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* [gcc r12-3324] c++, abi: Set DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on C++ zero width bitfields [PR102024]
@ 2021-09-03 7:53 Jakub Jelinek
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From: Jakub Jelinek @ 2021-09-03 7:53 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:e902136b310ee17d4b49eb42d9d5e487d5dcf4a1
commit r12-3324-ge902136b310ee17d4b49eb42d9d5e487d5dcf4a1
Author: Jakub Jelinek <jakub@redhat.com>
Date: Fri Sep 3 09:46:32 2021 +0200
c++, abi: Set DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on C++ zero width bitfields [PR102024]
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++.
While both the C and C++ FE had some code to remove zero width bitfields
after structure layout, in both FEs it was buggy and didn't really remove
any. In the C FE that code has been removed later on, while in the C++ FE
for GCC 4.5 in PR42217 it has been actually fixed, so the C++ FE started
to remove those bitfields.
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_CXX_ZERO_WIDTH_BIT_FIELD flag set.
I've reused the DECL_FIELD_ABI_IGNORED flag which is only used on non-bitfield
FIELD_DECLs right now, but the macros now check DECL_BIT_FIELD flag.
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 (i.e. incompatible
with G++ <= 4.4, compatible with G++ 4.5 .. 11), for that it would
ignore for the aggregate passing/returning decisions all
DECL_FIELD_CXX_ZERO_WIDTH_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_CXX_ZERO_WIDTH_BIT_FIELD
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 which the backends can then use.
2021-09-03 Jakub Jelinek <jakub@redhat.com>
PR target/102024
gcc/
* tree.h (DECL_FIELD_ABI_IGNORED): Changed into rvalue only macro
that is false if DECL_BIT_FIELD.
(SET_DECL_FIELD_ABI_IGNORED, DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD,
SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD): Define.
* tree-streamer-out.c (pack_ts_decl_common_value_fields): For
DECL_BIT_FIELD stream DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead
of DECL_FIELD_ABI_IGNORED.
* tree-streamer-in.c (unpack_ts_decl_common_value_fields): Use
SET_DECL_FIELD_ABI_IGNORED instead of writing to
DECL_FIELD_ABI_IGNORED and for DECL_BIT_FIELD use
SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead.
* lto-streamer-out.c (hash_tree): For DECL_BIT_FIELD hash
DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead of DECL_FIELD_ABI_IGNORED.
gcc/cp/
* class.c (build_base_field): Use SET_DECL_FIELD_ABI_IGNORED
instead of writing to DECL_FIELD_ABI_IGNORED.
(layout_class_type): Likewise. In the place where zero-width
bitfields used to be removed, use
SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on those fields instead.
gcc/lto/
* lto-common.c (compare_tree_sccs_1): Also compare
DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD values.
Diff:
---
gcc/cp/class.c | 21 +++++++++++++++++++--
gcc/lto-streamer-out.c | 5 ++++-
gcc/lto/lto-common.c | 1 +
gcc/tree-streamer-in.c | 6 +++++-
gcc/tree-streamer-out.c | 5 ++++-
gcc/tree.h | 26 ++++++++++++++++++++++----
6 files changed, 55 insertions(+), 9 deletions(-)
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 17876c724ab..fe225c61a62 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -4634,7 +4634,7 @@ build_base_field (record_layout_info rli, tree binfo, tree access,
DECL_FIELD_OFFSET (decl) = BINFO_OFFSET (binfo);
DECL_FIELD_BIT_OFFSET (decl) = bitsize_zero_node;
SET_DECL_OFFSET_ALIGN (decl, BITS_PER_UNIT);
- DECL_FIELD_ABI_IGNORED (decl) = 1;
+ SET_DECL_FIELD_ABI_IGNORED (decl, 1);
}
/* An empty virtual base causes a class to be non-empty
@@ -6658,7 +6658,7 @@ layout_class_type (tree t, tree *virtuals_p)
}
else if (might_overlap && is_empty_class (type))
{
- DECL_FIELD_ABI_IGNORED (field) = 1;
+ SET_DECL_FIELD_ABI_IGNORED (field, 1);
layout_empty_base_or_field (rli, field, empty_base_offsets);
}
else
@@ -6746,6 +6746,23 @@ layout_class_type (tree t, tree *virtuals_p)
normalize_rli (rli);
}
+ /* We used to remove zero width bitfields at this point since PR42217,
+ while the C FE never did that. That caused ABI differences on various
+ targets. Set the DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD 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))))
+ SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (field, 1);
+
if (CLASSTYPE_NON_LAYOUT_POD_P (t) || CLASSTYPE_EMPTY_P (t))
{
/* T needs a different layout as a base (eliding virtual bases
diff --git a/gcc/lto-streamer-out.c b/gcc/lto-streamer-out.c
index a26d4885800..643d9275a95 100644
