Re: -ffunction-sections, -fdata-sections and -Wl,--gc-sections

On 18/09/2013, at 4:22 PM, Konstantin Belousov <kostikbel_at_gmail.com> wrote:

> On Tue, Sep 17, 2013 at 11:45:19PM +0200, Ed Schouten wrote:
>> [ ... ]
>> Honestly, I think we can assume we'll never reach the point where all
>> the components listed above will properly have all functions
>> partitioned over separate compilation units.
>> 
>> I suspect that it would make a lot of sense to at least enable these
>> build flags for our core libraries (libc, libc++, libpthread,
>> libcompiler_rt, libcxxrt, etc). We could also enable it on
>> INTERNALLIBs (libraries that are not installed into /usr/lib), as for
>> these libraries, it would of course not come at any cost.
>> 
>> Would that sound okay?
> 
> I think this is a wrong direction. First, the split should be done at
> the source level, as it was usually done forever. One of the offender
> there was you, AFAIR.
> 
> Second, I would rather see init and devd, and in fact all other statically
> linked binaries from our base system, to become dynamically linked.  At
> least I added a knob for building toolchain dynamic, but avoided the
> fight of making this default.

Why do things by hand when there are good tools? Note "... as it was usually done forever" doesn't contain a good argument, and compilers and linkers on other platforms have been doing it like this for an awfully long time.

Adding the flags has a benefit in the case where there are many functions in a source file and minimal cost when everything is perfect. Not having the flags means paying a bigger price when things are not perfect. And things are very rarely perfect.

Having the structure of your source code driven by link-time considerations when there is a choice seems silly to me. Larger source files gives the compiler more scope for optimisation, and you can structure the code in a way useful to people working in the codebase.

If you have a moral argument about how code should be structured, I think that is separate discussion. Adding the flags has a benefit, regardless of how the code is structured. I can see all upside, and I am having trouble seeing a problem with adding them at all.

On the static linking vs. dynamic linking argument: I am strongly on the static linking side. But that is also a different discussion.

Regards,

Jan Mikkelsen