[llvm-dev] Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section

[Roland McGrath via llvm-dev]

On Mon, Dec 11, 2017 at 3:50 PM Rahul Chaudhry via gnu-gabi <
gnu-gabi at sourceware.org> wrote:

> A simple combination of delta-encoding and run_length-encoding is one of
> the
> first schemes we experimented with (32-bit entries with 24-bit 'delta' and
> an
> 8-bit 'count'). This gave really good results, but as Sri mentions, we
> observed
> several cases where the relative relocations were not on consecutive
> offsets.
> There were common cases where the relocations applied to alternate words,
> and
> that totally wrecked the scheme (a bunch of entries with delta==16 and
> count==1).
>

For the same issue in a different context, I recently implemented a scheme
using run-length-encoding but using a variable stride.  So for a run of
alternate words, you still get a single entry, but with stride 16 instead
of 8.  In my application, most cases of strides > 8 are a run of only 2 or
3 but there are a few cases of dozens or hundreds with a stride of 16.  My
case is a solution tailored to exactly one application (a kernel), so there
is a closed sample set that's all that matters and the trade-off between
simplicity of the analysis and compactness of the results is different than
the general case you're addressing (my "analysis" consists of a few lines
of AWK).  But I wonder if it might be worthwhile to study the effect a
variable-stride RLE scheme or adding the variable-stride ability into your
hybrid scheme has on your sample applications.

Since we're talking about specifying a new ABI that will be serving us for
many years to come and will be hard to change once deployed, it seems worth
spending quite a bit of effort up front to come to the most compact scheme
that's feasible.
-- 


Thanks,
Roland
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