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

[Iain Sandoe]

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***
> 
> 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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