[systemd-devel] [PATCH] build-sys: remove --gc-sections to fix debugging

[David Herrmann]

Hi

On Thu, Nov 27, 2014 at 6:35 PM, Peter Wu <peter at lekensteyn.nl> wrote:
> On Thursday 27 November 2014 13:42:27 Gustavo Sverzut Barbieri wrote:
>> I'm asking because from Peter Wu's email we're 26384 - 26380 = 4Kb
>> heavier in the whole installed set of binaries. If this is correct I
>> doubt the gc-sections is making any difference in the linkage of
>> static libraries (src/shared). In this case my *assumption* (as I have
>> no deep knowledge about the linker) is that it's selectively picking
>> the symbols it uses and leaving the other stuff out of it.
>>
>> Peter, could you provide more in-depth information, for instance you
>> could see:
>>         - what binaries are bigger;
>>         - what -Wl,--gc-sections -Wl,--print-gc-section says
>
> OK, here is a description of the test setup such you can reproduce it.
> Environment:
>  - Arch Linux x86_64 in a QEMU VM (live iso with systemd makedepends
>    installed).
>  - with systemd git in /tmp/systemd.
>  - master is at 9a20fcbcd1b010ad88bfbb8b7f0417bec7327fb4
>  - gcc 4.9.2
>  - binutils 2.24-8 (the broken one)
>
> Commands to build two versions:
>
>     cd /tmp/systemd
>     # Currently at 9a20fcbcd1b010ad88bfbb8b7f0417bec7327fb4 + patch to
>     # remove --gc-sections and -f{data,function}-sections
>     git clean -xfd && ./autogen.sh && mkdir build && cd $_ &&
>     ../configure && make -j6 && make install DESTDIR=$PWD/fs &&
>     (cd fs && find * -type f -printf '%h/%f\t%s\n') > /tmp/0.txt
>
>     # Now let's see what happens with --gc-sections
>     git checkout HEAD~ &&
>     git clean -xfd && ./autogen.sh && mkdir build && cd $_ &&
>     ../configure && make -j6 && make install DESTDIR=$PWD/fs &&
>     (cd fs && find * -type f -printf '%h/%f\t%s\n') > /tmp/1.txt
>
> Now build a table to compare the sizes:
>
>     join {0,1}.txt | awk '$2!=$3{print $2,$3,$2-$3,$2/$3,$1}' |
>     sort -k4n | column -t > results.txt
>
> The columns explained:
>  1. Size of binaries built without --gc-sections and -f...
>  2. Size of binaries built with --gc-sections and -f...
>  3. Absolute "saved" bytes when built with --gc-sections.
>  4. Ratio between the sizes, numbers smaller than 1 indicate a saving,
>     numbers larger than 1 show that --gc-sections increase the sizes.
>
> These results are strange, it suggests that --gc-sections make the
> binaries larger overall! I have repeated it three times just to make
> sure that I did not made a mistake.

Of course it gets bigger if every symbol is placed into it's own
section. It's -ffunction-section and -fdata-section that make it
bigger. But those are required for --gc-sections to work.

Btw., you run with LTO, which makes --gc-sections obsolete. But we
don't enable LTO for debug builds and I think distributions may
disable it, too. Until we decide to rely on LTO, I'd strongly suggest
keeping our --gc-sections + function/data-section. Either LTO _or_
--gc-sections is required!

My numbers were generated via debug-builds, which explicitly disables LTO.

I'd be fine with a patch that drops --gc-sections and friends if LTO
is enabled. But that requires us to check for LTO during ./configure
and really make sure we know that it's enabled. So far, we just enable
it if it's available, but never really check whether it was enabled.

Thanks
David