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 --- drivers/gpu/drm/radeon/r600_blit_kms.c | 51 +++++++++++++------------------- 1 file changed, 21 insertions(+), 30 deletions(-)
diff --git a/drivers/gpu/drm/radeon/r600_blit_kms.c b/drivers/gpu/drm/radeon/r600_blit_kms.c index 2bef854..e5a40ca 100644 --- a/drivers/gpu/drm/radeon/r600_blit_kms.c +++ b/drivers/gpu/drm/radeon/r600_blit_kms.c @@ -455,44 +455,35 @@ set_default_state(struct radeon_device *rdev) radeon_ring_write(ring, sq_stack_resource_mgmt_2); }
-#define I2F_MAX_BITS 15 -#define I2F_MAX_INPUT ((1 << I2F_MAX_BITS) - 1) -#define I2F_SHIFT (24 - I2F_MAX_BITS) +/* 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. - * Conversion is not universal and only works for the range from 0 - * to 2^I2F_MAX_BITS-1. Currently we only use it with inputs between - * 0 and 16384 (inclusive), so I2F_MAX_BITS=15 is enough. If necessary, - * I2F_MAX_BITS can be increased, but that will add to the loop iterations - * and slow us down. Conversion is done by shifting the input and counting - * down until the first 1 reaches bit position 23. The resulting counter - * and the shifted input are, respectively, the exponent and the fraction. - * The sign is always zero. + * 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.) */ -static uint32_t i2f(uint32_t input) +static uint32_t i2f(uint32_t x) { - u32 result, i, exponent, fraction; + uint32_t msb, exponent, fraction;
- WARN_ON_ONCE(input > I2F_MAX_INPUT); + /* Zero is special */ + if (!x) return 0;
- if ((input & I2F_MAX_INPUT) == 0) - result = 0; - else { - exponent = 126 + I2F_MAX_BITS; - fraction = (input & I2F_MAX_INPUT) << I2F_SHIFT; + /* Get location of the most significant bit */ + msb = __fls(x);
- for (i = 0; i < I2F_MAX_BITS; i++) { - if (fraction & 0x800000) - break; - else { - fraction = fraction << 1; - exponent = exponent - 1; - } - } - result = exponent << 23 | (fraction & 0x7fffff); - } - return result; + /* + * 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; }
int r600_blit_init(struct radeon_device *rdev)