[[PATCH][RESENT] 1/3] Replace i2f() in r600_blit.c with an optimized version.

[Steven Fuerst]

We use __fls() to find the most significant bit.  Using that, the
loop can be avoided.  A second trick is to use the mod(32)
behaviour of the rotate instructions on x86 to expand the range
of the unsigned int to float conversion to the full 32 bits.

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 at gmail.com>
---
 drivers/gpu/drm/radeon/r600_blit.c |   52 +++++++++++++++++++++---------------
 1 file changed, 30 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/radeon/r600_blit.c b/drivers/gpu/drm/radeon/r600_blit.c
index 3c031a4..f0ce441 100644
--- a/drivers/gpu/drm/radeon/r600_blit.c
+++ b/drivers/gpu/drm/radeon/r600_blit.c
@@ -489,29 +489,37 @@ set_default_state(drm_radeon_private_t *dev_priv)
 	ADVANCE_RING();
 }
 
-static uint32_t i2f(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.)
+ * This routine depends on the mod(32) behaviour of the rotate instructions
+ * on x86.
+ */
+static uint32_t i2f(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) beahviour.  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) & I2F_MASK;
+	exponent = (127 + msb) << I2F_FRAC_BITS;
+
+	return fraction + exponent;
 }
 
 
-- 
1.7.10.4