[Mesa-dev] [PATCH 2/3] nir: add conditional discard optimisation (v3)

[Eric Anholt]

Dave Airlie <airlied at gmail.com> writes:

> From: Dave Airlie <airlied at redhat.com>
>
> This is ported from GLSL and converts
>
> if (cond)
> 	discard;
>
> into
> discard_if(cond);
>
> This removes a block, but also is needed by radv
> to workaround a bug in the LLVM backend.
>
> v2: handle if (a) discard_if(b) (nha)
> cleanup and drop pointless loop (Matt)
> make sure there are no dependent phis (Eric)
> v3: make sure only one instruction in the then block.
>
> Signed-off-by: Dave Airlie <airlied at redhat.com>
> ---

> diff --git a/src/compiler/nir/nir_opt_conditional_discard.c b/src/compiler/nir/nir_opt_conditional_discard.c
> new file mode 100644
> index 0000000..6e90983
> --- /dev/null
> +++ b/src/compiler/nir/nir_opt_conditional_discard.c
> @@ -0,0 +1,125 @@
> +/*
> + * Copyright © 2016 Red Hat
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> + * copy of this software and associated documentation files (the "Software"),
> + * to deal in the Software without restriction, including without limitation
> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
> + * and/or sell copies of the Software, and to permit persons to whom the
> + * Software is furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice (including the next
> + * paragraph) shall be included in all copies or substantial portions of the
> + * Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
> + * IN THE SOFTWARE.
> + */
> +
> +#include "nir.h"
> +#include "nir_builder.h"
> +
> +/** @file nir_opt_conditional_discard.c
> + *
> + * Handles optimization of lowering if (cond) discard to discard_if(cond).
> + */

Maybe put some quotes around "if (cond) discard" to clarify what
statement is being lowered.

> +static bool
> +nir_opt_conditional_discard_block(nir_block *block, void *mem_ctx)
> +{
> +   nir_builder bld;
> +
> +   if (nir_cf_node_is_first(&block->cf_node))
> +      return false;
> +
> +   nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
> +   if (prev_node->type != nir_cf_node_if)
> +      return false;
> +
> +   nir_if *if_stmt = nir_cf_node_as_if(prev_node);
> +   nir_block *then_block = nir_if_first_then_block(if_stmt);
> +   nir_block *else_block = nir_if_first_else_block(if_stmt);
> +
> +   /* check there is only one else block and it is empty */
> +   if (nir_if_last_else_block(if_stmt) != else_block)
> +      return false;
> +   if (!exec_list_is_empty(&else_block->instr_list))
> +      return false;
> +
> +   /* check there is only one then block and it has only one instruction in it */
> +   if (nir_if_last_then_block(if_stmt) != then_block)
> +      return false;
> +   if (exec_list_is_empty(&then_block->instr_list))
> +      return false;
> +   if (exec_list_length(&then_block->instr_list) > 1)
> +      return false;
> +   /*
> +    * make sure no subsequent phi nodes point at this if.
> +    */
> +   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node));
> +   nir_foreach_instr_safe(instr, after) {
> +      if (instr->type != nir_instr_type_phi)
> +         break;
> +      nir_phi_instr *phi = nir_instr_as_phi(instr);
> +
> +      nir_foreach_phi_src(phi_src, phi) {
> +         if (phi_src->pred == then_block ||
> +             phi_src->pred == else_block)
> +            return false;
> +      }
> +   }
> +
> +   /* Get the first instruction in the then block and confirm it is
> +    * a discard or a discard_if
> +    */
> +   nir_instr *instr = nir_block_first_instr(then_block);
> +   if (instr->type != nir_instr_type_intrinsic)
> +      return false;
> +
> +   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
> +   if (intrin->intrinsic != nir_intrinsic_discard &&
> +       intrin->intrinsic != nir_intrinsic_discard_if)
> +      return false;
> +
> +   nir_src cond;
> +
> +   nir_builder_init(&bld, mem_ctx);

Missing bld.cursor initialization, so the adding-to-a-discard_if case
should crash.  With that fixed, this will be:

Reviewed-by: Eric Anholt <eric at anholt.net>
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