[PATCH] D17934: [ELF] Implement infrastructure for thunk code creation

[Rui Ueyama via llvm-commits]

On Sun, Mar 13, 2016 at 8:36 AM, Simon Atanasyan <simon at atanasyan.com>
wrote:

> atanasyan added a comment.
>
> In http://reviews.llvm.org/D17934#374020, @ruiu wrote:
>
> > This looks still a bit too intrusive. The code to handles thunks appears
> too many places.
> >
> > Maybe we should handle thunks as a separate output section? Then we
> don't need to add addThunk to the regular output section class and we don't
> have to update the code to calculate section size and offsets.
>
>
> I see some problems here:
>
> - We can get a really long out-of-range jumps. I remember that my initial
> solution had the same problem but it was my mistake.
> - The output section should have a name. Putting all thunks into the
> separate output section is in fact ABI extension because we have to choose
> dedicated unique name for such section. Moreover if somebody have a linker
> script to put output sections in exact places (common case for embedded
> software developers), it will be surprise that he or she have to handle one
> more unusual output section.
> - There is optimization used by bfd and gold linkers - if input section
> contains the only function and we need to create a thunk for it, we can
> place the thunk in the beginning of the section and use just two assembly
> instructions to save the function address into the $25 registers and do not
> use additional jump instructions at all. My patch does not implement this
> optimization yet but moving all thunks into the separate output section
> make such optimization impossible at all.
>
> I think the less intrusive solution is to concentrate the thunk handling
> in the InputSection class. We can add a vector of SymbolBody pointers right
> to this class. In that case only InputSection and TargetInfo descendant
> will know about synthetic code.
>

That might be a good idea. I'm not sure if it's going to be better than
this, but IMO it's worth trying.


> BTW MIPS is not the only target uses thunks. AFAIK ARM has concept of
> veneer.
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