--- a/gcc/lto-streamer-out.c
+++ b/gcc/lto-streamer-out.c
@@ -1271,7 +1271,10 @@ hash_tree (struct streamer_tree_cache_d *cache, hash_map<tree, hashval_t> *map,
hstate.add_flag (DECL_PACKED (t));
hstate.add_flag (DECL_NONADDRESSABLE_P (t));
hstate.add_flag (DECL_PADDING_P (t));
- hstate.add_flag (DECL_FIELD_ABI_IGNORED (t));
+ if (DECL_BIT_FIELD (t))
+ hstate.add_flag (DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (t));
+ else
+ hstate.add_flag (DECL_FIELD_ABI_IGNORED (t));
hstate.add_int (DECL_OFFSET_ALIGN (t));
}
else if (code == VAR_DECL)
diff --git a/gcc/lto/lto-common.c b/gcc/lto/lto-common.c
index bfe52a2e942..3eba431e5af 100644
--- a/gcc/lto/lto-common.c
+++ b/gcc/lto/lto-common.c
@@ -1187,6 +1187,7 @@ compare_tree_sccs_1 (tree t1, tree t2, tree **map)
compare_values (DECL_NONADDRESSABLE_P);
compare_values (DECL_PADDING_P);
compare_values (DECL_FIELD_ABI_IGNORED);
+ compare_values (DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD);
compare_values (DECL_OFFSET_ALIGN);
}
else if (code == VAR_DECL)
diff --git a/gcc/tree-streamer-in.c b/gcc/tree-streamer-in.c
index e0522bf2ac1..848981aa20b 100644
--- a/gcc/tree-streamer-in.c
+++ b/gcc/tree-streamer-in.c
@@ -256,7 +256,11 @@ unpack_ts_decl_common_value_fields (struct bitpack_d *bp, tree expr)
DECL_PACKED (expr) = (unsigned) bp_unpack_value (bp, 1);
DECL_NONADDRESSABLE_P (expr) = (unsigned) bp_unpack_value (bp, 1);
DECL_PADDING_P (expr) = (unsigned) bp_unpack_value (bp, 1);
- DECL_FIELD_ABI_IGNORED (expr) = (unsigned) bp_unpack_value (bp, 1);
+ unsigned val = (unsigned) bp_unpack_value (bp, 1);
+ if (DECL_BIT_FIELD (expr))
+ SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (expr, val);
+ else
+ SET_DECL_FIELD_ABI_IGNORED (expr, val);
expr->decl_common.off_align = bp_unpack_value (bp, 8);
}
diff --git a/gcc/tree-streamer-out.c b/gcc/tree-streamer-out.c
index 855d1cd59b9..1f9ce52d541 100644
--- a/gcc/tree-streamer-out.c
+++ b/gcc/tree-streamer-out.c
@@ -219,7 +219,10 @@ pack_ts_decl_common_value_fields (struct bitpack_d *bp, tree expr)
bp_pack_value (bp, DECL_PACKED (expr), 1);
bp_pack_value (bp, DECL_NONADDRESSABLE_P (expr), 1);
bp_pack_value (bp, DECL_PADDING_P (expr), 1);
- bp_pack_value (bp, DECL_FIELD_ABI_IGNORED (expr), 1);
+ if (DECL_BIT_FIELD (expr))
+ bp_pack_value (bp, DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (expr), 1);
+ else
+ bp_pack_value (bp, DECL_FIELD_ABI_IGNORED (expr), 1);
bp_pack_value (bp, expr->decl_common.off_align, 8);
}
diff --git a/gcc/tree.h b/gcc/tree.h
index 2c8973f34e2..1559fe060a0 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -2852,16 +2852,34 @@ extern void decl_value_expr_insert (tree, tree);
/* In a FIELD_DECL, indicates this field should be bit-packed. */
#define DECL_PACKED(NODE) (FIELD_DECL_CHECK (NODE)->base.u.bits.packed_flag)
+/* Nonzero in a FIELD_DECL means it is a bit field, and must be accessed
+ specially. */
+#define DECL_BIT_FIELD(NODE) (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_1)
+
/* In a FIELD_DECL, indicates this field should be ignored for ABI decisions
like passing/returning containing struct by value.
Set for C++17 empty base artificial FIELD_DECLs as well as
empty [[no_unique_address]] non-static data members. */
#define DECL_FIELD_ABI_IGNORED(NODE) \
- (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_0)
+ (!DECL_BIT_FIELD (NODE) && (NODE)->decl_common.decl_flag_0)
+#define SET_DECL_FIELD_ABI_IGNORED(NODE, VAL) \
+ do { \
+ gcc_checking_assert (!DECL_BIT_FIELD (NODE)); \
+ FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_0 = (VAL); \
+ } while (0)
-/* Nonzero in a FIELD_DECL means it is a bit field, and must be accessed
- specially. */
-#define DECL_BIT_FIELD(NODE) (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_1)
+/* In a FIELD_DECL, indicates C++ zero-width bitfield that used to be
+ removed from the IL since PR42217 until PR101539 and by that changed
+ the ABI on several targets. This flag is provided so that the backends
+ can decide on the ABI with zero-width bitfields and emit -Wpsabi
+ warnings. */
+#define DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD(NODE) \
+ (DECL_BIT_FIELD (NODE) && (NODE)->decl_common.decl_flag_0)
+#define SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD(NODE, VAL) \
+ do { \
+ gcc_checking_assert (DECL_BIT_FIELD (NODE)); \
+ FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_0 = (VAL); \
+ } while (0)
/* Used in a FIELD_DECL to indicate that we cannot form the address of
this component. This makes it possible for Type-Based Alias Analysis
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