### Author Topic: Premature optimization is the root of all evil, but...  (Read 2992 times)

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#### Kerame Pxel Nume

• Taronyu
•    • Posts: 736
• Karma: 6
• CPU follow my command since 1991 ##### Premature optimization is the root of all evil, but...
« on: June 07, 2010, 09:51:31 am »
when developing stuff one should at least keep his mind on things like L1 code cache behaviour:
http://x264dev.multimedia.cx/?p=201

Now take this small C++ snippet, which uses STL for only two things (a vector and a algorithm), compile it with
g++ -Os -c \$SRCFILE

Code: [Select]
`#include <vector>#include <algorithm>#include <iostream>struct vertex;void serialize_vertex(vertex const &);struct vertex {public: vertex(float x, float y, float z) { pos = x; pos = y; pos = z; } vertex& operator=(vertex const &v) { for(int i=0; i<3; ++i) pos[i] = v.pos[i]; return *this; }private: float pos; friend void serialize_vertex(vertex const &);};typedef std::vector<vertex> vertex_vector;void serialize_vertex(vertex const &v){ std::cout << v.pos << ' ' << v.pos << ' ' << v.pos << std::endl;}void serialize_vertex_vector(vertex_vector &v){ std::for_each(v.begin(), v.end(), serialize_vertex);}int main(){ vertex_vector V; for(int i=0; i < 50; ++i) { V.push_back(vertex(rand(), rand(), rand())); } serialize_vertex_vector(V); return 0;}`
And look at it's size. If I compile with gcc-4.3.4 for x86_64 it's about 0.8k in the .text segment.

Okay, that't not a problem yet, but now imagine if you'd implement a real algorithm, like eliminating all double vertices in a terrain grid (after LOD you may and up with exactly those). Aside the fact that STL algorithm is suboptimal for that ( O(n²) in a naive program, also a lot of data gets copied around due to RAII), You'll easily go over that L1 code cache limit of 32K.

Now let's look at the equivalent C program:
Code: [Select]
`#include <stdio.h>#include <stdlib.h>#include <string.h>typedef struct vertex {        float pos;} vertex;typedef struct vertex_array {        size_t len;        vertex *v;} vertex_array;void vertex_array_push_vertex(vertex_array * const VA, vertex const * const v){        vertex * new = 0;        if(!VA)                return;        new = realloc(VA->v, sizeof(vertex)*(VA->len + 1));        if(!new)                return;        VA->v = new;        memcpy(VA->v + VA->len, v, sizeof(vertex));        VA->len++;}void serialize_vertex(vertex const * const v){        printf("%f %f %f\n", v->pos, v->pos, v->pos);}void serialize_vertex_array(vertex_array const * const V){        int i;        for(i=0; i < V->len; ++i)                serialize_vertex(V->v + i);}int main(){        int i;        vertex_array V = {0,0};        for(i=0; i < 50; ++i) {                vertex v = {{rand(), rand(), rand()}};                vertex_array_push_vertex(&V, &v);        }        serialize_vertex_array(&V);        return 0;}`
This does the very same like the C++ equivalent, yet its .text segment size is only 0.3k. And since you keep working on the very same dataset (no RAII, no duplicates) as a side effect you also don't trash the L1 data cache and the L2 cache in total (which is shared by code and data). So if you've got a complex algorithm, doing it in pure C will give you that reduction in size of fit it into L1 code cache, resulting in a order of magnitude better performance.

#### bommel

• Palulukan Makto
•     • • • Posts: 3865
• Germany
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• Addicted to music! ##### Re: Premature optimization is the root of all evil, but...
« Reply #1 on: June 08, 2010, 11:58:50 am »
Sometimes I even break it down to asm level (for example when using SSE).
For all of you interested, I can recommend the Intel Software Optimization guide as a good reference. You can download it for free at Intel (don't have the link atm) or pm me.   LearnNavi is not affiliated with the official Avatar website,
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