LD: function-level linking

LD: function-level linking

[Tzu-Chien Chiu]

Hello all.

I'm looking for a similar feature available in Visual C++.
Function-level linking.

In Visual C++, there is an option "/Gy" which enables "function-level
linking". That is, each function is packaged separately as a COMDAT,
and later the linker option "/OPT:REF" eliminates the COMDATs which
are never referenced.

FYI: http://msdn2.microsoft.com/en-us/library/bxwfs976.aspx

In this way, the entire object file won't have to be linked as a whole
when only one of the functions in it is referenced. It's preferred
linking for embedded system software. Not all libraries are design ed
as glibc, which has a separate file for each file, and it's sometimes
not easy or feasible to modify the source code of the libraries.

I can not find similar compiler and linker options in the GNU tool
chain. What I missed?

-- 
Tzu-Chien Chiu - SMedia Technology Corp.
URL: http://www.csie.nctu.edu.tw/~jwchiu/

Re: LD: function-level linking

[Thiemo Seufer]

Tzu-Chien Chiu wrote:
> Hello all.
> 
> I'm looking for a similar feature available in Visual C++.
> Function-level linking.
> 
> In Visual C++, there is an option "/Gy" which enables "function-level
> linking". That is, each function is packaged separately as a COMDAT,
> and later the linker option "/OPT:REF" eliminates the COMDATs which
> are never referenced.
> 
> FYI: http://msdn2.microsoft.com/en-us/library/bxwfs976.aspx
> 
> In this way, the entire object file won't have to be linked as a whole
> when only one of the functions in it is referenced. It's preferred
> linking for embedded system software. Not all libraries are design ed
> as glibc, which has a separate file for each file, and it's sometimes
> not easy or feasible to modify the source code of the libraries.
> 
> I can not find similar compiler and linker options in the GNU tool
> chain. What I missed?

There's gcc's -ffunction-sections, and ld's -Wl,--gc-sections.


Thiemo

Re: LD: function-level linking

[Tzu-Chien Chiu]

I got a warning from ld: "gc-sections option ignored."

The compilation command:
[quote]
or32-elf-gcc -o a.o -c -ffunction-sections -fdata-sections a.c
[/quote]

The link command is:
[quote]
or32-elf-gcc -o program a.o b.o c.o -lfoo -Wl,-gc-sections, -Wl,-static
[/quote]

I've check the object files by objdump, and each function was in its
own section (.text.function name), so I'm probably sure that the
compiler did the correct job.

According to LD manual (2.16.x):

[quote]
--no-gc-sections
--gc-sections
    Enable garbage collection of unused input sections. It is ignored
on targets that do not support this option.
[/quote]

I'm using a cross gnu-tool-chain, it may not be very well ported. What
should I check if the target (or the toolchain) support this option.

Thank you.



2006/7/10, Thiemo Seufer <ths@networkno.de>:
> Tzu-Chien Chiu wrote:
> > Hello all.
> >
> > I'm looking for a similar feature available in Visual C++.
> > Function-level linking.
> >
> > In Visual C++, there is an option "/Gy" which enables "function-level
> > linking". That is, each function is packaged separately as a COMDAT,
> > and later the linker option "/OPT:REF" eliminates the COMDATs which
> > are never referenced.
> >
> > FYI: http://msdn2.microsoft.com/en-us/library/bxwfs976.aspx
> >
> > In this way, the entire object file won't have to be linked as a whole
> > when only one of the functions in it is referenced. It's preferred
> > linking for embedded system software. Not all libraries are design ed
> > as glibc, which has a separate file for each file, and it's sometimes
> > not easy or feasible to modify the source code of the libraries.
> >
> > I can not find similar compiler and linker options in the GNU tool
> > chain. What I missed?
>
> There's gcc's -ffunction-sections, and ld's -Wl,--gc-sections.
>
>
> Thiemo
>


