What a trip down a rabbit hole this bug has been.
First observation: You can't use deferred_expression in s_leb128.
deferred_expression implements the semantics of .eqv or '==', saving
an expression with minimal simplification for assignment to a symbol
so that the expression is evaluated at uses of the symbol. In
particular, the value of "dot" is not evaluated at the .eqv symbol
assignment, but later. When s_leb128 uses deferred_expression,
"later" is at the end of assembly, giving entirely the wrong value of
"dot". There is no way to fix this for the s_leb128 use without
breaking .equ (which incidentally was already somewhat broken, see
commit e4c2619ad1). So, don't use deferred_expression in s_leb128.
But that leads to the gas test elf/dwarf2-17 failing, because view
symbols are calculated with a chain of expression symbols. In the
dwarf2-17 .L1 case there is a "temp_sym_1 > temp_sym_2" expression,
with temp_sym_1 and temp_sym_2 on either side of a ".balign". Since
".balign" and many other directives moving "dot" are not calculated on
the first (and only) pass over source, .L1 cannot be calculated until
final addresses are assigned to frags. However, ".uleb128 .L1" *is*
calculated immediately, resulting in the wrong value.
The reason why .L1 is calculated immediately is that code in
expr.c:operand after the comment
/* If we have an absolute symbol or a reg, then we know its
value now. */
does as it says and fixes the value of .L1, because .L1 is assigned
to absolute_section in dwarf2dbg.c:set_or_check_view. So, correct
that to expr_section.
Unfortunately that fix leads to failure of the elf/dwarf2-5 test with
../gas/elf/dwarf2-5.s: Error: attempt to get value of unresolved symbol `.L5'
../gas/elf/dwarf2-5.s: Error: attempt to get value of unresolved symbol `.L11'
../gas/elf/dwarf2-5.s: Error: attempt to get value of unresolved symbol `.L12'
So why is that? Well, it turns out that .L5 is defined in terms of
.L4, and apparently .L4 is undefined. But .L4 clearly is defined,
otherwise we would hit an error when trying to use .L4 a little
earlier. There are two copies of .L4! So, symbols are cloned when
that should not happen.
Symbol cloning is a technique used by gas to support saving the value
of symbols that change between uses, but that isn't the case with
.L4. Only one value is set and used for .L4, but indeed .L4 was being
cloned by symbol_clone_if_forward_ref. This despite no forward refs
being present. Also, .L4 is a local symbol and a cursory glance at
symbol_clone_if_forward_ref "if (symbolP && !LOCAL_SYMBOL_CHECK (symbolP))"
would seem to prevent cloning of local symbols. All is not as it
seems though, a curse of using macros. LOCAL_SYMBOL_CHECK modifies
its argument if a "struct local_symbol" is converted to the larger
"struct symbol", as happens when assigning a view symbol value.
That fact results in the recursive call to symbol_clone_if_forward_ref
returning a different address for "add_symbol". This problem could
have been fixed by using symbol_same_p rather than comparing symbol
pointers, but I thought it better to use the real symbol throughout.
Note that symbol_find_exact also returns the real symbol for a
converted local symbol.
Finally, this patch does expose lack of support for forward symbol
definitions in various targets. For example:
alpha-linux +ERROR: ../ld/testsuite/ld-elf/pr11138-2.c: compilation failed
This is caused by view symbol uses. On alpha-linux-gcc (GCC) 8.1.1
20180502 they happen to occur in .byte directives so were silently
broken in cases like elf/dwarf2-17 anyway.
/tmp/ccvtsMfU.s: Assembler messages:
/tmp/ccvtsMfU.s: Fatal error: unhandled relocation type BFD_RELOC_8
/tmp/ccvtsMfU.s: Fatal error: unhandled relocation type BFD_RELOC_8
md_apply_fix on those targets needs to handle fixups that resolve down
to a constant.
PR 23040
* symbols.c (get_real_sym): New function.
(symbol_same_p): Use get_real_sym.
(symbol_clone_if_forward_ref): Save real original add_symbol and
op_symbol for comparison against that returned from lookup or
recursive calls.
* dwarf2dbg.c (set_or_check_view): Use expr_section for
expression symbols, not absolute_section.
(dwarf2_directive_loc): Check symbol_equated_p and tidy cloning
of view symbols.
* read.c (s_leb128): Don't use deferred_expression.
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README for GAS
A number of things have changed since version 1 and the wonderful
world of gas looks very different. There's still a lot of irrelevant
garbage lying around that will be cleaned up in time. Documentation
is scarce, as are logs of the changes made since the last gas release.
My apologies, and I'll try to get something useful.
