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lynedavid

Vertex shader slower than fixed function pipeline?

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On a GeForce 6800/7800, your vertex shader generates the following *native* code for the GPU:


401F9C6C 01CD400D 8106C0C3 60411F80 DP4 o[HPOS].x, v[OPOS], c[212];
401F9C6C 01CD500D 8106C0C3 60409F80 DP4 o[HPOS].y, v[OPOS], c[213];
401F9C6C 01CD600D 8106C0C3 60405F80 DP4 o[HPOS].z, v[OPOS], c[214];
401F9C6C 01CD700D 8106C0C3 60403F80 DP4 o[HPOS].w, v[OPOS], c[215];
401F9C6C 00400808 0106C083 60419F9D MOV o[TEX0].xy, v[TEX0].xyxx;


(The hexcodes are the 128-bit instruction words.) The "fixed-function" path produces exactly the same code, but with different constant register indexes.

If you want to do very accurate timing on the GPU, you can use the EXT_timer_query extension. If defines the following enums and functions.


#define GL_TIME_ELAPSED_EXT 0x88BF

typedef __int64 GLint64EXT;
typedef unsigned __int64 GLuint64EXT;

void glGetQueryObjecti64vEXT(GLuint id, GLenum pname, GLint64EXT *params);
void glGetQueryObjectui64vEXT(GLuint id, GLenum pname, GLuint64EXT *params);


Use the glBeginQuery/glEndQuery mechanism with the GL_TIME_ELAPSED_EXT target to specify a timing interval. A call to glGetQueryObjectui64vEXT with <pname> GL_QUERY_RESULT returns the elapsed time in nanoseconds.

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BTW, in case you're curious, your vertex shader produces the following native code on Radeon X800/X1800.


00100201 00D10002 00D10001 00D10005 DP4 o[0].x, c[0], v[0];
00200201 00D10022 00D10001 00D10005 DP4 o[0].y, c[1], v[0];
00400201 00D10042 00D10001 00D10005 DP4 o[0].z, c[2], v[0];
00800201 00D10062 00D10001 00D10005 DP4 o[0].w, c[3], v[0];
00F02203 01648000 01248000 01248005 MOV o[1], R0.0001;
00304203 00D10041 01248041 01248045 MOV o[2].xy, v[2];


The ATI driver inserts an extra instruction to move (0,0,0,1) into the primary color interpolant, but otherwise, it's the same native instruction sequence that Nvidia hardware uses.

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