-- 
Tzu-Chien Chiu - SMedia Technology Corp.
URL: http://www.csie.nctu.edu.tw/~jwchiu/

Re: LD: function-level linking

[Thiemo Seufer]

Tzu-Chien Chiu wrote:
> I got a warning from ld: "gc-sections option ignored."
> 
> The compilation command:
> [quote]
> or32-elf-gcc -o a.o -c -ffunction-sections -fdata-sections a.c
> [/quote]
> 
> The link command is:
> [quote]
> or32-elf-gcc -o program a.o b.o c.o -lfoo -Wl,-gc-sections, -Wl,-static
> [/quote]

You missed the second dash in front of --gc-sections.


Thiemo

Re: LD: function-level linking

[Tzu-Chien Chiu]

According to the manual at:

http://sourceware.org/binutils/docs-2.16/ld/Options.html#Options

[quote]
For options whose names are multiple letters, either one dash or two
can precede the option name; ...
[/quote]

By the way, I tested both (one and two dashes), ld complained both anway.


2006/7/10, Thiemo Seufer <ths@networkno.de>:
> Tzu-Chien Chiu wrote:
> > I got a warning from ld: "gc-sections option ignored."
> >
> > The compilation command:
> > [quote]
> > or32-elf-gcc -o a.o -c -ffunction-sections -fdata-sections a.c
> > [/quote]
> >
> > The link command is:
> > [quote]
> > or32-elf-gcc -o program a.o b.o c.o -lfoo -Wl,-gc-sections, -Wl,-static
> > [/quote]
>
> You missed the second dash in front of --gc-sections.
>
>
> Thiemo
>


-- 
Tzu-Chien Chiu - SMedia Technology Corp.
URL: http://www.csie.nctu.edu.tw/~jwchiu/

Re: LD: function-level linking

[Tzu-Chien Chiu]

I didn't port that binutils, and I'm neither a bintuil port expert,
but probably I should check some files in binutils to see if this port
_support_ GC sections?

The obj format is ELF.


FYI:
I'm using OpenRISC tool-chain, an open source RISC CPU.
http://www.opencores.org/pnews.cgi/list/or1k?no_loop=yes


2006/7/10, Tzu-Chien Chiu <tzuchien.chiu@gmail.com>:
> According to the manual at:
>
> http://sourceware.org/binutils/docs-2.16/ld/Options.html#Options
>
> [quote]
> For options whose names are multiple letters, either one dash or two
> can precede the option name; ...
> [/quote]
>
> By the way, I tested both (one and two dashes), ld complained both anway.
>
>
> 2006/7/10, Thiemo Seufer <ths@networkno.de>:
> > Tzu-Chien Chiu wrote:
> > > I got a warning from ld: "gc-sections option ignored."
> > >
> > > The compilation command:
> > > [quote]
> > > or32-elf-gcc -o a.o -c -ffunction-sections -fdata-sections a.c
> > > [/quote]
> > >
> > > The link command is:
> > > [quote]
> > > or32-elf-gcc -o program a.o b.o c.o -lfoo -Wl,-gc-sections, -Wl,-static
> > > [/quote]
> >
> > You missed the second dash in front of --gc-sections.
> >
> >
> > Thiemo
> >
>
>
> --
> Tzu-Chien Chiu - SMedia Technology Corp.
> URL: http://www.csie.nctu.edu.tw/~jwchiu/
>


-- 
Tzu-Chien Chiu - SMedia Technology Corp.
URL: http://www.csie.nctu.edu.tw/~jwchiu/

Re: LD: function-level linking

[Ian Lance Taylor]

"Tzu-Chien Chiu" <tzuchien.chiu@gmail.com> writes:

> I got a warning from ld: "gc-sections option ignored."

That means that your specific backend does not have support for
gc-sections.

> or32-elf-gcc -o a.o -c -ffunction-sections -fdata-sections a.c

You need to add gc_mark_hook and gc_sweep_hook functions to
bfd/elf32-or32.c.

Ian