llvm/lib/Target/BPF/BPFIRPeephole.cpp

   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
  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
  95 PreservedAnalyses BPFIRPeepholePass::run(Function &F,
  96                                          FunctionAnalysisManager &AM) {
  97   return BPFIRPeepholeImpl(F) ? PreservedAnalyses::none()
  98                               : PreservedAnalyses::all();
  99 }