[Mesa-dev] [PATCH 1/2] nir: Use alloca instead of variable length arrays.

[Brian Paul]

On 02/26/2015 09:51 AM, Jose Fonseca wrote:
> This is to enable the code to build with -Werror=vla in the short term,
> and enable the code to build with MSVC2013 soon after.
> ---
>   include/c99_alloca.h                 | 45 ++++++++++++++++++++++++++++++++++++
>   src/glsl/nir/nir_from_ssa.c          | 19 +++++++--------
>   src/glsl/nir/nir_live_variables.c    |  5 ++--
>   src/glsl/nir/nir_lower_vars_to_ssa.c | 13 +++++++----
>   4 files changed, 66 insertions(+), 16 deletions(-)
>   create mode 100644 include/c99_alloca.h
>
> diff --git a/include/c99_alloca.h b/include/c99_alloca.h
> new file mode 100644
> index 0000000..6d96d06
> --- /dev/null
> +++ b/include/c99_alloca.h
> @@ -0,0 +1,45 @@
> +/**************************************************************************
> + *
> + * Copyright 2015 VMware, Inc.
> + * All Rights Reserved.
> + *
> + * 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, sub license, 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 NON-INFRINGEMENT.
> + * IN NO EVENT SHALL VMWARE AND/OR ITS SUPPLIERS 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.
> + *
> + **************************************************************************/
> +
> +#ifndef _C99_ALLOCA_H_
> +#define _C99_ALLOCA_H_
> +
> +
> +#if defined(_MSC_VER)
> +
> +#  include <malloc.h>
> +
> +#  define alloca _alloca
> +
> +#else /* !defined(_MSC_VER) */
> +
> +#  include <alloca.h>
> +
> +#endif /* !defined(_MSC_VER) */
> +
> +
> +#endif
> diff --git a/src/glsl/nir/nir_from_ssa.c b/src/glsl/nir/nir_from_ssa.c
> index c695c95..66339f3 100644
> --- a/src/glsl/nir/nir_from_ssa.c
> +++ b/src/glsl/nir/nir_from_ssa.c
> @@ -26,6 +26,7 @@
>    */
>
>   #include "nir.h"
> +#include "c99_alloca.h"
>
>   /*
>    * This file implements an out-of-SSA pass as described in "Revisiting
> @@ -181,7 +182,7 @@ merge_merge_sets(merge_set *a, merge_set *b)
>   static bool
>   merge_sets_interfere(merge_set *a, merge_set *b)
>   {
> -   merge_node *dom[a->size + b->size];
> +   merge_node **dom = alloca((a->size + b->size) * sizeof *dom);
>      int dom_idx = -1;
>
>      struct exec_node *an = exec_list_get_head(&a->nodes);
> @@ -673,21 +674,21 @@ resolve_parallel_copy(nir_parallel_copy_instr *pcopy,
>      }
>
>      /* The register/source corresponding to the given index */
> -   nir_src values[num_copies * 2];
> -   memset(values, 0, sizeof values);
> +   nir_src *values = alloca(num_copies * 2 * sizeof *values);
> +   memset(values, 0, num_copies * 2 * sizeof *values);
>
>      /* The current location of a given piece of data */
> -   int loc[num_copies * 2];
> +   int *loc = alloca(num_copies * 2 * sizeof *loc);
>
>      /* The piece of data that the given piece of data is to be copied from */
> -   int pred[num_copies * 2];
> +   int *pred = alloca(num_copies * 2 * sizeof *pred);
>
>      /* Initialize loc and pred.  We will use -1 for "null" */
> -   memset(loc, -1, sizeof loc);
> -   memset(pred, -1, sizeof pred);
> +   memset(loc, -1, num_copies * 2 * sizeof *loc);
> +   memset(pred, -1, num_copies * 2 * sizeof *pred);
>
>      /* The destinations we have yet to properly fill */
> -   int to_do[num_copies * 2];
> +   int *to_do = alloca(num_copies * 2 * sizeof *to_do);
