1 //===--- WebAssemblyOptimizeLiveIntervals.cpp - LiveInterval processing ---===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 ///
10 /// \file
11 /// \brief Optimize LiveIntervals for use in a post-RA context.
12 //
13 /// LiveIntervals normally runs before register allocation when the code is
14 /// only recently lowered out of SSA form, so it's uncommon for registers to
15 /// have multiple defs, and then they do, the defs are usually closely related.
16 /// Later, after coalescing, tail duplication, and other optimizations, it's
17 /// more common to see registers with multiple unrelated defs. This pass
18 /// updates LiveIntervalAnalysis to distribute the value numbers across separate
19 /// LiveIntervals.
20 ///
21 //===----------------------------------------------------------------------===//
22
23 #include "WebAssembly.h"
24 #include "WebAssemblySubtarget.h"
25 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
26 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
27 #include "llvm/CodeGen/MachineRegisterInfo.h"
28 #include "llvm/CodeGen/Passes.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/Support/raw_ostream.h"
31 using namespace llvm;
32
33 #define DEBUG_TYPE "wasm-optimize-live-intervals"
34
35 namespace {
36 class WebAssemblyOptimizeLiveIntervals final : public MachineFunctionPass {
getPassName() const37 const char *getPassName() const override {
38 return "WebAssembly Optimize Live Intervals";
39 }
40
getAnalysisUsage(AnalysisUsage & AU) const41 void getAnalysisUsage(AnalysisUsage &AU) const override {
42 AU.setPreservesCFG();
43 AU.addRequired<LiveIntervals>();
44 AU.addPreserved<MachineBlockFrequencyInfo>();
45 AU.addPreserved<SlotIndexes>();
46 AU.addPreserved<LiveIntervals>();
47 AU.addPreservedID(LiveVariablesID);
48 AU.addPreservedID(MachineDominatorsID);
49 MachineFunctionPass::getAnalysisUsage(AU);
50 }
51
52 bool runOnMachineFunction(MachineFunction &MF) override;
53
54 public:
55 static char ID; // Pass identification, replacement for typeid
WebAssemblyOptimizeLiveIntervals()56 WebAssemblyOptimizeLiveIntervals() : MachineFunctionPass(ID) {}
57 };
58 } // end anonymous namespace
59
60 char WebAssemblyOptimizeLiveIntervals::ID = 0;
createWebAssemblyOptimizeLiveIntervals()61 FunctionPass *llvm::createWebAssemblyOptimizeLiveIntervals() {
62 return new WebAssemblyOptimizeLiveIntervals();
63 }
64
runOnMachineFunction(MachineFunction & MF)65 bool WebAssemblyOptimizeLiveIntervals::runOnMachineFunction(MachineFunction &MF) {
66 DEBUG(dbgs() << "********** Optimize LiveIntervals **********\n"
67 "********** Function: "
68 << MF.getName() << '\n');
69
70 MachineRegisterInfo &MRI = MF.getRegInfo();
71 LiveIntervals &LIS = getAnalysis<LiveIntervals>();
72
73 // We don't preserve SSA form.
74 MRI.leaveSSA();
75
76 assert(MRI.tracksLiveness() &&
77 "OptimizeLiveIntervals expects liveness");
78
79 // Split multiple-VN LiveIntervals into multiple LiveIntervals.
80 SmallVector<LiveInterval*, 4> SplitLIs;
81 for (unsigned i = 0, e = MRI.getNumVirtRegs(); i < e; ++i) {
82 unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
83 if (MRI.reg_nodbg_empty(Reg))
84 continue;
85
86 LIS.splitSeparateComponents(LIS.getInterval(Reg), SplitLIs);
87 SplitLIs.clear();
88 }
89
90 // In PrepareForLiveIntervals, we conservatively inserted IMPLICIT_DEF
91 // instructions to satisfy LiveIntervals' requirement that all uses be
92 // dominated by defs. Now that LiveIntervals has computed which of these
93 // defs are actually needed and which are dead, remove the dead ones.
94 for (auto MII = MF.begin()->begin(), MIE = MF.begin()->end(); MII != MIE; ) {
95 MachineInstr *MI = &*MII++;
96 if (MI->isImplicitDef() && MI->getOperand(0).isDead()) {
97 LiveInterval &LI = LIS.getInterval(MI->getOperand(0).getReg());
98 LIS.removeVRegDefAt(LI, LIS.getInstructionIndex(*MI).getRegSlot());
99 LIS.RemoveMachineInstrFromMaps(*MI);
100 MI->eraseFromParent();
101 }
102 }
103
104 return false;
105 }
106