[Mesa-dev] [PATCH 2/5] i965/cfg: Track prev_block and prev_inst explicitly in the whole function

[Francisco Jerez]

Ian Romanick <idr at freedesktop.org> writes:

> From: Ian Romanick <ian.d.romanick at intel.com>
>
> This provides a trivial simplification now, and it makes some future
> changes more straight forward.
>
> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
> ---
>  src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp b/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
> index 716e2bc..7aa72b1 100644
> --- a/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
> @@ -41,7 +41,9 @@ dead_control_flow_eliminate(backend_shader *s)
>     bool progress = false;
>  
>     foreach_block_safe (block, s->cfg) {
> +      bblock_t *prev_block = block->prev();
>        backend_instruction *const inst = block->start();
> +      backend_instruction *prev_inst = prev_block->end();
>  
Just noticed that this change is bogus when 'block' is the first block
of the program: block->prev() will return an incorrectly casted pointer
to the head sentinel and prev_block->end() will then read invalid
memory.

Any suggestions?

>        /* ENDIF instructions, by definition, can only be found at the start of
>         * basic blocks.
> @@ -52,20 +54,20 @@ dead_control_flow_eliminate(backend_shader *s)
>           backend_instruction *endif_inst = inst;
>  
>           backend_instruction *if_inst = NULL, *else_inst = NULL;
> -         backend_instruction *prev_inst = endif_block->prev()->end();
>           if (prev_inst->opcode == BRW_OPCODE_ELSE) {
>              else_inst = prev_inst;
>              else_block = endif_block->prev();
>              found = true;
>  
> -            if (else_block->start_ip == else_block->end_ip)
> -               prev_inst = else_block->prev()->end();
> +            if (else_block->start_ip == else_block->end_ip) {
> +               prev_block = prev_block->prev();
> +               prev_inst = prev_block->end();
> +            }
>           }
>  
>           if (prev_inst->opcode == BRW_OPCODE_IF) {
>              if_inst = prev_inst;
> -            if_block = else_block != NULL ? else_block->prev()
> -               : endif_block->prev();
> +            if_block = prev_block;
>              found = true;
>           } else {
>              /* Don't remove the ENDIF if we didn't find a dead IF. */
> -- 
> 2.5.0
>
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