On Fri, Jan 07, 2022 at 02:40:51PM -0500, David Edelsohn via Gcc-patches wrote:
> +#ifdef __LITTLE_ENDIAN__
> + /* Sum across four integers with two integer results. */
> + asm ("vsum2sws %0,%1,%2" : "=v" (result) : "v" (vsum), "v" (zero));
> + /* Note: vec_sum2s could be used here, but on little-endian, vector
> + shifts are added that are not needed for this use-case.
> + A vector shift to correctly position the 32-bit integer results
> + (currently at [0] and [2]) to [1] and [3] would then need to be
> + swapped back again since the desired results are two 64-bit
> + integers ([1]|[0] and [3]|[2]). Thus, no shift is performed. */
> +#else
> /* Sum across four integers with two integer results. */
> result = vec_sum2s (vsum, (__vector signed int) zero);
>
> If little-endian adds shifts to correct for the position and
> big-endian does not, why not use the inline asm without the shifts for
> both? It seems confusing to add the inline asm only for LE instead of
> always using it with the appropriate comment.
>
> It's a good and valuable optimization for LE. Fewer variants are less
> fragile, easier to test and easier to maintain. If you're going to go
> to the trouble of using inline asm for LE, use it for both.
BE (only) _does_ need a shift as seen on the next two lines after the
code snippet above:
/* Sum across four integers with two integer results. */
result = vec_sum2s (vsum, (__vector signed int) zero);
/* Rotate the sums into the correct position. */
result = vec_sld (result, result, 6);
So, when using {vec_sum2s;vec_sld}:
- LE gets an implicit shift in vec_sum2s which just needs to be undone
by the vec_sld, and those shifts don't "cancel out" and get removed
by GCC.
- BE does not get any implicit shifts, but needs one that comes from
vec_sld.
Are you saying use the asm(vsum2sws) and then conditionally call
vec_sld on BE only?
I viewed this change as a temporary bandage unless and until GCC can
remove the unnecessary swaps. It seems like the preferred code is
vec_sum2s/vec_sld, not the asm, but that currently is suboptimal for LE.
PC
