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authorSteven Fuerst <svfuerst@gmail.com>2012-08-16 02:07:15 +0400
committerAlex Deucher <alexander.deucher@amd.com>2012-09-20 21:10:34 +0400
commit747f49ba67b8895a5831ab539de551b916f3738c (patch)
treeb1b1561ae97f9f7fdab8ae44e019bd5dca0e62e4 /drivers/gpu/drm/radeon/r600_blit.c
parent7ff64fcaa7b7ba62d12758e49643f31dd9e90ece (diff)
downloadlinux-747f49ba67b8895a5831ab539de551b916f3738c.tar.xz
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>
Diffstat (limited to 'drivers/gpu/drm/radeon/r600_blit.c')
-rw-r--r--drivers/gpu/drm/radeon/r600_blit.c51
1 files changed, 28 insertions, 23 deletions
diff --git a/drivers/gpu/drm/radeon/r600_blit.c b/drivers/gpu/drm/radeon/r600_blit.c
index ee1b815b5689..7d8ac42e3846 100644
--- 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)
{