Optimize i2f()

[Alex Deucher]

On Mon, Jul 30, 2012 at 5:49 PM, Steven Fuerst <svfuerst at gmail.com> wrote:
> Looking through the kernel radeon drm source, it looks like the i2f()
> functions in r600_blit.c and r600_blit_ksm() can be optimized a bit.

Care to send a patch?

Thanks,

Alex

>
> The following extends the range to all unsigned 32bit integers, and avoids
> the slow loop by using the bsr instruction via __fls().  It provides an
> exact 1-1 correspondence up to 2^24.  Above that, there is the inevitable
> rounding.  This routine rounds towards zero (truncation).
>
> /* 23 bits of float fractional data */
> #define I2F_FRAC_BITS 23
> #define I2F_MASK ((1 << I2F_FRAC_BITS) - 1)
>
> /*
>  * Converts an 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.
>  */
> uint32_t i2f(uint32_t x)
> {
> 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;
> }
>
> Steven Fuerst
>
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