Replace int2float() with an optimized version.

We use __fls() to find the most significant bit.  Using that, the
loop can be avoided.  A second trick is to use the behaviour of the
rotate instructions to expand the range of the unsigned int to float
conversion to the full 32 bits in a branchless way.

The routine is now exact up to 2^24.  Above that, we truncate which
is equivalent to rounding towards zero.

Signed-off-by: Steven Fuerst <svfuerst@gmail.com>

diff --git a/drivers/gpu/drm/radeon/r600_blit.c b/drivers/gpu/drm/radeon/r600_blit.c
index ee1b815..7d8ac42 100644 (file)
--- a/drivers/gpu/drm/radeon/r600_blit.c
+++ b/drivers/gpu/drm/radeon/r600_blit.c
@@ -489,31 +489,36 @@ set_default_state(drm_radeon_private_t *dev_priv)
        ADVANCE_RING();
 }
 
-uint32_t int2float(uint32_t input)
+/* 23 bits of float fractional data */
+#define I2F_FRAC_BITS  23
+#define I2F_MASK ((1 << I2F_FRAC_BITS) - 1)
+
+/*
+ * Converts unsigned integer into 32-bit IEEE floating point representation.
+ * Will be exact from 0 to 2^24.  Above that, we round towards zero
+ * as the fractional bits will not fit in a float.  (It would be better to
+ * round towards even as the fpu does, but that is slower.)
+ */
+uint32_t int2float(uint32_t x)
 {
-       u32 result, i, exponent, fraction;
-
-       if ((input & 0x3fff) == 0)
-               result = 0; /* 0 is a special case */
-       else {
-               exponent = 140; /* exponent biased by 127; */
-               fraction = (input & 0x3fff) << 10; /* cheat and only
-                                                     handle numbers below 2^^15 */
-               for (i = 0; i < 14; i++) {
-                       if (fraction & 0x800000)
-                               break;
-                       else {
-                               fraction = fraction << 1; /* keep
-                                                            shifting left until top bit = 1 */
-                               exponent = exponent - 1;
-                       }
-               }
-               result = exponent << 23 | (fraction & 0x7fffff); /* mask
-                                                                   off top bit; assumed 1 */
-       }
-       return result;
-}
+       uint32_t msb, exponent, fraction;
+
+       /* Zero is special */
+       if (!x) return 0;
+
+       /* Get location of the most significant bit */
+       msb = __fls(x);
 
+       /*
+        * Use a rotate instead of a shift because that works both leftwards
+        * and rightwards due to the mod(32) behaviour.  This means we don't
+        * need to check to see if we are above 2^24 or not.
+        */
+       fraction = ror32(x, (msb - I2F_FRAC_BITS) & 0x1f) & I2F_MASK;
+       exponent = (127 + msb) << I2F_FRAC_BITS;
+
+       return fraction + exponent;
+}
 
 static int r600_nomm_get_vb(struct drm_device *dev)
 {