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//===-- WebAssemblyRegColoring.cpp - Register coloring --------------------===//
//
// 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 a virtual register coloring pass.
///
/// WebAssembly doesn't have a fixed number of registers, but it is still
/// desirable to minimize the total number of registers used in each function.
///
/// This code is modeled after lib/CodeGen/StackSlotColoring.cpp.
///
//===----------------------------------------------------------------------===//
#include "WebAssembly.h"
#include "WebAssemblyMachineFunctionInfo.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-reg-coloring"
namespace {
class WebAssemblyRegColoring final : public MachineFunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
WebAssemblyRegColoring() : MachineFunctionPass(ID) {}
StringRef getPassName() const override {
return "WebAssembly Register Coloring";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<LiveIntervals>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addPreserved<MachineBlockFrequencyInfo>();
AU.addPreservedID(MachineDominatorsID);
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
};
} // end anonymous namespace
char WebAssemblyRegColoring::ID = 0;
INITIALIZE_PASS(WebAssemblyRegColoring, DEBUG_TYPE,
"Minimize number of registers used", false, false)
FunctionPass *llvm::createWebAssemblyRegColoring() {
return new WebAssemblyRegColoring();
}
// Compute the total spill weight for VReg.
static float computeWeight(const MachineRegisterInfo *MRI,
const MachineBlockFrequencyInfo *MBFI,
unsigned VReg) {
float Weight = 0.0f;
for (MachineOperand &MO : MRI->reg_nodbg_operands(VReg))
Weight += LiveIntervals::getSpillWeight(MO.isDef(), MO.isUse(), MBFI,
*MO.getParent());
return Weight;
}
bool WebAssemblyRegColoring::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG({
dbgs() << "********** Register Coloring **********\n"
<< "********** Function: " << MF.getName() << '\n';
});
// If there are calls to setjmp or sigsetjmp, don't perform coloring. Virtual
// registers could be modified before the longjmp is executed, resulting in
// the wrong value being used afterwards. (See <rdar://problem/8007500>.)
// TODO: Does WebAssembly need to care about setjmp for register coloring?
if (MF.exposesReturnsTwice())
return false;
MachineRegisterInfo *MRI = &MF.getRegInfo();
LiveIntervals *Liveness = &getAnalysis<LiveIntervals>();
const MachineBlockFrequencyInfo *MBFI =
&getAnalysis<MachineBlockFrequencyInfo>();
WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
// Gather all register intervals into a list and sort them.
unsigned NumVRegs = MRI->getNumVirtRegs();
SmallVector<LiveInterval *, 0> SortedIntervals;
SortedIntervals.reserve(NumVRegs);
LLVM_DEBUG(dbgs() << "Interesting register intervals:\n");
for (unsigned I = 0; I < NumVRegs; ++I) {
unsigned VReg = TargetRegisterInfo::index2VirtReg(I);
if (MFI.isVRegStackified(VReg))
continue;
// Skip unused registers, which can use $drop.
if (MRI->use_empty(VReg))
continue;
LiveInterval *LI = &Liveness->getInterval(VReg);
assert(LI->weight == 0.0f);
LI->weight = computeWeight(MRI, MBFI, VReg);
LLVM_DEBUG(LI->dump());
SortedIntervals.push_back(LI);
}
LLVM_DEBUG(dbgs() << '\n');
// Sort them to put arguments first (since we don't want to rename live-in
// registers), by weight next, and then by position.
// TODO: Investigate more intelligent sorting heuristics. For starters, we
// should try to coalesce adjacent live intervals before non-adjacent ones.
llvm::sort(SortedIntervals, [MRI](LiveInterval *LHS, LiveInterval *RHS) {
if (MRI->isLiveIn(LHS->reg) != MRI->isLiveIn(RHS->reg))
return MRI->isLiveIn(LHS->reg);
if (LHS->weight != RHS->weight)
return LHS->weight > RHS->weight;
if (LHS->empty() || RHS->empty())
return !LHS->empty() && RHS->empty();
return *LHS < *RHS;
});
LLVM_DEBUG(dbgs() << "Coloring register intervals:\n");
SmallVector<unsigned, 16> SlotMapping(SortedIntervals.size(), -1u);
SmallVector<SmallVector<LiveInterval *, 4>, 16> Assignments(
SortedIntervals.size());
BitVector UsedColors(SortedIntervals.size());
bool Changed = false;
for (size_t I = 0, E = SortedIntervals.size(); I < E; ++I) {
LiveInterval *LI = SortedIntervals[I];
unsigned Old = LI->reg;
size_t Color = I;
const TargetRegisterClass *RC = MRI->getRegClass(Old);
// Check if it's possible to reuse any of the used colors.
if (!MRI->isLiveIn(Old))
for (unsigned C : UsedColors.set_bits()) {
if (MRI->getRegClass(SortedIntervals[C]->reg) != RC)
continue;
for (LiveInterval *OtherLI : Assignments[C])
if (!OtherLI->empty() && OtherLI->overlaps(*LI))
goto continue_outer;
Color = C;
break;
continue_outer:;
}
unsigned New = SortedIntervals[Color]->reg;
SlotMapping[I] = New;
Changed |= Old != New;
UsedColors.set(Color);
Assignments[Color].push_back(LI);
LLVM_DEBUG(
dbgs() << "Assigning vreg" << TargetRegisterInfo::virtReg2Index(LI->reg)
<< " to vreg" << TargetRegisterInfo::virtReg2Index(New) << "\n");
}
if (!Changed)
return false;
// Rewrite register operands.
for (size_t I = 0, E = SortedIntervals.size(); I < E; ++I) {
unsigned Old = SortedIntervals[I]->reg;
unsigned New = SlotMapping[I];
if (Old != New)
MRI->replaceRegWith(Old, New);
}
return true;
}
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