LLVM Data-structures overviewllvm.org/devmtg/2014-04/PDFs/LightningTalks/data...Data-structures Maps...

LLVM Data-structures overviewLLVM Data-structures

Marcello Maggioni

1Codeplay Software Ltd.

EuroLLVM 2014

Marcello Maggioni (Codeplay Software Ltd.) LLVM Data-structures overview EuroLLVM 2014 1 / 31

Outline

1 MotivationWhy having specific data-structuresLLVM Resources

2 Data-structuresVectorsMapsSets


Motivation Why having specific data-structures

Outline




Motivation Why having specific data-structures

Why not using standard structures?

C++ Standard data-structures have performance that is platformdependentC++ Standard might not have a specific kind of data structures (likeHashMaps. With C++11 this problem was solved)Specialized data-structures can be made faster than the Standardgeneric ones


Motivation LLVM Resources

Outline





Resources on LLVM Data-Structures

http://llvm.org/docs/ProgrammersManual.htmlhttp://llvm.org/docs/doxygen/html/


http://llvm.org/docs/ProgrammersManual.html

http://llvm.org/docs/doxygen/html/


Looking at Doxygen info

Look for methods in every subclassThe most exposed interface does not expose all methods indocumentation usually


Data-structures Vectors

Outline





Possible Choices

LLVM SmallVectorstd::vector



SmallVector

Vector-like data structureIs optimized to contain a fixed amount of elementsIt is flexible if more elements are addedInterface similar to std::vector



SmallVector

#inc lude " l l vm /ADT/ Sma l lVec to r . h"

Smal lVector<type , N> V;

void f oo ( Sma l lVec to r Imp l<type> &V) {}



SmallVector

Smal lVector<I n s t r u c t i o n ∗ , 10> WorkL is t ;

fo r ( . . . ) {I n s t r u c t i o n ∗ I = . . . ;WorkL is t . push_back ( I ) ;

}. . .while ( WorkL is t . empty ( ) ) {

I n s t r u c t i o n ∗ I = WorkL is t . pop_back_val ( ) ;. . .

}



SmallVector vs World


Data-structures Maps

Outline





Possible Choices

LLVM DenseMapLLVM StringMap (only for strings)std::mapstd::unordered_map



DenseMap

DenseMap is a quadratically probed HashMap.Keeps everything in a single memory allocationIterators potentially invalidated after insertionMatches pretty closely std::map interface

insert(std::pair<>)find(Key&)count(Key&)begin(), end() (unordered)



DenseMap Keys

Supports all pointers and integer types as keysAdditional Key types can be specified defining a customDenseMapInfo<> class

s t ruct Key In fo {s t a t i c i n l i n e T getEmptyKey ( ) { . . . }s t a t i c i n l i n e T getTombstoneKey ( ) { . . . }s t a t i c unsigned getHashValue ( const T &Val ) { . . . }s t a t i c bool i s E q u a l ( const T &LHS , const T &RHS)

{ . . . }} ;

DenseMap<Key , Value , KeyIn fo> M;



DenseMap vs World



StringMap

Specific implementation of an HashMap only for having strings as keysStrings are copied into the map. They don’t store the pointer to themap as a key.Similar interface to DenseMapInsert is different though ... it is actually called GetOrCreateValue()



StringMap

const char ∗ s t r = "__some_symbol" ;

Str ingMap<Data> Map ;Data D = { 10 , 5 } ;

Map . GetOrCreateVa lue ( s t r , D) ;Map [ s t r ] = D;Map . f i n d ( s t r ) ;Map . count ( s t r ) ;



StringMap vs World

Storing a 16 character wide random string



StringMap vs World


Data-structures Sets

Outline





Possible Choices

Sorted vectorsLLVM SmallSetstd::setstd::unordered_set



SmallSet

Replacement for set in LLVMIt is implemented as a small vector of fixed size that is not sortedSearches are linear in timeWhen exceding the specified size switches to a quadratically probedset for some keys and std::set for othersCannot be iterated, only for querying



SmallSet

Smal lSet<i n t ∗ , 10> Si n t a ;S . i n s e r t (&a ) ;S . count (&a ) ;



SmallSet vs World



Summary

Covered basic data structures and their performance comparisonsOther data structures are available for specific needs (BitVectors,SparseSet, ValueMap ...)Using LLVM data-structures can give performance portabiltyLVM data-structures are not always faster and may require parametertuning



Contacts

Marcello [email protected]@codeplay.com


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LLVM Data-structures overviewllvm.org/devmtg/2014-04/PDFs/LightningTalks/data...Data-structures Maps...

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