[llvm-dev] Would a spreadsheet be a good project using LLVM?

[mats petersson via llvm-dev]

This sounds like an interesting project, but I'm far from convinced that
LLVM will help much:

I have not TRIED to solve this, but my guess is that of the time to
recalculate a large spreadsheet, the majority of the time is not actually
spent calculating the values of the content, but actually inferring the
dependencies and other "work out what needs to be (re-)calculated". And
whilst LLVM can probably help in doing that, I'm far from convinced that it
will speed up the things you actually want to speed up.

Compilers (I'm including LLVM in "compilers" for this discussing) are
really best at dealing with things where you repeat the same exact thing
over and over. In a spreadsheet, the common case is that you calculate all
the fields ONCE, then only recalculate based on changes. Of course, some
change that is used EVERYWHERE (like if you change the base-interest rate
in your spreadsheet for calculating bank-rates and the earnings on various
bank-accounts, then ALL your cells will depend on that in some way, and
everything gets recalculated). And whilst the dependancy graph DOESN'T
change (except when you change the formulae in the cells). So you'd end up
spending a lot of time compiling the spreadsheet code, and probably lose
time rather than gain on the competing solutions.

So my feeling, without testing it, is that LLVM probably won't give you a
huge amount of benefit in the general case of spreadsheets.

If it was my project, I think it would be worth writing something that
simulates the general principle of spreadsheet calculations, and first of
all, figure out where the majority of the time is spent. I don't think this
experiment needs to have full support of all the different data types,
functons, etc, just the dependency and (basic) formula support.

No, I can't back up this with any facts - just my thoughts - hopefully it's
of some help, and I haven't just rambled away completely aimlessly.

--
Mats


On 21 August 2015 at 14:23, Jason Glazer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I am thinking about writing a new open source spreadsheet application
> since I think the spreadsheet applications out there (Microsoft Excel,
> LibreOffice Calc, etc.) do not have some features that I would really like
> to use. I would like the spreadsheet to recalculate very fast and wondered
> if it would make sense to use LLVM to calculate the cell values quickly.
> Each cell of a spreadsheet contains an expression, much like an expression
> in any programming language. The big difference is that the ordering of the
> expression evaluations is governed by the dependencies of each cell on
> other cells and end up being turned into a directed acyclic graph (DAG). It
> would be good if the actual conversion of cell expressions from strings
> that the user enters into a cell, would be converted into a representation
> that can be recalculated very quickly. I was thinking that LLVM machine
> code might be a good target.
>
> Overall, is this a good project to use LLVM?
>
> Is there existing open source code for compiling expressions using LLVM
> that you would recommend for this project?
>
> Any suggestions or concerns about this approach?
>
> Is anyone interested in helping out?
>
> Thanks
>
> Jason
>
> --
> Jason Glazer, P.E., GARD Analytics, 90.1 ECB chair
> Admin for onebuilding.org building performance mailing lists
>
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