Re: [Valgrind-developers] vex: r1418 - trunk/priv/guest-ppc32

[Oswald B. <os...@kd...>]

On Sat, Oct 08, 2005 at 06:49:52PM -0500, Nicholas Nethercote wrote:
> On Sat, 8 Oct 2005, Josef Weidendorfer wrote:
> 
> >Hmmm... this leads to a further question: Could a persistant
> >translation cache speed up Valgrind, by simply executing the
> >"pre-translated" code chunks from an earlier run?  In multiple runs,
> >shared objects can be mapped to different addresses. So the
> >translation cache should be separated by shared objects. But this
> >would need a (object/offset) tuple as lookup key instead of a simple
> >code address.
> 
> See this paper from the recent WBIA workshop:
> 
>   http://rogue.colorado.edu/draco/papers/wbia05-persistence.pdf
> 
i have the impression that these guys were told to produce six pages, so
they did ... with the content for max four pages. oh, well. :)

a critical aspect: valgrind is a debugging tool. that means that a
particular binary is usually executed exactly once. consequently a
wholesale consistency check would drastically lower the effect of
persistence (there still would be a gain for separately cached dynamic
objects). so we need function-level granularity, based on function
signatures. however, static linking will most often yield different
binary images for unmodified functions due to offset changes stemming
from unrelated code changes. i *think* such locations are easy to spot -
they are either dynamic relocations of some type or they are involved
into PLT or GOT references, and debug info certainly provides the info
directly. they would be made wildcards in the signatures, and their
values would have to be propagated into the translated code. fetching
from the cache could be done at the function translation stage, but i
guess the global view (function order) one has at object load time would
help improving the overall overhead.

an orthogonal issue: V could detect tight loops and optimize them
further (anybody willing to incorporate gcc's optimizer into V? :).
with persistent translations, expensive optimizations could be applied
more liberally.

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