Re: [PATCH] Support SHF_GNU_RETAIN ELF section flag

[Jozef Lawrynowicz <jozef.l@mittosystems.com>]

On Tue, Sep 29, 2020 at 01:18:31PM +0000, Michael Matz wrote:
> Hello,
> 
> On Mon, 28 Sep 2020, Pedro Alves wrote:
> 
> > >>> The overall intention for this new flag is to enable a new "retain"
> > >>> attribute to be applied to declarations of functions and data in the
> > >>> source code. This attribute can be used to ensure the definition
> > >>> associated with the declaration is present in the linked output file,
> > >>> even if linker garbage collection would normally remove the containing
> > >>> section because it is unused.
> > >>
> > >> On a high level, this sounds pretty much like __attribute__((used)).
> > >> Couldn't the new section flag be wired to that attribute?  
> > >>
> > >> I mean, isn't it a bug if linker garbage collection eliminates a
> > >> function marked with __attribute__((used)) ?
> > >>
> > >>  "used
> > >>
> > >>    This attribute, attached to a function, means that code must be emitted 
> > >>    for the function even if it appears that the function is not referenced."
> > >>
> > >> I was surprised to not see any mention of the "used" attribute in the
> > >> proposal, neither here, nor in gABI mailing list discussion linked.
> > >> But maybe I missed it.
> > > 
> > > In an early version of the implementation, I tried tying the
> > > behavior to the "used" attribute, but it caused some problems.
> > > 
> > > I believe the issues mainly came from the usage of "used" in
> > > libgcc/crtstuff.c. The functions there are static and unused, so have
> > > "used" applied to prevent removal by the compiler. However, that does
> > > not mean that all of those functions should be included in every program
> > > linking against libgcc.
> > > 
> > > I don't remember the exact failure mode, I think it may have been that
> > > lots of tests were failing due to "multiple definition of ..." errors,
> > > or perhaps "undefined reference to ...".
> > 
> > That's really strange, and to be honest, not very convincing.
> > 
> > If there are multiple static functions with the same name in the same
> > translation unit, when you should be getting errors at compile time.
> > 
> > If OTOH, your early implementation resulted in undefined references,
> > then it just sounds like a bug in your implementation, since as you say,
> > __attribute__((retain)) is a superset of __attribute__((used)).  As in,
> > undefined references suggests the function wasn't emitted, contrary to
> > the point of the attribute.
> > 
> > > 
> > > The "retain" attribute implies the "used" attribute, so if the user
> > > wants to prevent compiler optimization AND linker optimization, "retain"
> > > can be used. To prevent only compiler optimization, "used" should be
> > > applied.
> > 
> > I'm not convinced such a distinction makes sense.  Maybe a small self
> > contained use case where the distinction makes a difference and is
> > desirable would help.
> 
> We should remember that this thread is about the addition of the section 
> flag, which affects but isn't directly related to how it's going to be 
> used in source code in a high level language.  If it's the pre-existing 
> "used" or a new "retain" attribute, or an attribute at all or just left to 
> section markers or pragmas: a discussion about that doesn't need to hold 
> up the addition of the ELF feature.

Yep, thanks Michael.

I'm working on the GCC implementation at the moment, I'll review what
happens if "used" implies SHF_GNU_RETAIN before I submit to gcc-patches,
and that aspect of the implementation can be discussed there.

Jozef

> 
> 
> Ciao,
> Michael.