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

Jakub Jelinek jakub@redhat.com
Tue Aug 31 07:44:30 GMT 2021


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?

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



More information about the Gcc-patches mailing list