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Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section

On 04/26/2017 06:06 AM, Cary Coutant wrote:
4. Evaluate some alternate representations to improve upon the simple
list of addresses. An idea I've been considering is emitting a bitmap
-- an alternating series of pairs (starting address, bits), where
"bits" is just a fixed-size bit vector describing each word starting
at the given starting address. The "bits" field could be, say, 128 or
256 bits in length, or could begin with a byte that defines its
length. I think this would work well, as the relocations are often
clustered, but haven't yet done any testing of that hypothesis. A
simple run-length encoding, as previously suggested, would also work,
of course. I'm not in favor of using a general-purpose compression
algorithm on the relocations.

It might also be possible to sort the relocated objects so that they are more contiguous.

It could be beneficial to arrange things so that vector instructions can be used for the relocation. I'm not sure if it is worthwhile to design a compression scheme based on bit fiddling for this because those who want compression can probably use a compressed file system anyway.


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