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mris_fix_topology has a hot loop, which does unnecessary calculations of the face normals |
==== mris_fix_topology has a hot loop, which does unnecessary calculations of the face normals ==== |
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To simplify, the original code does this |
==== To simplify, the original code does this ==== |
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Since only a few of the face normals are used, it is a waste of time to calculate all of them every time around the loop! | ''Since only a few of the face normals are used, it is a waste of time to calculate all of them every time around the loop! '' |
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This is replaced by code that does |
==== This is replaced by code that does ==== |
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==== How can we test it gets the right answers? ==== When checking code is enabled, the normal[fno] is computed from the inputs twice - once when it is deferred, and again when it is undeferred. It is checked that these two values are the same. |
A variety of techniques have been used.
Deferring calculations until needed
mris_fix_topology has a hot loop, which does unnecessary calculations of the face normals
mrisComputeOptimalRetessellation and mrisComputeRandomRetessellation have a similar structure
_:_: it loops over a set of patches, or iterates on one patch. For each patch it calls
_:_:_: mrisDefectPatchFitness, which calls
_:_:_:_: mrisComputeDefectLogLikelihood, which calls
_:_:_:_:_: mrisComputeDefectMRILogUnlikelihood, which
_:_:_:_:_:_: does an expensive computation all the face normals for ALL the faces
_:_:_:_:_:_: does two other expensive steps, which only use a few of the face normals
To simplify, the original code does this
_: loop
_:_: for all fno normal[fno] = f (inputs)
_:_: for a few fno use normal[fno]
_: end loop
_: change some inputs
_: use some normal[fno]
Since only a few of the face normals are used, it is a waste of time to calculate all of them every time around the loop!
This is replaced by code that does
_: loop
_:_: for all fno: normal[fno].deferred = true
_:_: for a few fno: use normal[fno], computing it from f (inputs) if is deferred, and changing it to being no-longer deferred
_: end loop
_: compute any remaining deferred normal[fno] before their inputs can change
_: change some inputs
_: use some normal[fno]
How can we test it gets the right answers?
When checking code is enabled, the normal[fno] is computed from the inputs twice - once when it is deferred, and again when it is undeferred. It is checked that these two values are the same.