draw,llvmpipe: use exponent manipulation instead of exp2 for polygon offset

Since we explicitly require a integer input we should avoid using exp2 math
(even if we were using optimized versions), which turns the exp2 into a int
sub (plus some casts).

v2: fix bogus uint (needs to be int) math spotted by Matthew, fix comments

Reviewed-by: Jose Fonseca <[email protected]>

1 files changed, 13 insertions, 8 deletions

diff --git a/src/gallium/auxiliary/draw/draw_pipe_offset.c b/src/gallium/auxiliary/draw/draw_pipe_offset.c
index 8071bc77b80..34114035ac8 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_offset.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_offset.c
@@ -90,18 +90,23 @@ static void do_offset_tri( struct draw_stage *stage,
    float dzdx = fabsf(a * inv_det);
    float dzdy = fabsf(b * inv_det);
 
-   float zoffset, maxz, bias, mult;
+   float zoffset, mult;
 
    mult = MAX2(dzdx, dzdy) * offset->scale;
 
    if (stage->draw->floating_point_depth) {
-      maxz = MAX3(v0[2], v1[2], v2[2]);
-
-      /**
-       * XXX: TODO optimize this to quickly resolve a pow2 number through
-       *      an exponent only operation.
-       */
-      bias = offset->units * util_fast_exp2(util_get_float32_exponent(maxz) - 23);
+      float bias;
+      union fi maxz;
+      maxz.f = MAX3(v0[2], v1[2], v2[2]);
+      /* just do the math directly on shifted number */
+      maxz.ui &= 0xff << 23;
+      maxz.i -= 23 << 23;
+      /* Clamping to zero means mrd will be zero for very small numbers,
+       * but specs do not indicate this should be prevented by clamping
+       * mrd to smallest normal number instead. */
+      maxz.i = MAX2(maxz.i, 0);
+
+      bias = offset->units * maxz.f;
       zoffset = bias + mult;
    } else {
       zoffset = offset->units + mult;