When generating notoc call and branch stubs without the benefit of
pc-relative insns, the stubs need to use LR to access the run time PC.
All LR changes must be described in .eh_frame if we're to support
unwinding through asynchronous exceptions. That's what this patch
does.
The patch has gone through way too many iterations. At first I
attempted to add multiple FDEs, one for each stub. That ran into
difficulties with do_plt_fde_location which is only capable of setting
the address of a single FDE per Output_data section, and with removing
any FDEs added on a previous do_relax pass. Removing FDEs (git commit
be897fb774) went overboard in matching the FDE contents. That means
either stashing the contents created for add_eh_frame_for_plt to use
when calling remove_eh_frame_for_plt, or recreating contents on the
fly (*) just to remove FDEs. In fact, FDE content matching is quite
unnecesary. FDEs added by a previous do_relax pass are those with
u_.from_linker.post_map set. So they can easily be recognised just by
looking at that flag. This patch keeps that part of the multiple FDE
changes.
In the end I went for just one FDE per stub group to describe the call
stubs. That's reasonably efficient for the common case of only
needing to describe the __tls_get_addr_opt call stub. We don't expect
to be making many calls using notoc stubs without pc-relative insns.
*) Which has it's own set of problems. The contents must be recreated
using the old stub layout, but .eh_frame size can affect stub
requirements so you need to temporarily keep the old .eh_frame size
when creating new stubs, then reset .eh_frame size before adding new
FDEs.
* ehframe.cc (Fde::operator==): Delete.
(Cie::remove_fde): Delete.
(Eh_frame::remove_ehframe_for_plt): Delete fde_data and fde_length
parameters. Remove all post-map plt FDEs.
* ehframe.h (Fde:post_map): Make const, add variant to compare plt.
(Fde::operator==): Delete.
(Cie::remove_fde): Implement here.
(Cie::last_fde): New accessor.
(Eh_frame::remove_ehframe_for_plt): Update prototype.
* layout.cc (Layout::remove_eh_frame_for_plt): Delete fde_data and
fde_length parameters.
* layout.h (Layout::remove_eh_frame_for_plt): Update prototype.
* powerpc.cc (Stub_table::tls_get_addr_opt_bctrl_): Delete.
(Stub_table::plt_fde_len_, plt_fde_, init_plt_fde): Delete.
(Stub_table::add_plt_call_entry): Don't set tls_get_addr_opt_bctrl_.
(eh_advance): New function.
(stub_sort): New function.
(Stub_table::add_eh_frame): Emit eh_frame for notoc plt calls and
branches as well as __tls_get_addr_opt plt call stub.
(Stub_table::remove_eh_frame): Update to suit.
This file contains invisible Unicode characters that are indistinguishable to humans but may be processed differently by a computer. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.
gold is an ELF linker. It is intended to have complete support for
ELF and to run as fast as possible on modern systems. For normal use
it is a drop-in replacement for the older GNU linker.
gold is part of the GNU binutils. See ../binutils/README for more
general notes, including where to send bug reports.
gold was originally developed at Google, and was contributed to the
Free Software Foundation in March 2008. At Google it was designed by
Ian Lance Taylor, with major contributions by Cary Coutant, Craig
Silverstein, and Andrew Chatham.
The existing GNU linker manual is intended to be accurate
documentation for features which gold supports. gold supports most of
the features of the GNU linker for ELF targets. Notable
omissions--features of the GNU linker not currently supported in
gold--are:
* MRI compatible linker scripts
* cross-reference reports (--cref)
* various other minor options
Notes on the code
=================
These are some notes which may be helpful to people working on the
source code of gold itself.
gold is written in C++. It is a GNU program, and therefore follows
the GNU formatting standards as modified for C++. Source documents in
order of decreasing precedence:
http://www.gnu.org/prep/standards/
http://gcc.gnu.org/onlinedocs/libstdc++/manual/source_code_style.html
http://www.zembu.com/eng/procs/c++style.html
The linker is intended to have complete support for cross-compilation,
while still supporting the normal case of native linking as fast as
possible. In order to do this, many classes are actually templates
whose parameter is the ELF file class (e.g., 32 bits or 64 bits). The
C++ code is the same, but we don't pay the execution time cost of
always using 64-bit integers if the target is 32 bits. Many of these
class templates also have an endianness parameter: true for
big-endian, false for little-endian.
The linker is multi-threaded. The Task class represents a single unit
of work. Task objects are stored on a single Workqueue object. Tasks
communicate via Task_token objects. Task_token objects are only
manipulated while holding the master Workqueue lock. Relatively few
mutexes are used.
Build requirements
==================
The gold source code uses templates heavily. Building it requires a
recent version of g++. g++ 4.0.3 and 4.1.3 are known to work. g++
3.2, 3.4.3, and 4.1.2 are known to fail.
The linker script parser uses features which are only in newer
versions of bison. bison 2.3 is known to work. bison 1.26 is known
to fail. If you are building gold from an official binutils release,
the bison output should already be included.
Copyright (C) 2012-2019 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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