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

[Nick Kledzik]

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.  

-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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