Hello,
I performed several tests of the logbase2() function.
This function is defined and used in these files:
src/mesa/main/teximage.c
src/mesa/drivers/dri/intel/intel_tex_image.c
src/mesa/drivers/dri/unichrome/via_tex.c
src/mesa/drivers/dri/i965/intel_tex_image.c
src/mesa/drivers/dri/tdfx/tdfx_tex.c
There is also a util_logbase2() function in
src/gallium/auxiliary/util/u_math.c which do the same thing.
Both functions from "src/mesa/main/teximage.c" and
"src/gallium/auxiliary/util/u_math.c" are called when I play to ETQW
with r600g.
First thing: is it possible to merge this function in one place? The
same function is implement in different place. I have no sufficient
experience of the Mesa architecture to answer this myself.
Second thing: there is a C implementation in Wikipedia (
http://en.wikipedia.org/wiki/Binary_logarithm#Algorithm ), I merged it
in Mesa and I noted that it is *at least* twice faster than logbase2()
and util_logbase2() (with or without GCC optimization).
Third thing: I think it's better to use an unsigned int than a int for
the logbase2's parameter. At least in "teximage.c" and
"intel_tex_image.c", the result is stored in a GLuint. But not in
"via_tex.c"
Here is a patch to merge it, at least in "src/mesa/main/teximage.c". I
will understand any comments, especially on the relevance of this patch.
(The patch still uses a integer parameter).
Regards.
Benjamin Bellec
diff --git a/src/mesa/main/teximage.c b/src/mesa/main/teximage.c
index 4ea9a48..e2eeaa0 100644
--- a/src/mesa/main/teximage.c
+++ b/src/mesa/main/teximage.c
@@ -81,31 +81,25 @@ _mesa_free_texmemory(void *m)
/*
- * Compute floor(log_base_2(n)).
+ * Returns the floor form of binary logarithm for a 32 bit integer.
* If n < 0 return -1.
*/
static int
logbase2( int n )
{
- GLint i = 1;
- GLint log2 = 0;
+ if (n == 0)
+ return 0;
if (n < 0)
return -1;
- if (n == 0)
- return 0;
-
- while ( n > i ) {
- i *= 2;
- log2++;
- }
- if (i != n) {
- return log2 - 1;
- }
- else {
- return log2;
- }
+ int pos = 0;
+ if (n >= 1<<16) { n >>= 16; pos += 16; }
+ if (n >= 1<< 8) { n >>= 8; pos += 8; }
+ if (n >= 1<< 4) { n >>= 4; pos += 4; }
+ if (n >= 1<< 2) { n >>= 2; pos += 2; }
+ if (n >= 1<< 1) { pos += 1; }
+ return pos;
}
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