Re: RFC: Sort input section by list

[Arjan van de Ven <arjan at linux dot intel dot com>]

Christophe LYON wrote:
> > > What is the real purpose of this feature ?  Ie do you know why it has 
> > > been requested ?  The implication of the example you have given is 
> > > that the user wants to put certain name-mangled functions at the 
> > > start of the .text section, but why ?
> >
> > because it allows the developer to put the commonly used functions at 
> > the start of the file,
> > improving density in the page cache for example, and generally 
> > improving startup time.
> > (less pagefaults and less TLB pressure)
>
> Why not have a tool dedicated to this kind of optimization?
> For one of our targets, we have developed a post linker tool which 
> reorders functions based on call graph analysis.

cool! is this downloadable somewhere ? ;)
> The compiler driver asks the linker to keep all the relocations in the 
> generated executable, and then calls the optimizer, which parses the ELF 
> file and optionally a "layout file" to reorder code. By default it is 
> used to improve locality (we have a direct mapped cache) based on static 
> analysis, but can take advantage of profiling, or the user can even 
> force the function order.
>
> If the purpose is really to reorder functions, why should the developer 
> need to deal with sections? (I mean, not all developers master the 
> concept of section, do they?)

this is what -ffunction-sections is for

there are some complications like.. does the linker know what the end of a function is?
With tail packing but without sections.. I kind of fear the answer to be "no" ;(