[PATCH] Do not use layout-before to layout atoms.

[Rui Ueyama]

On Wed, Mar 26, 2014 at 4:11 PM, Shankar Easwaran
<shankare at codeaurora.org>wrote:

>  On 3/26/2014 5:55 PM, Nick Kledzik wrote:
>
>
>  On Mar 26, 2014, at 3:37 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>   On Wed, Mar 26, 2014 at 3:09 PM, Nick Kledzik <kledzik at apple.com> wrote:
>
>>
>>  On Mar 26, 2014, at 2:56 PM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>   On Wed, Mar 26, 2014 at 2:51 PM, Nick Kledzik <kledzik at apple.com>wrote:
>>
>>>
>>>  On Mar 26, 2014, at 2:44 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>   On Wed, Mar 26, 2014 at 2:23 PM, kledzik at apple.com <kledzik at apple.com>wrote:
>>>
>>>>
>>>>   Given your description of COFF, I'm not sure follow-on
>>>> atom/references is the right model.
>>>>
>>>>   We probably want a way to order sections.  For instance, the darwin
>>>> linker automatically arranges sections for best performance.
>>>>
>>>
>>>  Yes, follow-on atom/references model is not probably the best model
>>> for COFF. It's too fine grained. The unit of layout is not an atom (symbol)
>>> but a section. We never want to rearrange or dead-strip each atom. AFAIK so
>>> is true for ELF and Mach-O.
>>>
>>>
>>>  The darwin linker very much dead strips and re-orders mach-o at the
>>> atom level.
>>>
>>
>>  Maybe silly question, but how are dependencies between symbols (atoms)
>> represented in Mach-O? I mean, if function A in .text have a relative call
>> instructions to function B in the same .text section, both functions needs
>> to be in the final executable keeping the same offset.
>>
>>  The call instruction has a relocation on it, so if the target moves
>> (relatively), the linker updates the call instruction.
>>
>>  In more detail, the linker parses the call instruction and relocation
>> and adds to A, a Reference to B.  Then everything proceeds as an atom
>> graph, including dead stripping and re-ordering.  In the writer, after
>> atoms that remain are assigned addresses, the writer sees the reference in
>> A to B, it knows the address of both, so it fixes up the call instruction
>> given those final addresses.
>>
>
>  Thank you for the explanation. That sounds like a different semantics
> than COFF indeed. In COFF we don't usually have relocations within a
> section, so a section is a basic unit and we can't reorder or remove data
> in it. If you want to reorder and dead-strip each function, you have to
> specify /Gy (equivalent to -ffunction-section) to put each function in its
> own section.
>
>  Do you know if we really use layout-after's in ELF to order atoms? It
> seems to me that the default sorting function orders them correctly,
> according to atom's ordinal, file ordinal, etc. Is there any case that we
> are doing more than that using layout-after edges?
>
> You’ll have to ask Shankar what problem he was trying to solve with the
> layout before/after stuff.
>
>   ELF does not use layout-before references, it uses only layout-after
> and in-group references. The reason the ELF linker tried to do this was
>
> a) Prevent reordering of atoms and maintain section relationship. All
> atoms in a section have to be tied together. Anything that tries to reorder
> has to reorder the whole section.
> b) This also would clearly model linker scripts in the future that moving
> a function would make sure the section that contains the function is moved,
> and not the individual function.
>
> This was earlier discussed in the thread :
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20121022/154212.html
>
> I would like to have the kindLayoutBefore/kindLayoutAfter/kindInGroup
> references to exist and do the functionalities that it is trying to
> accomplish.
>
> This could be used when we want to implement the order file in the near
> future.
>

Is there anything that is presently depending on layout-after and in-group,
which does not work with the default ordering function, compareAtomsSub()?
I don't intend to say that that wouldn't be needed for ELF, but just trying
to understand.

As to the feature set that LayoutPass provides, we shouldn't keep this
complexity as is as I repeatedly stated, because as I sayd multiple times
it seems that this level of complexity is not (and will not) needed.

Specifically, layout-before relationship is not really needed for any
occasion to layout atoms -- in all cases when you want to layout atom A
before B, you can just add "layout-after to A" to B instead. Dead-stripping
path would still need doubly-linked graph, but it does not mean that
layout-pass needs to use it too.

Also the piece of code handling layout-before is buggy. It sometimes falls
into an infinite loop for a valid object file. Fixing it would be a waste
of time if the feature is not needed.

 Thanks
>
> Shankar Easwaran
>
>
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation
>
>
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