98 : PreservedAnalyses::all();
99 }
100