[PATCH] c++, abi: Set DECL_FIELD_ABI_IGNORED on C++ zero width bitfields [PR102024]

Richard Biener rguenther@suse.de
Tue Aug 31 07:57:44 GMT 2021


On Tue, 31 Aug 2021, Jakub Jelinek wrote:

> Hi!
> 
> The removal of remove_zero_width_bitfields function and its call from
> C++ FE layout_class_type (which I've done in the P0466R5
> layout-compatible helper intrinsics patch, so that the FE can actually
> determine what is and isn't layout-compatible according to the spec)
> unfortunately changed the ABI on various platforms.
> The C FE has been keeping zero-width bitfields in the types, while
> the C++ FE has been removing them after structure layout, so in various
> cases when passing such structures in registers we had different ABI
> between C and C++.
> 
> The following patch doesn't change anything ABI-wise, but allows the
> targets to decide what to do, emit -Wpsabi warnings etc.
> Non-C zero width bitfields will be seen by the backends as normal
> zero width bitfields, C++ zero width bitfields that used to be previously
> removed will have DECL_FIELD_ABI_IGNORED flag set.  It is ok to reuse
> this flag, as it has been before used only on aggregate types and C++
> bitfields are always scalar (and DECL_BIT_FIELD too).
> 
> Each backend can then decide what it wants, whether it wants to keep
> different ABI between C and C++ as in GCC 11 and older (for that it would
> ignore for the homogenous aggregate decisions all DECL_FIELD_ABI_IGNORED
> DECL_BIT_FIELD FIELD_DECLs), whether it wants to never ignore zero
> width bitfields (no changes needed for that case, except perhaps -Wpsabi
> warning should be added and for that DECL_FIELD_ABI_IGNORED can be tested),
> or whether it wants to always ignore zero width bitfields (I think e.g.
> riscv in GCC 10+ does that).
> 
> All this patch does is set the flag and adjust backends so that nothing
> changes for now.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

Just to clarify - in the C++ FE these fields are meaningful for
layout purposes but they are only supposed to influence layout
but not ABI (but why does the C++ FE say that?) and thus the
'DECL_FIELD_ABI_IGNORED' is a good term to use?  But we still want
to have the backends decide whether to actually follow this advice
and we do expect some to not do this?

Thanks,
Richard.

