From e902136b310ee17d4b49eb42d9d5e487d5dcf4a1 Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Fri, 3 Sep 2021 09:46:32 +0200 Subject: [PATCH] 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 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. --- 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 *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