[Mesa-dev] [PATCH 2/5] intel/fs: Lower integer multiply correctly when destination stride equals 4.

[Jason Ekstrand]

On Fri, Jan 18, 2019 at 6:09 PM Francisco Jerez <currojerez at riseup.net>
wrote:

> Because the "low" temporary needs to be accessed with word type and
> twice the original stride, attempting to preserve the alignment of the
> original destination can potentially lead to instructions with illegal
> destination stride greater than four.  Because the CHV/BXT alignment
> restrictions are now being enforced by the regioning lowering pass run
> after lower_integer_multiplication(), there is no real need to
> preserve the original strides anymore.
>
> Note that this bug can be reproduced on stable branches, but
> back-porting would be non-trivial, because the fix relies on the
> regioning lowering pass recently introduced.
> ---
>  src/intel/compiler/brw_fs.cpp | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
> index f475b617df2..5768e0d6542 100644
> --- a/src/intel/compiler/brw_fs.cpp
> +++ b/src/intel/compiler/brw_fs.cpp
> @@ -3962,13 +3962,11 @@ fs_visitor::lower_integer_multiplication()
>                  regions_overlap(inst->dst, inst->size_written,
>                                  inst->src[0], inst->size_read(0)) ||
>                  regions_overlap(inst->dst, inst->size_written,
> -                                inst->src[1], inst->size_read(1))) {
> +                                inst->src[1], inst->size_read(1)) ||
> +                inst->dst.stride >= 4) {
>

It would be nice to throw in a quick comment as to why we're adding a
temporary when stride >= 4.


>                 needs_mov = true;
> -               /* Get a new VGRF but keep the same stride as inst->dst */
>                 low = fs_reg(VGRF, alloc.allocate(regs_written(inst)),
>                              inst->dst.type);
> -               low.stride = inst->dst.stride;
> -               low.offset = inst->dst.offset % REG_SIZE;
>              }
>
>              /* Get a new VGRF but keep the same stride as inst->dst */
> --
> 2.19.2
>
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