[llvm][patch] Adjust behavior of FDE cross-section relocs for targets that don't support abs-differences.

[Iain Sandoe]

Hi Nick,

On 17 Dec 2013, at 22:28, Nick Kledzik wrote:

> On Dec 12, 2013, at 3:57 PM, Iain Sandoe <iain at codesourcery.com> wrote:
>> bump
>> 
>> @jim - although this was reported against an old target, I believe it potentially effects any target using non-trivial relocs for Start-PC and LSDAs in FDEs.
>> Iain
>> 
>> On 3 Dec 2013, at 15:59, Iain Sandoe wrote:
>> 
>>> Hi, 
>>> (I hope I have directed this to the correct reviewers)
>>> 
>>> At the dev. conf. David reported to me that EH was not working on ppc-darwin8.  Once we checked a few more platforms, it turns out it's not working with any ld64 < 97.17 (so OSX < 10.6 on both x86/ppc).
>>> 
>>> The reason is that the older ld/ld64 versions require proper scattered relocs for expressions in CIE/FDE symbol refs that cross section boundaries.
>>> 
>>> Later versions of the ld64 allow that these may be replaced by an absolute difference***
> I can tell you the motivation for the linker change.  ld64 takes the “smart linker” approach.  If fully breaks up the __eh_frame section and associates each FDE with its owning function.  Since it knows how to parse an FDE, a relocation is redundant and only slows down the linker.  On the other hand, a “dumb linker” which does not know anything special about the __eh_frame section would need relocations.  

indeed, but (as far as I am aware) the only such smart linker is ld64 >= 97.1 …
…  so should the patch be applied to allow other linkers to function?

Iain


> 
> -Nick
> 
>>> 
>>> AFAIK, this behaviour is specific to Darwin/OSX (and ld64 >= 97.17), I suspect that any other target that elected to use a non-trivial FDE symbol encoding would likely be surprised by the abs values.  However, these are hardwired in MCDwarf.cpp, as of now.
>>> 
>>> NOTE1: in general, this would not cause a linkage error for the target - 'just' a failure to unwind at runtime.
>>> NOTE2: it also seems that the abs-ification is not done even for x86-64-darwin12 for compact unwind.
>>> 
>>> The solution I am proposing is to introduce a "DwarfFDESymbolsUseAbsDiff" boolean value in MCAsmInfo and to use that to control this behaviour.  In the patch, this is only switched on for X86 Darwin/OSX & for OSX >= 10.6.  It should really be contingent on -target-linker-version >= 97.17, but this information is not yet passed to the target.
>>> 
>>> With the patch EH works on *-darwin9 with the native linker (and still emits the same set of relocs on darwin12).
>>> 
>>> Is this a reasonable solution?
>>> Iain
>>> 
>>> *** P.S.
>>> I assume that these extra hoops and inconsistencies (and presumably matching ones in ld64) are really worthwhile?
>>> I.E. that there is some measurable performance gain?
>>> (if not, we could just elect to emit the relocs and drop this special casing).
>>> 
>>> <llvm-eh-fde-relocs-diff-v1.txt>
>>> 
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>> 
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