HotColdSplitting.cpp File Reference

The goal of hot/cold splitting is to improve the memory locality of code. More...

#include "llvm/Transforms/IPO/HotColdSplitting.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/EHPersonalities.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/CodeExtractor.h"
#include <cassert>
#include <limits>
#include <string>

Go to the source code of this file.

Macros
#define	DEBUG_TYPE   "hotcoldsplit"

Typedefs
using	BlockTy = std::pair< BasicBlock *, unsigned >
	A pair of (basic block, score).

Functions
	STATISTIC (NumColdRegionsFound, "Number of cold regions found.")
	STATISTIC (NumColdRegionsOutlined, "Number of cold regions outlined.")
static InstructionCost	getOutliningBenefit (ArrayRef< BasicBlock * > Region, TargetTransformInfo &TTI)
	Get the benefit score of outlining Region.
static int	getOutliningPenalty (ArrayRef< BasicBlock * > Region, unsigned NumInputs, unsigned NumOutputs)
	Get the penalty score for outlining Region.

Variables
static cl::opt< bool >	EnableStaticAnalysis ("hot-cold-static-analysis", cl::init(true), cl::Hidden)
static cl::opt< int >	SplittingThreshold ("hotcoldsplit-threshold", cl::init(2), cl::Hidden, cl::desc("Base penalty for splitting cold code (as a " "multiple of TCC_Basic)"))
static cl::opt< bool >	EnableColdSection ("enable-cold-section", cl::init(false), cl::Hidden, cl::desc("Enable placement of extracted cold functions" " into a separate section after hot-cold splitting."))
static cl::opt< std::string >	ColdSectionName ("hotcoldsplit-cold-section-name", cl::init("__llvm_cold"), cl::Hidden, cl::desc("Name for the section containing cold functions " "extracted by hot-cold splitting."))
static cl::opt< int >	MaxParametersForSplit ("hotcoldsplit-max-params", cl::init(4), cl::Hidden, cl::desc("Maximum number of parameters for a split function"))
static cl::opt< int >	ColdBranchProbDenom ("hotcoldsplit-cold-probability-denom", cl::init(100), cl::Hidden, cl::desc("Divisor of cold branch probability." "BranchProbability = 1/ColdBranchProbDenom"))

Detailed Description

The goal of hot/cold splitting is to improve the memory locality of code.

The splitting pass does this by identifying cold blocks and moving them into separate functions.

When the splitting pass finds a cold block (referred to as "the sink"), it grows a maximal cold region around that block. The maximal region contains all blocks (post-)dominated by the sink [*]. In theory, these blocks are as cold as the sink. Once a region is found, it's split out of the original function provided it's profitable to do so.

[*] In practice, there is some added complexity because some blocks are not safe to extract.

TODO: Use the PM to get domtrees, and preserve BFI/BPI. TODO: Reorder outlined functions.

Definition in file HotColdSplitting.cpp.

Macro Definition Documentation

DEBUG_TYPE

#define DEBUG_TYPE   "hotcoldsplit"

Definition at line 60 of file HotColdSplitting.cpp.

Typedef Documentation

BlockTy

using BlockTy = std::pair<BasicBlock *, unsigned>

A pair of (basic block, score).

Definition at line 456 of file HotColdSplitting.cpp.

Function Documentation

getOutliningBenefit()

static InstructionCost getOutliningBenefit ( ArrayRef< BasicBlock * >  Region, TargetTransformInfo &  TTI  )

static

Get the benefit score of outlining Region.

Definition at line 284 of file HotColdSplitting.cpp.

References llvm::TargetTransformInfo::getInstructionCost(), llvm::BasicBlock::getTerminator(), I, llvm::BasicBlock::instructionsWithoutDebug(), and llvm::TargetTransformInfo::TCK_CodeSize.

getOutliningPenalty()

static int getOutliningPenalty ( ArrayRef< BasicBlock * >  Region, unsigned  NumInputs, unsigned  NumOutputs  )

static

Get the penalty score for outlining Region.

Definition at line 299 of file HotColdSplitting.cpp.

References llvm::dbgs(), llvm::BasicBlock::getTerminator(), llvm::SmallPtrSetImpl< PtrType >::insert(), llvm::is_contained(), LLVM_DEBUG, MaxParametersForSplit, llvm::SmallPtrSetImplBase::size(), SplittingThreshold, llvm::succ_empty(), llvm::successors(), and llvm::TargetTransformInfo::TCC_Basic.

STATISTIC() [1/2]

STATISTIC	(	NumColdRegionsFound	,
		"Number of cold regions found."
	)

STATISTIC() [2/2]

STATISTIC	(	NumColdRegionsOutlined	,
		"Number of cold regions outlined."
	)

Variable Documentation

ColdBranchProbDenom

cl::opt< int > ColdBranchProbDenom("hotcoldsplit-cold-probability-denom", cl::init(100), cl::Hidden, cl::desc("Divisor of cold branch probability." "BranchProbability = 1/ColdBranchProbDenom")) ( "hotcoldsplit-cold-probability-denom"  , cl::init(100)  , cl::Hidden  , cl::desc("Divisor of cold branch probability." "BranchProbability = 1/ColdBranchProbDenom")    )

static

ColdSectionName

cl::opt< std::string > ColdSectionName("hotcoldsplit-cold-section-name", cl::init("__llvm_cold"), cl::Hidden, cl::desc("Name for the section containing cold functions " "extracted by hot-cold splitting.")) ( "hotcoldsplit-cold-section-name"  , cl::init("__llvm_cold")  , cl::Hidden  , cl::desc("Name for the section containing cold functions " "extracted by hot-cold splitting.")    )

static

EnableColdSection

cl::opt< bool > EnableColdSection("enable-cold-section", cl::init(false), cl::Hidden, cl::desc("Enable placement of extracted cold functions" " into a separate section after hot-cold splitting.")) ( "enable-cold-section"  , cl::init(false)  , cl::Hidden  , cl::desc("Enable placement of extracted cold functions" " into a separate section after hot-cold splitting.")    )

static

EnableStaticAnalysis

cl::opt< bool > EnableStaticAnalysis("hot-cold-static-analysis", cl::init(true), cl::Hidden) ( "hot-cold-static-analysis"  , cl::init(true)  , cl::Hidden    )

static

MaxParametersForSplit

cl::opt< int > MaxParametersForSplit("hotcoldsplit-max-params", cl::init(4), cl::Hidden, cl::desc("Maximum number of parameters for a split function")) ( "hotcoldsplit-max-params"  , cl::init(4)  , cl::Hidden  , cl::desc("Maximum number of parameters for a split function")    )

static

Referenced by getOutliningPenalty().

SplittingThreshold

cl::opt< int > SplittingThreshold("hotcoldsplit-threshold", cl::init(2), cl::Hidden, cl::desc("Base penalty for splitting cold code (as a " "multiple of TCC_Basic)")) ( "hotcoldsplit-threshold"  , cl::init(2)  , cl::Hidden  , cl::desc("Base penalty for splitting cold code (as a " "multiple of TCC_Basic)")    )

static

Referenced by getOutliningPenalty().