>      int to_do_idx = -1;
>
>      /* Now we set everything up:
> @@ -737,7 +738,7 @@ resolve_parallel_copy(nir_parallel_copy_instr *pcopy,
>      }
>
>      /* Currently empty destinations we can go ahead and fill */
> -   int ready[num_copies * 2];
> +   int *ready = alloca(num_copies * 2 * sizeof *ready);
>      int ready_idx = -1;
>
>      /* Mark the ones that are ready for copying.  We know an index is a
> diff --git a/src/glsl/nir/nir_live_variables.c b/src/glsl/nir/nir_live_variables.c
> index 7402dc0..b57ca3a 100644
> --- a/src/glsl/nir/nir_live_variables.c
> +++ b/src/glsl/nir/nir_live_variables.c
> @@ -26,6 +26,7 @@
>
>   #include "nir.h"
>   #include "nir_worklist.h"
> +#include "c99_alloca.h"
>
>   /*
>    * Basic liveness analysis.  This works only in SSA form.
> @@ -130,8 +131,8 @@ static bool
>   propagate_across_edge(nir_block *pred, nir_block *succ,
>                         struct live_variables_state *state)
>   {
> -   BITSET_WORD live[state->bitset_words];
> -   memcpy(live, succ->live_in, sizeof live);
> +   BITSET_WORD *live = alloca(state->bitset_words * sizeof *live);
> +   memcpy(live, succ->live_in, state->bitset_words * sizeof *live);
>
>      nir_foreach_instr(succ, instr) {
>         if (instr->type != nir_instr_type_phi)
> diff --git a/src/glsl/nir/nir_lower_vars_to_ssa.c b/src/glsl/nir/nir_lower_vars_to_ssa.c
> index 8af7530..f54d1b7 100644
> --- a/src/glsl/nir/nir_lower_vars_to_ssa.c
> +++ b/src/glsl/nir/nir_lower_vars_to_ssa.c
> @@ -27,6 +27,9 @@
>
>   #include "nir.h"
>
> +#include "c99_alloca.h"
> +
> +
>   struct deref_node {
>      struct deref_node *parent;
>      const struct glsl_type *type;
> @@ -899,8 +902,8 @@ rename_variables_block(nir_block *block, struct lower_variables_state *state)
>   static void
>   insert_phi_nodes(struct lower_variables_state *state)
>   {
> -   unsigned work[state->impl->num_blocks];
> -   unsigned has_already[state->impl->num_blocks];
> +   unsigned *work = alloca(state->impl->num_blocks * sizeof *work);
> +   unsigned *has_already = alloca(state->impl->num_blocks * sizeof *has_already);
>
>      /*
>       * Since the work flags already prevent us from inserting a node that has
> @@ -910,10 +913,10 @@ insert_phi_nodes(struct lower_variables_state *state)
>       * function. So all we need to handle W is an array and a pointer to the
>       * next element to be inserted and the next element to be removed.
>       */
> -   nir_block *W[state->impl->num_blocks];
> +   nir_block **W = alloca(state->impl->num_blocks * sizeof *W);
>
> -   memset(work, 0, sizeof work);
> -   memset(has_already, 0, sizeof has_already);
> +   memset(work, 0, state->impl->num_blocks * sizeof *work);
> +   memset(has_already, 0, state->impl->num_blocks * sizeof *has_already);
>
>      unsigned w_start, w_end;
>      unsigned iter_count = 0;
>

Looks OK to me.

One thing I might have done would be instead of:

unsigned *work = alloca(state->impl->num_blocks * sizeof *work);
...
memset(work, 0, state->impl->num_blocks * sizeof *work);

do

const int work_size = state->impl->num_blocks * sizeof *work;
unsigned *work = alloca(work_size);
...
memset(work, 0, work_size);


AFAIK, there's no zeroing version of alloca().

Reviewed-by: Brian Paul <brianp at vmware.com>