[Mesa-dev] [PATCH 11/28] replace LOG2 with util_fast_log2

[Dylan Baker]

The implementation is somewhat different, although if you go back in
time far enough they're the same, but the one in u_math was changed a
long time back to be faster.
---
 src/mesa/main/imports.h       | 29 -----------------------------
 src/mesa/swrast/s_span.c      |  4 ++--
 src/mesa/swrast/s_texfilter.c |  2 +-
 3 files changed, 3 insertions(+), 32 deletions(-)

diff --git a/src/mesa/main/imports.h b/src/mesa/main/imports.h
index 7e067dd23e3..461e801bc31 100644
--- a/src/mesa/main/imports.h
+++ b/src/mesa/main/imports.h
@@ -88,35 +88,6 @@ typedef union { GLfloat f; GLint i; GLuint u; } fi_type;
 /*@}*/
 
 
-/***
- *** LOG2: Log base 2 of float
- ***/
-static inline GLfloat LOG2(GLfloat x)
-{
-#if 0
-   /* This is pretty fast, but not accurate enough (only 2 fractional bits).
-    * Based on code from http://www.stereopsis.com/log2.html
-    */
-   const GLfloat y = x * x * x * x;
-   const GLuint ix = *((GLuint *) &y);
-   const GLuint exp = (ix >> 23) & 0xFF;
-   const GLint log2 = ((GLint) exp) - 127;
-   return (GLfloat) log2 * (1.0 / 4.0);  /* 4, because of x^4 above */
-#endif
-   /* Pretty fast, and accurate.
-    * Based on code from http://www.flipcode.com/totd/
-    */
-   fi_type num;
-   GLint log_2;
-   num.f = x;
-   log_2 = ((num.i >> 23) & 255) - 128;
-   num.i &= ~(255 << 23);
-   num.i += 127 << 23;
-   num.f = ((-1.0f/3) * num.f + 2) * num.f - 2.0f/3;
-   return num.f + log_2;
-}
-
-
 
 /**
  * finite macro.
diff --git a/src/mesa/swrast/s_span.c b/src/mesa/swrast/s_span.c
index f50b549a97f..e0d60deea7b 100644
--- a/src/mesa/swrast/s_span.c
+++ b/src/mesa/swrast/s_span.c
@@ -426,7 +426,7 @@ _swrast_compute_lambda(GLfloat dsdx, GLfloat dsdy, GLfloat dtdx, GLfloat dtdy,
    GLfloat x = sqrtf(dudx * dudx + dvdx * dvdx);
    GLfloat y = sqrtf(dudy * dudy + dvdy * dvdy);
    GLfloat rho = MAX2(x, y);
-   GLfloat lambda = LOG2(rho);
+   GLfloat lambda = util_fast_log2(rho);
    return lambda;
 }
 
@@ -453,7 +453,7 @@ _swrast_compute_lambda(GLfloat dsdx, GLfloat dsdy, GLfloat dtdx, GLfloat dtdy,
    maxU = MAX2(dsdx2, dsdy2) * texW;
    maxV = MAX2(dtdx2, dtdy2) * texH;
    rho = MAX2(maxU, maxV);
-   lambda = LOG2(rho);
+   lambda = util_fast_log2(rho);
    return lambda;
 }
 #endif
diff --git a/src/mesa/swrast/s_texfilter.c b/src/mesa/swrast/s_texfilter.c
index 314170fc751..acc470f46ef 100644
--- a/src/mesa/swrast/s_texfilter.c
+++ b/src/mesa/swrast/s_texfilter.c
@@ -1961,7 +1961,7 @@ sample_lambda_2d_aniso(struct gl_context *ctx,
       /* note: we need to have Pmin=sqrt(Pmin2) here, but we can avoid
        * this since 0.5*log(x) = log(sqrt(x))
        */
-      lod = 0.5f * LOG2(Pmin2);
+      lod = 0.5f * util_fast_log2(Pmin2);
       
       if (adjustLOD) {
          /* from swrast/s_texcombine.c _swrast_texture_span */
-- 
2.19.1