forked from Imagelibrary/binutils-gdb
21b25b168d
Internal AdaCore testing using -gdwarf-4 found a spot where GCC will
emit a negative DW_AT_bit_offset. However, my recent signed/unsigned
changes assumed that this value had to be positive.
I feel this bug somewhat invalidates my previous thinking about how
DWARF attributes should be handled.
In particular, both GCC and LLVM at understand that a negative bit
offset can be generated -- but for positive offsets they might use a
smaller "data" form, which is expected not to be sign-extended. LLVM
has similar code but GCC does:
if (bit_offset < 0)
add_AT_int (die, DW_AT_bit_offset, bit_offset);
else
add_AT_unsigned (die, DW_AT_bit_offset, (unsigned HOST_WIDE_INT) bit_offset);
What this means is that this attribute is "signed but default
unsigned".
To fix this, I've added a new attribute::confused_constant method.
This should be used when a constant value might be signed, but where
narrow forms (e.g., DW_FORM_data1) should *not* cause sign extension.
I examined the GCC and LLVM DWARF writers to come up with the list of
attributes where this applies, namely DW_AT_bit_offset,
DW_AT_const_value and DW_AT_data_member_location (GCC only, but LLVM
always emits it as unsigned, so we're safe here).
This patch corrects the bug and imports the relevant test case.
Regression tested on x86-64 Fedora 41.
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680
Bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118837
Approved-By: Simon Marchi <simon.marchi@efficios.com>
37 lines
1.2 KiB
Plaintext
37 lines
1.2 KiB
Plaintext
# Copyright 2025 Free Software Foundation, Inc.
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#
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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# Test negative DW_AT_bit_offset.
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load_lib "ada.exp"
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require allow_ada_tests
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standard_ada_testfile prog
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# This particular output is only generated with -gdwarf-4.
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if {[gdb_compile_ada "${srcfile}" "${binfile}" executable \
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{debug additional_flags=-gdwarf-4}] != ""} {
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return
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}
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clean_restart ${testfile}
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set bp_location [gdb_get_line_number "STOP" ${testdir}/prog.adb]
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runto "prog.adb:$bp_location"
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gdb_test "print xp" \
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[string_to_regexp "(x => 21, y => (-1, -2, -3, -4, -5, -6, -7, -8, -9, -10))"]
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