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When a variable's location is expressed as DW_OP_implicit_value, but the given value is longer than needed, which bytes should be used? GDB's current logic was introduced with a patch from 2011 and uses the "least significant" bytes: https://sourceware.org/ml/gdb-patches/2011-08/msg00123.html Now consider a sub-value from such a location at a given offset, accessed through DW_OP_implicit_pointer. Which bytes should be used for that? The patch above *always* uses the last bytes on big-endian targets, ignoring the offset. E.g., given the code snippet const char foo[] = "Hello, world!"; const char *a = &foo[0]; const char *b = &foo[7]; assume that `foo' is described as DW_OP_implicit_value and `a' and `b' each as DW_OP_implicit_pointer into that value. Then with current GDB `*a' and `*b' yield the same result -- the string's zero terminator. This patch basically reverts the portion of the patch above that deals with DW_OP_implicit_value. This fixes the offset handling and also goes back to dropping the last instead of the first bytes on big-endian targets if the implicit value is longer than needed. The latter aspect of the change probably doesn't matter for actual programs, but simplifies the logic. The patch also cleans up the original code a bit and adds appropriate test cases. gdb/testsuite/ChangeLog: * gdb.dwarf2/dw2-op-stack-value.exp: Adjust expected result of taking a 2-byte value out of a 4-byte DWARF implicit value on big-endian targets. * gdb.dwarf2/nonvar-access.exp: Add more comments to existing logic. Add test cases for DW_OP_implicit. gdb/ChangeLog: * dwarf2loc.c (dwarf2_evaluate_loc_desc_full): For DWARF_VALUE_LITERAL, no longer ignore the offset on big-endian targets. And if the implicit value is longer than needed, extract the first bytes instead of the "least significant" ones. |
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.