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//===--- WebAssemblyOptimizeLiveIntervals.cpp - LiveInterval processing ---===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// Optimize LiveIntervals for use in a post-RA context.
/// LiveIntervals normally runs before register allocation when the code is
/// only recently lowered out of SSA form, so it's uncommon for registers to
/// have multiple defs, and when they do, the defs are usually closely related.
/// Later, after coalescing, tail duplication, and other optimizations, it's
/// more common to see registers with multiple unrelated defs. This pass
/// updates LiveIntervals to distribute the value numbers across separate
/// LiveIntervals.
#include "WebAssembly.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.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-optimize-live-intervals"
namespace {
class WebAssemblyOptimizeLiveIntervals final : public MachineFunctionPass {
StringRef getPassName() const override {
return "WebAssembly Optimize Live Intervals";
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnMachineFunction(MachineFunction &MF) override;
static char ID; // Pass identification, replacement for typeid
WebAssemblyOptimizeLiveIntervals() : MachineFunctionPass(ID) {}
} // end anonymous namespace
char WebAssemblyOptimizeLiveIntervals::ID = 0;
INITIALIZE_PASS(WebAssemblyOptimizeLiveIntervals, DEBUG_TYPE,
"Optimize LiveIntervals for WebAssembly", false, false)
FunctionPass *llvm::createWebAssemblyOptimizeLiveIntervals() {
return new WebAssemblyOptimizeLiveIntervals();
bool WebAssemblyOptimizeLiveIntervals::runOnMachineFunction(
MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** Optimize LiveIntervals **********\n"
"********** Function: "
<< MF.getName() << '\n');
MachineRegisterInfo &MRI = MF.getRegInfo();
auto &LIS = getAnalysis<LiveIntervals>();
// We don't preserve SSA form.
assert(MRI.tracksLiveness() && "OptimizeLiveIntervals expects liveness");
// Split multiple-VN LiveIntervals into multiple LiveIntervals.
SmallVector<LiveInterval *, 4> SplitLIs;
for (unsigned I = 0, E = MRI.getNumVirtRegs(); I < E; ++I) {
unsigned Reg = Register::index2VirtReg(I);
auto &TRI = *MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
if (MRI.reg_nodbg_empty(Reg))
LIS.splitSeparateComponents(LIS.getInterval(Reg), SplitLIs);
if (Reg == TRI.getFrameRegister(MF) && SplitLIs.size() > 0) {
// The live interval for the frame register was split, resulting in a new
// VReg. For now we only support debug info output for a single frame base
// value for the function, so just use the last one. It will certainly be
// wrong for some part of the function, but until we are able to track
// values through live-range splitting and stackification, it will have to
// do.
// In PrepareForLiveIntervals, we conservatively inserted IMPLICIT_DEF
// instructions to satisfy LiveIntervals' requirement that all uses be
// dominated by defs. Now that LiveIntervals has computed which of these
// defs are actually needed and which are dead, remove the dead ones.
for (auto MII = MF.begin()->begin(), MIE = MF.begin()->end(); MII != MIE;) {
MachineInstr *MI = &*MII++;
if (MI->isImplicitDef() && MI->getOperand(0).isDead()) {
LiveInterval &LI = LIS.getInterval(MI->getOperand(0).getReg());
LIS.removeVRegDefAt(LI, LIS.getInstructionIndex(*MI).getRegSlot());
return true;