[Mesa-dev] [PATCH] intel/fs: Do the grf127 hack on SIMD8 instructions in SIMD16 mode

[Matt Turner]

On Tue, Jan 15, 2019 at 8:58 AM Jason Ekstrand <jason at jlekstrand.net> wrote:
>
> Previously, we only applied the fix to shaders with a dispatch mode of
> SIMD8 but the code it relies on for SIMD16 mode only applies to SIMD16
> instructions.  If you have a SIMD8 instruction in a SIMD16 shader,
> neither would trigger and the restriction could still be hit.
>
> Cc: Jose Maria Casanova Crespo <jmcasanova at igalia.com>
> Fixes: 232ed8980217dd "i965/fs: Register allocator shoudn't use grf127..."
> ---
>  src/intel/compiler/brw_fs_reg_allocate.cpp | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/src/intel/compiler/brw_fs_reg_allocate.cpp b/src/intel/compiler/brw_fs_reg_allocate.cpp
> index 5db5242452e..ec743f9b5bf 100644
> --- a/src/intel/compiler/brw_fs_reg_allocate.cpp.
> +++ b/src/intel/compiler/brw_fs_reg_allocate.cpp
> @@ -667,15 +667,14 @@ fs_visitor::assign_regs(bool allow_spilling, bool spill_all)
>         * messages adding a node interference to the grf127_send_hack_node.
>         * This node has a fixed asignment to grf127.
>         *
> -       * We don't apply it to SIMD16 because previous code avoids any register
> -       * overlap between sources and destination.
> +       * We don't apply it to SIMD16 instructions because previous code avoids
> +       * any register overlap between sources and destination.
>         */
>        ra_set_node_reg(g, grf127_send_hack_node, 127);
> -      if (dispatch_width == 8) {
> -         foreach_block_and_inst(block, fs_inst, inst, cfg) {
> -            if (inst->is_send_from_grf() && inst->dst.file == VGRF)
> -               ra_add_node_interference(g, inst->dst.nr, grf127_send_hack_node);
> -         }
> +      foreach_block_and_inst(block, fs_inst, inst, cfg) {
> +         if (inst->exec_size < 16 && inst->is_send_from_grf() &&
> +             inst->dst.file == VGRF)
> +            ra_add_node_interference(g, inst->dst.nr, grf127_send_hack_node);
>        }
>

Did the code in brw_eu_validate.c catch the case you found?

In fact, that code looks wrong:

|                  (brw_inst_dst_da_reg_nr(devinfo, inst) +
|                   brw_inst_rlen(devinfo, inst) > 127) &&

I think > should be >=. And maybe we should have a separate case
earlier that checks that dst_nr+rlen actually fits in registers, and
then change > to just ==. FFS :(

Not sure what I was thinking letting that patch through without a unit
test. I'll do that.