Unpacking and Installation - Summary
====================================
See ../binutils/README.
To build just the assembler, make the target all-gas.
Documentation
=============
The GAS release includes texinfo source for its manual, which can be processed
into `info' or `dvi' forms.
The DVI form is suitable for printing or displaying; the commands for doing
this vary from system to system. On many systems, `lpr -d' will print a DVI
file. On others, you may need to run a program such as `dvips' to convert the
DVI file into a form your system can print.
If you wish to build the DVI file, you will need to have TeX installed on your
system. You can rebuild it by typing:
cd gas/doc
make as.dvi
The Info form is viewable with the GNU Emacs `info' subsystem, or the
stand-alone `info' program, available as part of the GNU Texinfo distribution.
To build the info files, you will need the `makeinfo' program. Type:
cd gas/doc
make info
Specifying names for hosts and targets
======================================
The specifications used for hosts and targets in the `configure'
script are based on a three-part naming scheme, but some short
predefined aliases are also supported. The full naming scheme encodes
three pieces of information in the following pattern:
ARCHITECTURE-VENDOR-OS
For example, you can use the alias `sun4' as a HOST argument or in a
`--target=TARGET' option. The equivalent full name is
`sparc-sun-sunos4'.
The `configure' script accompanying GAS does not provide any query
facility to list all supported host and target names or aliases.
`configure' calls the Bourne shell script `config.sub' to map
abbreviations to full names; you can read the script, if you wish, or
you can use it to test your guesses on abbreviations--for example:
% sh config.sub i386v
i386-unknown-sysv
% sh config.sub i786v
Invalid configuration `i786v': machine `i786v' not recognized
`configure' options
===================
Here is a summary of the `configure' options and arguments that are
most often useful for building GAS. `configure' also has several other
options not listed here.
configure [--help]
[--prefix=DIR]
[--srcdir=PATH]
[--host=HOST]
[--target=TARGET]
[--with-OPTION]
[--enable-OPTION]
You may introduce options with a single `-' rather than `--' if you
prefer; but you may abbreviate option names if you use `--'.
`--help'
Print a summary of the options to `configure', and exit.
`-prefix=DIR'
Configure the source to install programs and files under directory
`DIR'.
`--srcdir=PATH'
Look for the package's source code in directory DIR. Usually
`configure' can determine that directory automatically.
`--host=HOST'
Configure GAS to run on the specified HOST. Normally the
configure script can figure this out automatically.
There is no convenient way to generate a list of all available
hosts.
`--target=TARGET'
Configure GAS for cross-assembling programs for the specified
TARGET. Without this option, GAS is configured to assemble .o files
that run on the same machine (HOST) as GAS itself.
There is no convenient way to generate a list of all available
targets.
`--enable-OPTION'
These flags tell the program or library being configured to
configure itself differently from the default for the specified
host/target combination. See below for a list of `--enable'
options recognized in the gas distribution.
`configure' accepts other options, for compatibility with configuring
other GNU tools recursively; but these are the only options that affect
GAS or its supporting libraries.
The `--enable' options recognized by software in the gas distribution are:
`--enable-targets=...'
This causes one or more specified configurations to be added to those for
which BFD support is compiled. Currently gas cannot use any format other
than its compiled-in default, so this option is not very useful.
`--enable-bfd-assembler'
This causes the assembler to use the new code being merged into it to use
BFD data structures internally, and use BFD for writing object files.
For most targets, this isn't supported yet. For most targets where it has
been done, it's already the default. So generally you won't need to use
this option.
Compiler Support Hacks
======================
On a few targets, the assembler has been modified to support a feature
that is potentially useful when assembling compiler output, but which
may confuse assembly language programmers. If assembler encounters a
.word pseudo-op of the form symbol1-symbol2 (the difference of two
symbols), and the difference of those two symbols will not fit in 16
bits, the assembler will create a branch around a long jump to
symbol1, and insert this into the output directly before the next
label: The .word will (instead of containing garbage, or giving an
error message) contain (the address of the long jump)-symbol2. This
allows the assembler to assemble jump tables that jump to locations
very far away into code that works properly. If the next label is
more than 32K away from the .word, you lose (silently); RMS claims
this will never happen. If the -K option is given, you will get a
warning message when this happens.
REPORTING BUGS IN GAS
=====================
Bugs in gas should be reported to:
bug-binutils@gnu.org.
They may be cross-posted to gcc-bugs@gnu.org if they affect the use of
gas with gcc. They should not be reported just to gcc-bugs, since not
all of the maintainers read that list.
See ../binutils/README for what we need in a bug report.
Copyright (C) 2012-2018 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
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