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Date: Fri, 10 Feb 2023 22:16:36 +0100
From: Jan Hubicka <hubicka@ucw.cz>
To: Richard Biener <rguenther@suse.de>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] tree-optimization/106722 - fix CD-DCE edge marking
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> The following fixes a latent issue when we mark control edges but
> end up with marking a block with no stmts necessary.  In this case
> we fail to mark dependent control edges of that block.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
> 
> Does this look OK?
> 
> Thanks,
> Richard.
> 
> 	PR tree-optimization/106722
> 	* tree-ssa-dce.cc (mark_last_stmt_necessary): Return
> 	whether we marked a stmt.
> 	(mark_control_dependent_edges_necessary): When
> 	mark_last_stmt_necessary didn't mark any stmt make sure
> 	to mark its control dependent edges.
> 	(propagate_necessity): Likewise.
> 
> 	* gcc.dg/torture/pr108737.c: New testcase.
> diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc
> index b2fe9f4f55e..21b3294fc86 100644
> --- a/gcc/tree-ssa-dce.cc
> +++ b/gcc/tree-ssa-dce.cc
> @@ -327,17 +327,23 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool aggressive)
>  
>  /* Mark the last statement of BB as necessary.  */
>  
> -static void
> +static bool
>  mark_last_stmt_necessary (basic_block bb)
>  {
>    gimple *stmt = last_stmt (bb);
>  
> -  bitmap_set_bit (last_stmt_necessary, bb->index);
> +  if (!bitmap_set_bit (last_stmt_necessary, bb->index))
> +    return true;
> +
>    bitmap_set_bit (bb_contains_live_stmts, bb->index);
>  
>    /* We actually mark the statement only if it is a control statement.  */
>    if (stmt && is_ctrl_stmt (stmt))
> -    mark_stmt_necessary (stmt, true);
> +    {
> +      mark_stmt_necessary (stmt, true);
> +      return true;
> +    }
> +  return false;
>  }
>  
>  
> @@ -369,8 +375,8 @@ mark_control_dependent_edges_necessary (basic_block bb, bool ignore_self)
>  	  continue;
>  	}
>  
> -      if (!bitmap_bit_p (last_stmt_necessary, cd_bb->index))
> -	mark_last_stmt_necessary (cd_bb);
> +      if (!mark_last_stmt_necessary (cd_bb))
> +	mark_control_dependent_edges_necessary (cd_bb, false);

Makes sense to me, though I am bit surprised it took such a long time to
show up.

Honza