[RFC] Removing DefinedSynthetic

[Rui Ueyama via llvm-commits]

On Wed, Feb 22, 2017 at 10:33 AM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:

>
>
> On February 22, 2017 1:20:44 PM EST, Rui Ueyama <ruiu at google.com> wrote:
> >On Wed, Feb 22, 2017 at 9:43 AM, Rafael Avila de Espindola <
> >rafael.espindola at gmail.com> wrote:
> >
> >> I am looking at https://bugs.llvm.org/show_bug.cgi?id=32031 and there
> >> are a couple of ways to interpret it.
> >>
> >> One way is that we are just computing symbol values too early. When
> >we
> >> do it some output sections still don't have their address set and
> >some
> >> input sections still don't know what output section they will be in.
> >>
> >> The other way to look at it is just that our representation of symbol
> >> values in the linker script is wrong. I started experimenting with
> >using
> >> a ExprValue struct that is similar in idea to a MCValue. It is what
> >Expr
> >> returns and it contains a PointerUnion of an input or output section
> >and
> >> the offset. This allows us to avoid having to check the OutSec field
> >> before it is set and solves the problem.
> >>
> >> But that got me thinking, can we remove DefinedSynthetic completely?
> >> With the addition of a few dummy input sections and potentially a
> >couple
> >> more conversions to SyntheticSection I think we can.
> >>
> >
> >Regular Defined symbols are defined relative to input sections because
> >they
> >are both read from input files. Regular symbols get their addresses
> >when
> >the associated input sections get addresses.
> >
> >DefinedSynthetic is being used to define linker-synthesized symbols
> >that
> >are relative to output sections. _end or _edata are a few examples.
> >
> >So, if you remove DefinedSynthetic, how do you represent _end, for
> >example?
>
> By attaching it to a empty synthetic section at the end of the
> corresponding output section.
>

Not 100% sure, but it should work. The number of places we are using
DefinedSynthetic is limited, so it shouldn't be hard to rewrite it.

But you can update LinkerScript.cpp to verify that your idea works first,
right? Then you can update Writer.cpp to remove DefinedSynthetic if it
works.


> Cheers,
> Rafael
>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>
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