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//== WebAssemblyMemIntrinsicResults.cpp - Optimize memory intrinsic results ==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file implements an optimization pass using memory intrinsic results.
///
/// Calls to memory intrinsics (memcpy, memmove, memset) return the destination
/// address. They are in the form of
/// %dst_new = call @memcpy %dst, %src, %len
/// where %dst and %dst_new registers contain the same value.
///
/// This is to enable an optimization wherein uses of the %dst register used in
/// the parameter can be replaced by uses of the %dst_new register used in the
/// result, making the %dst register more likely to be single-use, thus more
/// likely to be useful to register stackifying, and potentially also exposing
/// the call instruction itself to register stackifying. These both can reduce
/// local.get/local.set traffic.
///
/// The LLVM intrinsics for these return void so they can't use the returned
/// attribute and consequently aren't handled by the OptimizeReturned pass.
///
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssembly.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-mem-intrinsic-results"
namespace {
class WebAssemblyMemIntrinsicResults final : public MachineFunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
WebAssemblyMemIntrinsicResults() : MachineFunctionPass(ID) {}
StringRef getPassName() const override {
return "WebAssembly Memory Intrinsic Results";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addPreserved<MachineBlockFrequencyInfo>();
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
AU.addRequired<LiveIntervals>();
AU.addPreserved<SlotIndexes>();
AU.addPreserved<LiveIntervals>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
};
} // end anonymous namespace
char WebAssemblyMemIntrinsicResults::ID = 0;
INITIALIZE_PASS(WebAssemblyMemIntrinsicResults, DEBUG_TYPE,
"Optimize memory intrinsic result values for WebAssembly",
false, false)
FunctionPass *llvm::createWebAssemblyMemIntrinsicResults() {
return new WebAssemblyMemIntrinsicResults();
}
// Replace uses of FromReg with ToReg if they are dominated by MI.
static bool replaceDominatedUses(MachineBasicBlock &MBB, MachineInstr &MI,
unsigned FromReg, unsigned ToReg,
const MachineRegisterInfo &MRI,
MachineDominatorTree &MDT,
LiveIntervals &LIS) {
bool Changed = false;
LiveInterval *FromLI = &LIS.getInterval(FromReg);
LiveInterval *ToLI = &LIS.getInterval(ToReg);
SlotIndex FromIdx = LIS.getInstructionIndex(MI).getRegSlot();
VNInfo *FromVNI = FromLI->getVNInfoAt(FromIdx);
SmallVector<SlotIndex, 4> Indices;
for (MachineOperand &O :
llvm::make_early_inc_range(MRI.use_nodbg_operands(FromReg))) {
MachineInstr *Where = O.getParent();
// Check that MI dominates the instruction in the normal way.
if (&MI == Where || !MDT.dominates(&MI, Where))
continue;
// If this use gets a different value, skip it.
SlotIndex WhereIdx = LIS.getInstructionIndex(*Where);
VNInfo *WhereVNI = FromLI->getVNInfoAt(WhereIdx);
if (WhereVNI && WhereVNI != FromVNI)
continue;
// Make sure ToReg isn't clobbered before it gets there.
VNInfo *ToVNI = ToLI->getVNInfoAt(WhereIdx);
if (ToVNI && ToVNI != FromVNI)
continue;
Changed = true;
LLVM_DEBUG(dbgs() << "Setting operand " << O << " in " << *Where << " from "
<< MI << "\n");
O.setReg(ToReg);
// If the store's def was previously dead, it is no longer.
if (!O.isUndef()) {
MI.getOperand(0).setIsDead(false);
Indices.push_back(WhereIdx.getRegSlot());
}
}
if (Changed) {
// Extend ToReg's liveness.
LIS.extendToIndices(*ToLI, Indices);
// Shrink FromReg's liveness.
LIS.shrinkToUses(FromLI);
// If we replaced all dominated uses, FromReg is now killed at MI.
if (!FromLI->liveAt(FromIdx.getDeadSlot()))
MI.addRegisterKilled(FromReg, MBB.getParent()
->getSubtarget<WebAssemblySubtarget>()
.getRegisterInfo());
}
return Changed;
}
static bool optimizeCall(MachineBasicBlock &MBB, MachineInstr &MI,
const MachineRegisterInfo &MRI,
MachineDominatorTree &MDT, LiveIntervals &LIS,
const WebAssemblyTargetLowering &TLI,
const TargetLibraryInfo &LibInfo) {
MachineOperand &Op1 = MI.getOperand(1);
if (!Op1.isSymbol())
return false;
StringRef Name(Op1.getSymbolName());
bool CallReturnsInput = Name == TLI.getLibcallName(RTLIB::MEMCPY) ||
Name == TLI.getLibcallName(RTLIB::MEMMOVE) ||
Name == TLI.getLibcallName(RTLIB::MEMSET);
if (!CallReturnsInput)
return false;
LibFunc Func;
if (!LibInfo.getLibFunc(Name, Func))
return false;
Register FromReg = MI.getOperand(2).getReg();
Register ToReg = MI.getOperand(0).getReg();
if (MRI.getRegClass(FromReg) != MRI.getRegClass(ToReg))
report_fatal_error("Memory Intrinsic results: call to builtin function "
"with wrong signature, from/to mismatch");
return replaceDominatedUses(MBB, MI, FromReg, ToReg, MRI, MDT, LIS);
}
bool WebAssemblyMemIntrinsicResults::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG({
dbgs() << "********** Memory Intrinsic Results **********\n"
<< "********** Function: " << MF.getName() << '\n';
});
MachineRegisterInfo &MRI = MF.getRegInfo();
auto &MDT = getAnalysis<MachineDominatorTree>();
const WebAssemblyTargetLowering &TLI =
*MF.getSubtarget<WebAssemblySubtarget>().getTargetLowering();
const auto &LibInfo =
getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(MF.getFunction());
auto &LIS = getAnalysis<LiveIntervals>();
bool Changed = false;
// We don't preserve SSA form.
MRI.leaveSSA();
assert(MRI.tracksLiveness() &&
"MemIntrinsicResults expects liveness tracking");
for (auto &MBB : MF) {
LLVM_DEBUG(dbgs() << "Basic Block: " << MBB.getName() << '\n');
for (auto &MI : MBB)
switch (MI.getOpcode()) {
default:
break;
case WebAssembly::CALL:
Changed |= optimizeCall(MBB, MI, MRI, MDT, LIS, TLI, LibInfo);
break;
}
}
return Changed;
}
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