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From: Richard Sandiford <richard.sandiford@arm.com>
To: Richard Biener <rguenther@suse.de>
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 richard.sandiford@arm.com
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Avoid setting TREE_ADDRESSABLE on stack vars during RTL
 expansion
References: <80p16ss1-o961-1r5p-rpss-45r4n1so371o@fhfr.qr>
Date: Tue, 18 May 2021 10:58:06 +0100
In-Reply-To: <80p16ss1-o961-1r5p-rpss-45r4n1so371o@fhfr.qr> (Richard Biener's
 message of "Tue, 18 May 2021 11:41:13 +0200 (CEST)")
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Richard Biener <rguenther@suse.de> writes:
> @@ -6621,9 +6637,31 @@ pass_expand::execute (function *fun)
>  		 (int) param_ssp_buffer_size);
>      }
>  
> +  /* Temporarily mark PARM_DECLs and RESULT_DECLs we need to expand to
> +     memory addressable so expand_function_start can emit the required
> +     copies.  */
> +  for (tree parm = DECL_ARGUMENTS (current_function_decl); parm;
> +       parm = DECL_CHAIN (parm))
> +    if (bitmap_bit_p (forced_stack_vars, DECL_UID (parm)))
> +      TREE_ADDRESSABLE (parm) = 1;
> +  if (DECL_RESULT (current_function_decl)
> +      && bitmap_bit_p (forced_stack_vars,
> +		       DECL_UID (DECL_RESULT (current_function_decl))))
> +    TREE_ADDRESSABLE (DECL_RESULT (current_function_decl)) = 1;
> +
>    /* Set up parameters and prepare for return, for the function.  */
>    expand_function_start (current_function_decl);
>  
> +  /* Clear TREE_ADDRESSABLE again.  */
> +  for (tree parm = DECL_ARGUMENTS (current_function_decl); parm;
> +       parm = DECL_CHAIN (parm))
> +    if (bitmap_bit_p (forced_stack_vars, DECL_UID (parm)))
> +      TREE_ADDRESSABLE (parm) = 0;
> +  if (DECL_RESULT (current_function_decl)
> +      && bitmap_bit_p (forced_stack_vars,
> +		       DECL_UID (DECL_RESULT (current_function_decl))))
> +    TREE_ADDRESSABLE (DECL_RESULT (current_function_decl)) = 0;
> +
>    /* If we emitted any instructions for setting up the variables,
>       emit them before the FUNCTION_START note.  */
>    if (var_seq)

Is TREE_ADDRESSABLE guaranteed to be 0 for these decls before the code
is hit?  I was surprised that we didn't need to protect against net
1->0 transitions.

Thanks,
Richard