> 2021-08-31  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR target/102024
> gcc/
> 	* config/arm/arm.c (aapcs_vfp_sub_candidate): Ignore
> 	DECL_FIELD_ABI_IGNORED on DECL_BIT_FIELD fields.
> 	* config/s390/s390.c (s390_function_arg_vector,
> 	s390_function_arg_float): Likewise.
> 	* config/ia64/ia64.c (hfa_element_mode): Likewise.
> 	* config/aarch64/aarch64.c (aapcs_vfp_sub_candidate): Likewise.
> 	* config/rs6000/rs6000.c (rs6000_special_adjust_field_align,
> 	rs6000_special_round_type_align): Likewise.
> 	* config/rs6000/rs6000-call.c (rs6000_aggregate_candidate): Likewise.
> gcc/cp/
> 	* class.c (layout_class_type): Set DECL_FIELD_ABI_IGNORED on zero
> 	width bitfields we used to remove in GCC 11 and earlier.
> 
> --- gcc/config/arm/arm.c.jj	2021-08-30 08:36:11.027519310 +0200
> +++ gcc/config/arm/arm.c	2021-08-30 13:36:57.068845279 +0200
> @@ -6332,7 +6332,7 @@ aapcs_vfp_sub_candidate (const_tree type
>  	    if (TREE_CODE (field) != FIELD_DECL)
>  	      continue;
>  
> -	    if (DECL_FIELD_ABI_IGNORED (field))
> +	    if (DECL_FIELD_ABI_IGNORED (field) && !DECL_BIT_FIELD (field))
>  	      {
>  		/* See whether this is something that earlier versions of
>  		   GCC failed to ignore.  */
> --- gcc/config/s390/s390.c.jj	2021-08-05 10:26:15.611554712 +0200
> +++ gcc/config/s390/s390.c	2021-08-30 13:38:29.493548311 +0200
> @@ -12167,7 +12167,7 @@ s390_function_arg_vector (machine_mode m
>  	  if (TREE_CODE (field) != FIELD_DECL)
>  	    continue;
>  
> -	  if (DECL_FIELD_ABI_IGNORED (field))
> +	  if (DECL_FIELD_ABI_IGNORED (field) && !DECL_BIT_FIELD (field))
>  	    {
>  	      if (lookup_attribute ("no_unique_address",
>  				    DECL_ATTRIBUTES (field)))
> @@ -12251,7 +12251,7 @@ s390_function_arg_float (machine_mode mo
>  	{
>  	  if (TREE_CODE (field) != FIELD_DECL)
>  	    continue;
> -	  if (DECL_FIELD_ABI_IGNORED (field))
> +	  if (DECL_FIELD_ABI_IGNORED (field) && !DECL_BIT_FIELD (field))
>  	    {
>  	      if (lookup_attribute ("no_unique_address",
>  				    DECL_ATTRIBUTES (field)))
> --- gcc/config/ia64/ia64.c.jj	2021-05-18 14:26:10.193220099 +0200
> +++ gcc/config/ia64/ia64.c	2021-08-30 13:37:32.855343096 +0200
> @@ -4665,7 +4665,8 @@ hfa_element_mode (const_tree type, bool
>      case QUAL_UNION_TYPE:
>        for (t = TYPE_FIELDS (type); t; t = DECL_CHAIN (t))
>  	{
> -	  if (TREE_CODE (t) != FIELD_DECL || DECL_FIELD_ABI_IGNORED (t))
> +	  if (TREE_CODE (t) != FIELD_DECL
> +	      || (DECL_FIELD_ABI_IGNORED (t) && !DECL_BIT_FIELD (t)))
>  	    continue;
>  
>  	  mode = hfa_element_mode (TREE_TYPE (t), 1);
> --- gcc/config/aarch64/aarch64.c.jj	2021-08-17 13:58:10.652245152 +0200
> +++ gcc/config/aarch64/aarch64.c	2021-08-30 13:36:28.268249427 +0200
> @@ -19019,7 +19019,7 @@ aapcs_vfp_sub_candidate (const_tree type
>  	    if (TREE_CODE (field) != FIELD_DECL)
>  	      continue;
>  
> -	    if (DECL_FIELD_ABI_IGNORED (field))
> +	    if (DECL_FIELD_ABI_IGNORED (field) && !DECL_BIT_FIELD (field))
>  	      {
>  		/* See whether this is something that earlier versions of
>  		   GCC failed to ignore.  */
> --- gcc/config/rs6000/rs6000.c.jj	2021-08-30 08:36:11.110518142 +0200
> +++ gcc/config/rs6000/rs6000.c	2021-08-30 13:39:42.790519755 +0200
> @@ -8023,7 +8023,8 @@ rs6000_special_adjust_field_align (tree
>        /* Skip all non field decls */
>        while (field != NULL
>  	     && (TREE_CODE (field) != FIELD_DECL
> -		 || DECL_FIELD_ABI_IGNORED (field)))
> +		 || (DECL_FIELD_ABI_IGNORED (field)
> +		     && !DECL_BIT_FIELD (field))))
>  	field = DECL_CHAIN (field);
>  
>        if (! field)
> @@ -8068,7 +8069,8 @@ rs6000_special_round_type_align (tree ty
>        /* Skip all non field decls */
>        while (field != NULL
>  	     && (TREE_CODE (field) != FIELD_DECL
> -		 || DECL_FIELD_ABI_IGNORED (field)))
> +		 || (DECL_FIELD_ABI_IGNORED (field)
> +		     && !DECL_BIT_FIELD (field))))
>  	field = DECL_CHAIN (field);
>  
>        if (! field)
> @@ -8110,7 +8112,8 @@ darwin_rs6000_special_round_type_align (
>      /* Skip all non field decls */
>      while (field != NULL
>  	   && (TREE_CODE (field) != FIELD_DECL
> -	       || DECL_FIELD_ABI_IGNORED (field)))
> +	       || (DECL_FIELD_ABI_IGNORED (field)
> +		   && !DECL_BIT_FIELD (field))))
>        field = DECL_CHAIN (field);
>      if (! field)
>        break;
> --- gcc/config/rs6000/rs6000-call.c.jj	2021-08-30 08:36:11.106518198 +0200
> +++ gcc/config/rs6000/rs6000-call.c	2021-08-30 13:40:02.261246527 +0200
> @@ -6335,7 +6335,7 @@ rs6000_aggregate_candidate (const_tree t
>  	    if (TREE_CODE (field) != FIELD_DECL)
>  	      continue;
>  
> -	    if (DECL_FIELD_ABI_IGNORED (field))
> +	    if (DECL_FIELD_ABI_IGNORED (field) && !DECL_BIT_FIELD (field))
>  	      {
>  		if (lookup_attribute ("no_unique_address",
>  				      DECL_ATTRIBUTES (field)))
> --- gcc/cp/class.c.jj	2021-08-19 11:42:27.269423736 +0200
> +++ gcc/cp/class.c	2021-08-30 12:43:43.371658433 +0200
> @@ -6744,6 +6744,22 @@ layout_class_type (tree t, tree *virtual
>        normalize_rli (rli);
>      }
>  
> +  /* We used to remove zero width bitfields at this point, while the C FE
> +     never did that.  That caused ABI differences on various targets.
> +     Set the DECL_FIELD_ABI_IGNORED flag on them instead, so that the backends
> +     can emit -Wpsabi warnings in the cases where the ABI changed.  */
> +  for (field = TYPE_FIELDS (t); field; field = DECL_CHAIN (field))
> +    if (TREE_CODE (field) == FIELD_DECL
> +        && DECL_C_BIT_FIELD (field)
> +	/* We should not be confused by the fact that grokbitfield
> +	   temporarily sets the width of the bit field into
> +	   DECL_BIT_FIELD_REPRESENTATIVE (field).
> +	   check_bitfield_decl eventually sets DECL_SIZE (field)
> +	   to that width.  */
> +	&& (DECL_SIZE (field) == NULL_TREE
> +	    || integer_zerop (DECL_SIZE (field))))
> +      DECL_FIELD_ABI_IGNORED (field) = 1;
> +
>    if (CLASSTYPE_NON_LAYOUT_POD_P (t) || CLASSTYPE_EMPTY_P (t))
>      {
>        /* T needs a different layout as a base (eliding virtual bases
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)


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