CppProfiling.md

#C++性能Profiling ##Content ###Intro cache locality, memory alignment, cache associativity, (false) sharing , vectorization, branch prediction, denomals 都都来自于硬件，而不是programming language

###Cache Locality

• 2d array
• struct related

class Baz
{
	Foo foo;
	Bar bar;
}

std::vector<Baz> bazs(1000);

doSomething(bazs[i]);

• L1 cache: data cache, instruction cache

###Memory Alignment

• packed structs(if not aligned, require to do bit-wise operation)

struct Foo
{
	char c;
	short s;
	int i;
	double d;
}
__attribute__((pack));

• benchmark: comparing the aligned struct with packed struct

• cast between different memroy alignments

struct Foo{

}

• could use union

union Foo{
	char c[8];
	double d;
}

• overalignment

struct alignas (32) Foo{
}

• alignment on the heap

• new synatx(std::max_align_T since c++11)

• convince the phone align does not wrok

struct alignas (8) Foo
{
	char ch;
}
auto* a = new Foo;

• c++11 introductionaligned_storage, aligned_union, not work on heap

• use std::align is messy

• many use operating system specific system call, but c++ standard not support yet

• align to cache lines, to avoid extracting two cache lines

###False-Sharing

###Cache-Associativity

###Vectorization & Simd

• simd instruction -> CPU <-> Simd Registers
• Auto-vectoriser, -O3
• Simd library

Vec4<float> a;
a* =f;

• SSE intrinsics(not portable)

• inline assembly

• how to tell the compiler the pointer is aligned

##References

• YouTube Vidio by Tgimur Doumler