blob: ede1a944493116d918279711b18fa0bfcd524f5b [file] [log] [blame]
//===- subzero/src/IceSwitchLowering.cpp - Switch lowering ----------------===//
// The Subzero Code Generator
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
/// \file
/// \brief Implements platform independent analysis of switch cases to improve
/// the generated code.
#include "IceSwitchLowering.h"
#include "IceCfgNode.h"
#include "IceTargetLowering.h"
#include <algorithm>
namespace Ice {
CaseClusterArray CaseCluster::clusterizeSwitch(Cfg *Func,
const InstSwitch *Instr) {
const SizeT NumCases = Instr->getNumCases();
CaseClusterArray CaseClusters;
// Load the cases
for (SizeT I = 0; I < NumCases; ++I)
CaseClusters.emplace_back(Instr->getValue(I), Instr->getLabel(I));
// Sort the cases
std::sort(CaseClusters.begin(), CaseClusters.end(),
[](const CaseCluster &x, const CaseCluster &y) {
return x.High < y.Low;
// Merge adjacent case ranges
auto Active = CaseClusters.begin();
std::for_each(Active + 1, CaseClusters.end(),
[&Active](const CaseCluster &x) {
if (!Active->tryAppend(x))
*(++Active) = x;
CaseClusters.erase(Active + 1, CaseClusters.end());
// TODO(ascull): Merge in a cycle i.e. -1(=UINTXX_MAX) to 0. This depends on
// the types for correct wrap around behavior.
// A small number of cases is more efficient without a jump table
if (CaseClusters.size() < Func->getTarget()->getMinJumpTableSize())
return CaseClusters;
// Test for a single jump table. This can be done in constant time whereas
// finding the best set of jump table would be quadratic, too slow(?). If
// jump tables were included in the search tree we'd first have to traverse
// to them. Ideally we would have an unbalanced tree which is biased towards
// frequently executed code but we can't do this well without profiling data.
// So, this single jump table is a good starting point where you can get to
// the jump table quickly without figuring out how to unbalance the tree.
const uint64_t MaxValue = CaseClusters.back().High;
const uint64_t MinValue = CaseClusters.front().Low;
// Don't +1 yet to avoid (INT64_MAX-0)+1 overflow
const uint64_t Range = MaxValue - MinValue;
// Might be too sparse for the jump table
if (NumCases * 2 <= Range)
return CaseClusters;
// Unlikely. Would mean can't store size of jump table.
if (Range == UINT64_MAX)
return CaseClusters;
const uint64_t TotalRange = Range + 1;
// Replace everything with a jump table
auto *JumpTable =
InstJumpTable::create(Func, TotalRange, Instr->getLabelDefault());
for (const CaseCluster &Case : CaseClusters) {
// Case.High could be UINT64_MAX which makes the loop awkward. Unwrap the
// last iteration to avoid wrap around problems.
for (uint64_t I = Case.Low; I < Case.High; ++I)
JumpTable->addTarget(I - MinValue, Case.Target);
JumpTable->addTarget(Case.High - MinValue, Case.Target);
CaseClusters.emplace_back(MinValue, MaxValue, JumpTable);
return CaseClusters;
bool CaseCluster::tryAppend(const CaseCluster &New) {
// Can only append ranges with the same target and are adjacent
const bool CanAppend =
this->Target == New.Target && this->High + 1 == New.Low;
if (CanAppend)
this->High = New.High;
return CanAppend;
} // end of namespace Ice