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 <ja...@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
