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 /// 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 when 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 LiveIntervals to distribute the value numbers across separate
19 /// LiveIntervals.
20 ///
21 //===----------------------------------------------------------------------===//
22
23 #include "WebAssembly.h"
24 #include "WebAssemblySubtarget.h"
25 #include "llvm/CodeGen/LiveIntervals.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 StringRef 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;
61 INITIALIZE_PASS(WebAssemblyOptimizeLiveIntervals, DEBUG_TYPE,
62 "Optimize LiveIntervals for WebAssembly", false, false)
63
createWebAssemblyOptimizeLiveIntervals()64 FunctionPass *llvm::createWebAssemblyOptimizeLiveIntervals() {
65 return new WebAssemblyOptimizeLiveIntervals();
66 }
67
runOnMachineFunction(MachineFunction & MF)68 bool WebAssemblyOptimizeLiveIntervals::runOnMachineFunction(
69 MachineFunction &MF) {
70 LLVM_DEBUG(dbgs() << "********** Optimize LiveIntervals **********\n"
71 "********** Function: "
72 << MF.getName() << '\n');
73
74 MachineRegisterInfo &MRI = MF.getRegInfo();
75 LiveIntervals &LIS = getAnalysis<LiveIntervals>();
76
77 // We don't preserve SSA form.
78 MRI.leaveSSA();
79
80 assert(MRI.tracksLiveness() && "OptimizeLiveIntervals expects liveness");
81
82 // Split multiple-VN LiveIntervals into multiple LiveIntervals.
83 SmallVector<LiveInterval *, 4> SplitLIs;
84 for (unsigned i = 0, e = MRI.getNumVirtRegs(); i < e; ++i) {
85 unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
86 if (MRI.reg_nodbg_empty(Reg))
87 continue;
88
89 LIS.splitSeparateComponents(LIS.getInterval(Reg), SplitLIs);
90 SplitLIs.clear();
91 }
92
93 // In PrepareForLiveIntervals, we conservatively inserted IMPLICIT_DEF
94 // instructions to satisfy LiveIntervals' requirement that all uses be
95 // dominated by defs. Now that LiveIntervals has computed which of these
96 // defs are actually needed and which are dead, remove the dead ones.
97 for (auto MII = MF.begin()->begin(), MIE = MF.begin()->end(); MII != MIE;) {
98 MachineInstr *MI = &*MII++;
99 if (MI->isImplicitDef() && MI->getOperand(0).isDead()) {
100 LiveInterval &LI = LIS.getInterval(MI->getOperand(0).getReg());
101 LIS.removeVRegDefAt(LI, LIS.getInstructionIndex(*MI).getRegSlot());
102 LIS.RemoveMachineInstrFromMaps(*MI);
103 MI->eraseFromParent();
104 }
105 }
106
107 return false;
108 }
109