CODEWITHSUNDEEP
c++cachingmicro-optimizationmemory-layout
matrixMule
matrixMule

Reputation: 11

How to group static/global variables together for improved cache locality in C++

My understanding is that the C++ compiler is free to place program-wide variables wherever it wants, as long as they do not appear together in a struct.

//in .h
struct Singleton {
   static int hotvar1;
   static int hotvar2;
};

//in .cpp
//is placed at location x
int Singleton::hotvar1 = 0;
//is next to hotvar2, but placed at a random location
int Singleton::hotvar2 = 0;

versus

struct MyStruct {
   //is placed at memory address x
   int hotvar1;
   //is placed at memory address x+4, therefore probably in the same cacheline
   int hotvar2;
};

My program uses a lot of singletons, which contain many high-frequency read, low-frequency write variables. I have read in another thread, that many compilers use some sort of hashing algorithm for the placement of these program-wide variables in .bss or .data section, which is more than sub-optimal for cache locality.

Is there any method for grouping these variables together without losing data encapsulation by just putting all of these static vars into a single global struct?

Idea:

struct HotVars {
   int Singleton1::hotvar1 = 0;
   int Singleton2::hotvar2 = 0;
   int Singleton1::apiHandle = 0;
};

I know that this is some kind of micro optimisation, but my program is severely back-end bound so improving cache locality is definitely a good idea.

Upvotes: 1

Views: 108

Answers (0)

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