When generating binaries, gcc will always add information of what it
calls "the exception handler framework" into its own section: .eh_frame.
This section is based on the DWARF format's call frame information (CFI) [1]
and holds information that can be useful for debuggers but also for language
constructs that relies on always having stack unwinding information (i.e. exceptions).
Such constructs, however, are pretty much useless for the C language and are
mainly just used on C++. Furthermore, this section is one of the loadable sections
of a binary, meaning it will take extra space on flash.
When .eh_frame is not present, debuggers can still get the exact same information
they need for unwinding a stack frame and for restoring registers thanks to yet
another section: .debug_frame. This section is generated by '-g' gcc option and
friends. It is actually defined by DWARF and, as opposed to .eh_frame, is not a
loadable section. In other words, it is 'strippable' while .eh_frame is not.
Since all we need is the debug information we can get from .debug_frame, we can
disable the generation of these large and unused information tables by using gcc's
'-fno-asynchronous-unwind-tables'. The .eh_frame section stays around but the code
size issue is heavily tackled. This is the same approach taken on other projects
that target small code size generation [2] [3].
Pratically speaking, on a DEBUG build of the all-timers appplication, before this
patch we had:
text data bss dec hex filename
21319 1188 12952 35459 8a83 all-timers.galileo
And now, after this patch:
text data bss dec hex filename
16347 1188 12952 30487 7717 all-timers.galileo
This means a ~5Kb reduction on the loadable text segment (.text + .rodata + .eh_frame).
The flag is applied regardless of build type, DEBUG or RELEASE, since it benefits both.
Note that when release builds apply --gc-sections, they will remove .eh_frame section entirely.
[1] http://comments.gmane.org/gmane.comp.standards.dwarf/222
[2] 0d74ad383b
[3] http://git.musl-libc.org/cgit/musl/commit/?id=b439c051c7eee4eb4b93fc382f993aa6305ce530
[4] https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>
This patch slightly revises CFLAGS and LDFLAGS to specify the
optimization and debugging options and linker script in a way that is
compatible with using Clang as the C compiler and to invoke the linker
(i.e. CC = clang and LD = clang).
This patch appends some gcc options to CFLAGS when building the default
image in order to improve the debugging experience on GDB.
We use the '-ggdb' option which produces debugging information used by
GDB (including GDB extensions) with level 3 which includes preprocessor
macros information. We also use '-Og' which enables optimizations that
do not interfere with debugging. According to gcc manpage, it should be
the optimization level of choice for the standard edit-compile-debug
cycle, offering a reasonable level of optimization while maintaining
fast compilation and a good debugging experience.
Also, this patch removes the '-g' option from the default CFLAGS because
there is no point in using it when BUILD_RELEASE=1.
As expected, the overall ELF image increases (due to -ggdb3 option) while
the .text section is reduced (due to -Og). For the sake of comparison,
below follows the output of 'size'.
Before patch:
$ size -A hello-world.galileo
hello-world.galileo :
section size addr
.text 13766 1048576
.rodata 241 1064960
.eh_frame 5160 1065204
.eh_frame_hdr 1212 1070364
.data 1188 1073152
.bss 12808 1077248
.debug_info 14351 0
.debug_abbrev 6281 0
.debug_aranges 768 0
.debug_line 6443 0
.debug_str 4805 0
.comment 17 0
.note 40 0
.debug_ranges 24 0
Total 67104
After patch:
$ size -A hello-world.galileo
hello-world.galileo :
section size addr
.text 11718 1048576
.rodata 249 1060864
.eh_frame 5496 1061116
.eh_frame_hdr 1204 1066612
.data 1156 1069056
.bss 12808 1073152
.debug_info 16727 0
.debug_abbrev 7254 0
.debug_loc 2083 0
.debug_aranges 768 0
.debug_macro 17273 0
.debug_line 13433 0
.debug_str 42192 0
.comment 17 0
.note 40 0
Total 132418
This patch adds support for building release images. The main difference
between release images and default images is that the former is optimized
for size while the latter is "optimized" for debugging. To build a release
image, the BUILD_RELEASE variable should be set to 1. For instance, the
following command build a release image from the hello-world application:
$ cd examples/hello-world && make TARGET=galileo BUILD_RELEASE=1
To optimize for size we use the '-Os' option from gcc. This option also
enables the strict aliasing optimization. This generates lots of warning
messages since we use the '-Wall' option and lots of code in core/net/
break the strict-aliasing rules. Some test have shown that the strict
aliasing optimization it not taking effect in the final binary. For that
reasons, this patch manually disables the optimization. Also, the release
image is stripped.
For the sake of comparison, below follows the output from 'wc' and 'size'
for both debugging (default) and release images.
Default image:
$ wc -c hello-world.galileo
71112 hello-world.galileo
$ size hello-world.galileo
text data bss dec hex filename
20379 1188 12808 34375 8647 hello-world.galileo
Release image:
$ wc -c hello-world.galileo
26320 hello-world.galileo
$ size hello-world.galileo
text data bss dec hex filename
18146 1156 12808 32110 7d6e hello-world.galileo
This patch does several cleanups in Makefile.x86_common file. The
changes are described above.
1) The CFLAGNO variable was removed since it is used only to assign
the CFLAGS variable. Also, CFLAGNO is not used outside Makefile.x86_
common.
2) The "-I/usr/local/include" option was removed since we provide manually
the include path from newlib in the bsp/ directory.
3) We only support building x86-based platforms on Linux so there is no
point in setting LDFLAGS conditionally.
4) The '-export-dynamic' option was removed from LDFLAGS since we are not
creating a dynamically linked executable.
5) Makefile.x86_quarkX1000 is the only one that includes Makefile.x86_
common. Since it doesn't use the custom rules from Makefile.x86_
common we remove them.
This patch adds "--build-id=none" to default LDFLAGS so
.note.gnu.build-id section is not generated. This section
contains unique identification for the built files what is
not important to us (at least at this moment).
This change simplifies all linker scripts for SoCs based on x86
(at this moment we only have Quark X1000) since we don't have to
care about it anymore.
The x86/init/common/ folder holds all cpu initialization
code - idt and gdt setup, interrupts and cpu initialization.
On this folder will also sit any SoC specific implementation of
the functions called from cpu_init().
Now the cpu/x86/ provides a Makefile.x86_common and a
Makefile.x86_pc. The former includes the common Makefile
and adds legacy pc specific implementations (currently,
drivers only) into the building context, while the latter
has everything that defines the bootstrap of a x86 CPU.
This commit also fixes platform/galileo/ so it includes the
correct makefile - Makefile.x86_quarkX1000. Galileo uses
a Quark X1000 SoC which is not an IBM Generic PC-like CPU,
but it does provide most of a PCs peripherals through
its "Legacy Bridge". Thus, it makes sense that QuarkX1000's
Makefile includes code from the legacy_pc x86 cpu.
