LLVM  14.0.0git
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MachineOutliner.cpp File Reference
#include "llvm/CodeGen/MachineOutliner.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Mangler.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/SuffixTree.h"
#include "llvm/Support/raw_ostream.h"
#include <functional>
#include <tuple>
#include <vector>
Include dependency graph for MachineOutliner.cpp:

Go to the source code of this file.


 This file implements support for optimizing divisions by a constant.


#define DEBUG_TYPE   "machine-outliner"


 STATISTIC (NumOutlined, "Number of candidates outlined")
 STATISTIC (FunctionsCreated, "Number of functions created")
ModulePassllvm::createMachineOutlinerPass (bool RunOnAllFunctions=true)
 This pass performs outlining on machine instructions directly before printing assembly. More...
 INITIALIZE_PASS (MachineOutliner, DEBUG_TYPE, "Machine Function Outliner", false, false) void MachineOutliner


static cl::opt< bool > EnableLinkOnceODROutlining ("enable-linkonceodr-outlining", cl::Hidden, cl::desc("Enable the machine outliner on linkonceodr functions"), cl::init(false))
static cl::opt< unsigned > OutlinerReruns ("machine-outliner-reruns", cl::init(0), cl::Hidden, cl::desc("Number of times to rerun the outliner after the initial outline"))
 Number of times to re-run the outliner. More...

Detailed Description

Replaces repeated sequences of instructions with function calls.

This works by placing every instruction from every basic block in a suffix tree, and repeatedly querying that tree for repeated sequences of instructions. If a sequence of instructions appears often, then it ought to be beneficial to pull out into a function.

The MachineOutliner communicates with a given target using hooks defined in TargetInstrInfo.h. The target supplies the outliner with information on how a specific sequence of instructions should be outlined. This information is used to deduce the number of instructions necessary to

Targets must implement

in order to make use of the MachineOutliner.

This was originally presented at the 2016 LLVM Developers' Meeting in the talk "Reducing Code Size Using Outlining". For a high-level overview of how this pass works, the talk is available on YouTube at


The slides for the talk are available at


The talk provides an overview of how the outliner finds candidates and ultimately outlines them. It describes how the main data structure for this pass, the suffix tree, is queried and purged for candidates. It also gives a simplified suffix tree construction algorithm for suffix trees based off of the algorithm actually used here, Ukkonen's algorithm.

For the original RFC for this pass, please see


For more information on the suffix tree data structure, please see https://www.cs.helsinki.fi/u/ukkonen/SuffixT1withFigs.pdf

Definition in file MachineOutliner.cpp.

Macro Definition Documentation


#define DEBUG_TYPE   "machine-outliner"

Definition at line 79 of file MachineOutliner.cpp.

Function Documentation


INITIALIZE_PASS ( MachineOutliner  ,
"Machine Function Outliner ,
false  ,

Definition at line 451 of file MachineOutliner.cpp.

References DEBUG_TYPE, llvm::numbers::e, getDebugLoc(), i, and MORE.

◆ STATISTIC() [1/2]

STATISTIC ( FunctionsCreated  ,
"Number of functions created"   

◆ STATISTIC() [2/2]

STATISTIC ( NumOutlined  ,
"Number of candidates outlined"   

Variable Documentation

◆ EnableLinkOnceODROutlining

cl::opt<bool> EnableLinkOnceODROutlining("enable-linkonceodr-outlining", cl::Hidden, cl::desc("Enable the machine outliner on linkonceodr functions"), cl::init(false))

◆ OutlinerReruns

cl::opt<unsigned> OutlinerReruns("machine-outliner-reruns", cl::init(0), cl::Hidden, cl::desc( "Number of times to rerun the outliner after the initial outline"))

Number of times to re-run the outliner.

This is not the total number of runs as the outliner will run at least one time. The default value is set to 0, meaning the outliner will run one time and rerun zero times after that.