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//===--- SwitchBuilder.h ----------------------------------------*- C++ -*-===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
#ifndef SWIFT_IRGEN_SWITCHBUILDER_H
#define SWIFT_IRGEN_SWITCHBUILDER_H
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "IRBuilder.h"
#include "IRGenFunction.h"
namespace swift {
namespace irgen {
/// A builder that produces an LLVM branch or switch instruction for a set
/// of destinations.
class SwitchBuilder {
private:
#ifndef NDEBUG
/// Track whether we added as many cases as we expected to.
unsigned CasesToAdd;
#endif
protected:
IRBuilder Builder;
llvm::Value *Subject;
/// Protected initializer. Clients should use the `create` factory method.
SwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject, unsigned NumCases)
:
#ifndef NDEBUG
CasesToAdd(NumCases),
#endif
// Create our own IRBuilder, so that the SwitchBuilder is always able to
// generate the branch instruction at the right point, even if it needs
// to collect multiple cases before building an instruction.
Builder(IGF.IGM.getLLVMContext(), /*DebugInfo*/ false),
Subject(Subject) {
// Start our builder off at IGF's current insertion point.
Builder.SetInsertPoint(IGF.Builder.GetInsertBlock(),
IGF.Builder.GetInsertPoint());
}
public:
virtual ~SwitchBuilder() {
#ifndef NDEBUG
assert(CasesToAdd == 0 && "Did not add enough cases");
#endif
}
// Create a SwitchBuilder instance for a switch that will have the given
// number of cases.
static std::unique_ptr<SwitchBuilder> create(IRGenFunction &IGF,
llvm::Value *Subject,
SwitchDefaultDest Default,
unsigned NumCases);
/// Add a case to the switch.
virtual void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) {
#ifndef NDEBUG
assert(CasesToAdd > 0 && "Added too many cases");
--CasesToAdd;
#endif
}
};
/// A builder that emits an unconditional branch for a "switch" with only
/// one destination.
class BrSwitchBuilder final : public SwitchBuilder {
private:
friend class SwitchBuilder;
BrSwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject, unsigned NumCases)
: SwitchBuilder(IGF, Subject, NumCases) {
assert(NumCases == 1 && "should have only one branch");
}
public:
void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) override {
SwitchBuilder::addCase(value, dest);
Builder.CreateBr(dest);
}
};
/// A builder that produces a conditional branch for a "switch" with two
/// defined destinations.
class CondBrSwitchBuilder final : public SwitchBuilder {
private:
friend class SwitchBuilder;
llvm::BasicBlock *FirstDest;
CondBrSwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject,
SwitchDefaultDest Default, unsigned NumCases)
: SwitchBuilder(IGF, Subject, NumCases),
FirstDest(Default.getInt() == IsUnreachable ? nullptr
: Default.getPointer()) {
assert(NumCases + (Default.getInt() == IsNotUnreachable) == 2 &&
"should have two branches total");
}
public:
void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) override {
SwitchBuilder::addCase(value, dest);
// If we don't have a first destination yet, save it.
// We don't need to save the value in this case since we assume that the
// subject must be one of the case values. We only have to test one or
// the other.
//
// TODO: It may make sense to save both case values, and pick which one
// to compare based on which value is more likely to be efficiently
// representable in the target machine language. For example, zero
// or a small immediate is usually cheaper to materialize than a larger
// value that may require multiple instructions or a bigger encoding.
if (!FirstDest) {
FirstDest = dest;
return;
}
// Otherwise, we have both destinations for the branch and a value to
// test against. We can make the instruction now.
if (cast<llvm::IntegerType>(Subject->getType())->getBitWidth() == 1) {
// If the subject is already i1, we can use it directly as the branch
// condition.
if (value->isZero())
Builder.CreateCondBr(Subject, FirstDest, dest);
else
Builder.CreateCondBr(Subject, dest, FirstDest);
return;
}
// Otherwise, compare against the case value we have.
auto test = Builder.CreateICmpNE(Subject, value);
Builder.CreateCondBr(test, FirstDest, dest);
}
};
/// A builder that produces a switch instruction for a switch with many
/// destinations.
class SwitchSwitchBuilder final : public SwitchBuilder {
private:
friend class SwitchBuilder;
llvm::SwitchInst *TheSwitch;
SwitchSwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject,
SwitchDefaultDest Default, unsigned NumCases)
: SwitchBuilder(IGF, Subject, NumCases) {
TheSwitch =
IGF.Builder.CreateSwitch(Subject, Default.getPointer(), NumCases);
}
public:
void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) override {
SwitchBuilder::addCase(value, dest);
TheSwitch->addCase(value, dest);
}
};
inline std::unique_ptr<SwitchBuilder>
SwitchBuilder::create(IRGenFunction &IGF, llvm::Value *Subject,
SwitchDefaultDest Default, unsigned NumCases) {
// Pick a builder based on how many total reachable destinations we intend
// to have.
switch (NumCases + (Default.getInt() == IsNotUnreachable)) {
case 0:
// No reachable destinations. We can emit an unreachable and go about
// our business.
IGF.Builder.CreateUnreachable();
return nullptr;
case 1:
// One reachable destination. We can emit an unconditional branch.
// If that one branch is the default, we're done.
if (Default.getInt() == IsNotUnreachable) {
IGF.Builder.CreateBr(Default.getPointer());
return nullptr;
}
// Otherwise, use a builder to emit the one case.
return std::unique_ptr<SwitchBuilder>(
new BrSwitchBuilder(IGF, Subject, NumCases));
case 2:
// Two reachable destinations. We can emit a single conditional branch.
return std::unique_ptr<SwitchBuilder>(
new CondBrSwitchBuilder(IGF, Subject, Default, NumCases));
default:
// Anything more, fall over into a switch.
// TODO: Since fast isel doesn't support switch insns, we may want to
// also support a "pre-lowered-switch" builder that builds a jump tree
// for unoptimized builds.
return std::unique_ptr<SwitchBuilder>(
new SwitchSwitchBuilder(IGF, Subject, Default, NumCases));
}
}
} // end irgen namespace
} // end swift namespace
#endif
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