[lld] r254461 - [ELF] MIPS paired R_MIPS_HI16/LO16 relocations support

[Sean Silva via llvm-commits]

On Wed, Dec 2, 2015 at 2:19 AM, Simon Atanasyan via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> On Wed, Dec 2, 2015 at 12:51 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> > ----- Original Message -----
> >> From: "Simon Atanasyan via llvm-commits" <llvm-commits at lists.llvm.org>
> >> To: llvm-commits at lists.llvm.org
> >> Sent: Tuesday, December 1, 2015 3:24:46 PM
> >> Subject: [lld] r254461 - [ELF] MIPS paired R_MIPS_HI16/LO16 relocations
> support
> >>
> >> Author: atanasyan
> >> Date: Tue Dec  1 15:24:45 2015
> >> New Revision: 254461
> >>
> >> URL: http://llvm.org/viewvc/llvm-project?rev=254461&view=rev
> >> Log:
> >> [ELF] MIPS paired R_MIPS_HI16/LO16 relocations support
>
> [...]
>
> > If I'm reading this correctly, this search is O(N^2/2) in the worst case.
> > Is there some reason you don't get that behavior in practice?
> >
> > It seems like if we presorted a list by <type, offset>, we could easily
> > make this O(N*ln(N)) instead.
>
> Usually a distance between R_MIPS_HI16 and corresponding R_MIPS_LO16
> relocation are 1-3 steps. So for each R_MIPS_HI16 relocation we need
> to check a few next entries in a relocation list. If think a cost of
> building and maintaining complete sorted relocations list is much
> higher than this lookup.
>

This makes sense. I think it is worth commenting.

-- Sean Silva


>
> There is one more method of handling R_MIPS_HI16/LO16 pairs. We can
> save R_MIPS_HI16 relocations in a container and postpone calculation
> When we get R_MIPS_LO16 relocation, we iterate over the list and handle
> all postponed HI16 relocations. This method does not require lookup
> at all. But in this case it might be difficult to organize relocations
> handling in parallel.
>
> I am going to return to this point of the code later, measure performance
> of various approaches on a real data and select the best variant.
>
> --
> Simon Atanasyan
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