Snapshot diff merging fixes #174
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csegarragonz
approved these changes
Nov 11, 2021
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| if (p > 0 && dirtyPages.at(p - 1) == thisPageNum - 1) { | ||
| // Previous page was also dirty, just update last region | ||
| regions.back().second = thisPageEnd; |
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Could regions here be empty, thus back() cause undefined behaviour?
I am aware this may not have sense logically, but maybe worth catching?
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Yes good point, I'll add an assert as that empty back() problem can be hard to diagnose.
Actually, it's impossible for that to occur as the else branch of this if will always add the first element to the list.
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These changes were part of the effort to get pthreads working in Faasm again. The original issue was an off-by-one error in the diff detection loop, but to make diagnosis simpler I refactored the diff detection logic.
Changes:
getDirtyRegionsfunction. This is preferable to the oldgetDirtyPageswhich would return a list of page numbers that the diffing function eventually converted into regions. This logic is now isolated and tested separately.MergeRegionclass (now all the grimness is held in once place 😄)Overwritemerge regions to specifylength=0, which means they will extend to the end of the memory. This is necessary to merge back changes from memory allocated outside the original snapshot size.