Re: [PATCH] configure: add gcc flags for better link-time optimization

[Denys Vlasenko <vda.linux@googlemail.com>]

On Fri, Oct 23, 2015 at 3:12 PM, Szabolcs Nagy <nsz@port70.net> wrote:
>> +# When linker merges sections, a tiny section (such as one resulting
>> +# from "static char flag_var") with no alignment restrictions
>> +# can end up logded between two more strongly aligned ones (say,
>> +# "static int global_cnt1/2", both of which want 32-bit alignment).
>> +# Then this byte-sized "flag_var" gets 3 bytes of padding.
>> +#
>> +# With section sorting by alignment, one-byte flag variables have
>> +# higher chance of being grouped together and not require padding.
>> +# (It can be made even better. Linker is too dumb.
>> +# ld needs to grow -Wl,--pack-sections-optimally)
>> +#
>> +# For us, this affects the size of only one file: libc.so
>> +#
>> +tryldflag LDFLAGS_AUTO -Wl,--sort-section=alignment
>> +tryldflag LDFLAGS_AUTO -Wl,--sort-common
>
> i think this came up before
> https://sourceware.org/bugzilla/show_bug.cgi?id=14156
>
> it was also noted at some point that the optimal sorting
> is 'sort by use' so all the unused legacy functions end
> up on the same page so they never need to be loaded.

Sure, but that would be quite hard to do.
How would you reliably know who uses which part of libc
code?

OTOH, we don't _need_ to kill ourselves trying to optimize
that. Optimizing code size is not the big thing here.
Even though data and bss shrinkage is smaller,
it is more important.

Minimizing the number of data pages is more important
than text pages. A text page is shared among all processes linked
to this libc.so; data page is allocated in every process
(as soon as even one byte in this page is written to.
With only 4 pages in total like in this example, I'm pretty sure
all of them get dirtied by libc init, use of stdio or malloc).

Make libc (.data + .bss) fit into one page less and you get about
as many pages saved as you have processes running.