[Mesa-dev] [PATCH] gallivm: eliminate a unnecessary AND with unorm lerps

[Jose Fonseca]

On 13/05/16 11:08, sroland at vmware.com wrote:
> From: Roland Scheidegger <sroland at vmware.com>
>
> Instead of doing a add and then mask out the upper bits, we can
> simply do a add with a half wide type (this, of course, assumes
> the hw can actually do it...), so we'll get the required zero
> in the upper bits automatically.
> ---
>   src/gallium/auxiliary/gallivm/lp_bld_arit.c | 45 ++++++++++++++++++++++-------
>   1 file changed, 35 insertions(+), 10 deletions(-)
>
> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_arit.c b/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> index 9c78837..11a1e7d 100644
> --- a/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> +++ b/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> @@ -1182,16 +1182,41 @@ lp_build_lerp_simple(struct lp_build_context *bld,
>         res = lp_build_mul(bld, x, delta);
>      }
>
> -   res = lp_build_add(bld, v0, res);
> -
> -   if (((flags & LP_BLD_LERP_WIDE_NORMALIZED) && !bld->type.sign) ||
> -       bld->type.fixed) {
> -      /* We need to mask out the high order bits when lerping 8bit normalized colors stored on 16bits */
> -      /* XXX: This step is necessary for lerping 8bit colors stored on 16bits,
> -       * but it will be wrong for true fixed point use cases. Basically we need
> -       * a more powerful lp_type, capable of further distinguishing the values
> -       * interpretation from the value storage. */
> -      res = LLVMBuildAnd(builder, res, lp_build_const_int_vec(bld->gallivm, bld->type, (1 << half_width) - 1), "");
> +   if ((flags & LP_BLD_LERP_WIDE_NORMALIZED) && !bld->type.sign) {
> +      /*
> +       * At this point both res and v0 only use the lower half of the bits,
> +       * the rest is zero. Instead of add / mask, do add with half wide type.
> +       */
> +      struct lp_type narrow_type;
> +      struct lp_build_context narrow_bld;
> +
> +      memset(&narrow_type, 0, sizeof narrow_type);
> +      narrow_type.sign   = bld->type.sign;
> +      narrow_type.width  = bld->type.width/2;
> +      narrow_type.length = bld->type.length*2;
> +
> +      lp_build_context_init(&narrow_bld, bld->gallivm, narrow_type);
> +      res = LLVMBuildBitCast(builder, res, narrow_bld.vec_type, "");
> +      v0 = LLVMBuildBitCast(builder, v0, narrow_bld.vec_type, "");
> +      res = lp_build_add(&narrow_bld, v0, res);
> +      res = LLVMBuildBitCast(builder, res, bld->vec_type, "");
> +   } else {
> +      res = lp_build_add(bld, v0, res);
> +
> +      if (bld->type.fixed) {
> +         /*
> +          * We need to mask out the high order bits when lerping 8bit
> +          * normalized colors stored on 16bits
> +          */
> +         /* XXX: This step is necessary for lerping 8bit colors stored on
> +          * 16bits, but it will be wrong for true fixed point use cases.
> +          * Basically we need a more powerful lp_type, capable of further
> +          * distinguishing the values interpretation from the value storage.
> +          */
> +         LLVMValueRef low_bits;
> +         low_bits = lp_build_const_int_vec(bld->gallivm, bld->type, (1 << half_width) - 1);
> +         res = LLVMBuildAnd(builder, res, low_bits, "");
> +      }
>      }
>
>      return res;
>

Reviewed-by: Jose Fonseca <jfonseca at vmware.com>

Jose