Re: [eigen] RFC: making a deterministic and reproducable product codepath with Eigen

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 2016-09-06 16:09, Peter wrote:
> It's beyond my knowledge, whether scalar products will always be
> scheduled in the same way on the FPUs by the hardware,
> especially if the scalar product appears after an if-statement.
> I'm just sceptical, but maybe I just got surprised too often.

I would be very surprised, if a CPU would give different results for the 
very same machine instructions (given the same inputs and 
FPU-configuration, of course), solely based on internal scheduling.

Overall, I'm not sure if your overall goal is really feasible to 
achieve. If you don't care (too much) about performance, then maybe by 
providing a "always use simple 3-loop-matrix-products"-path, and always 
evaluate as if we followed C++ operator precedence (which takes away 
many optimization opportunities, e.g. for (Scalar * A * B + C) ).
Certainly, many element-wise operations (R= A+B+C) could still be lazily 
evaluated (and even vectorized), but allowing vectorization even for 
simple reductions will depend on the packet-size.
Things like blocking, expression-tree-reordering, vectorization will 
likely change for different Eigen-versions (and depending on the CPU). 
Making all of that user-configurable, will likely become a maintenance 
nightmare over time.

For the given use-case, I guess the simplest solution would be to 
hand-craft a simple library with naive but stable implementations.

Alternatively, if you mostly care about hardware independence (but not 
Eigen-version independence), it might be possible to compile to some 
intermediate code which is then translated to equivalent code for each 
architecture (which is one of the main ideas of LLVM, I guess).
Of course, you'd also need to get rid of any run-time dependent 
decisions (like determining block-sizes from CPU cache sizes).



Christoph






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