From 420d030e88d2c8bbad6044278000fbd9efbf65d9 Mon Sep 17 00:00:00 2001 From: Tom Tromey Date: Fri, 18 Apr 2025 09:28:13 -0600 Subject: Handle field with dynamic bit offset I discovered that GCC emitted incorrect DWARF for the test case included in this patch. Eric wrote a fix for GCC, but then he found that gdb crashed on the resulting file. This test has a field that is at a non-constant bit offset from the start of the type. DWARF 5 does not allow for this situation (I've sent a report to the DWARF list), but DWARF 3 did allow for this via a combination of an expression for the byte offset and then the use of DW_AT_bit_offset. This looks like: <5><117a>: Abbrev Number: 17 (DW_TAG_member) <117b> DW_AT_name : (indirect string, offset: 0x1959): another_field ... <1188> DW_AT_bit_offset : 6 <1189> DW_AT_data_member_location: 6 byte block: 99 3d 1 0 0 22 (DW_OP_call4: <0x1193>; DW_OP_plus) ... <3><1193>: Abbrev Number: 2 (DW_TAG_dwarf_procedure) <1194> DW_AT_location : 15 byte block: 97 94 1 37 1a 32 1e 23 7 38 1b 31 1c 23 3 (DW_OP_push_object_address; DW_OP_deref_size: 1; DW_OP_lit7; DW_OP_and; DW_OP_lit2; DW_OP_mul; DW_OP_plus_uconst: 7; DW_OP_lit8; DW_OP_div; DW_OP_lit1; DW_OP_minus; DW_OP_plus_uconst: 3) Now, that combination is not fully general, in that the bit offset must be a constant -- only the byte offset may really vary. However, I couldn't come up with a situation where full generality is needed, mainly because GNAT won't seem to pack fields into the padding of a variable-length array. Meanwhile, the reason for the gdb crash is that the code handling DW_AT_bit_offset assumes that the byte offset is a constant. This causes an assertion failure. This patch arranges for DW_AT_bit_offset to be applied during field resolution, when needed. --- gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb | 45 +++++++++++++++++++++++++++ 1 file changed, 45 insertions(+) create mode 100644 gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb (limited to 'gdb/testsuite/gdb.ada/dyn-bit-offset') diff --git a/gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb b/gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb new file mode 100644 index 0000000..a882afd --- /dev/null +++ b/gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb @@ -0,0 +1,45 @@ +-- Copyright 2025 Free Software Foundation, Inc. +-- +-- This program is free software; you can redistribute it and/or modify +-- it under the terms of the GNU General Public License as published by +-- the Free Software Foundation; either version 3 of the License, or +-- (at your option) any later version. +-- +-- This program is distributed in the hope that it will be useful, +-- but WITHOUT ANY WARRANTY; without even the implied warranty of +-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +-- GNU General Public License for more details. +-- +-- You should have received a copy of the GNU General Public License +-- along with this program. If not, see . + +procedure Exam is + type Small is range -7 .. -4; + for Small'Size use 2; + + type Packed_Array is array (Integer range <>) of Small; + pragma pack (Packed_Array); + + subtype Range_Int is Natural range 0 .. 7; + + type Some_Packed_Record (Discr : Range_Int := 3) is record + Array_Field : Packed_Array (1 .. Discr); + Field: Small; + case Discr is + when 3 => + Another_Field : Small; + when others => + null; + end case; + end record; + pragma Pack (Some_Packed_Record); + pragma No_Component_Reordering (Some_Packed_Record); + + SPR : Some_Packed_Record := (Discr => 3, + Field => -4, + Another_Field => -6, + Array_Field => (-5, -6, -7)); + +begin + null; -- STOP +end Exam; -- cgit v1.1