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Simon Peyton Jones authored
Matthew Pickering uncovered a bad performance hole in the way that single-method dictionaries work, described in Trac #14955. See Note [Do not unpack class dictionaries] in WwLib. I tried to fix this 6 years ago, but got it slightly wrong. This patch fixes it, which makes a dramatic improvement in the test case. Nofib highlights: not much happening: Program Size Allocs Runtime Elapsed TotalMem ----------------------------------------------------------------- VSM -0.3% +2.7% -7.4% -7.4% 0.0% cacheprof -0.0% +0.1% +0.3% +0.7% 0.0% integer -0.0% +1.1% +7.5% +7.5% 0.0% tak -0.1% -0.2% 0.024 0.024 0.0% ----------------------------------------------------------------- Min -4.4% -0.2% -7.4% -7.4% -8.0% Max +0.6% +2.7% +7.5% +7.5% 0.0% Geom Mean -0.1% +0.0% +0.1% +0.1% -0.2% I investigated VSM. The patch unpacks class dictionaries a bit more than before (i.e. does so if there is no INLINABLE pragma). And that gives better code in VSM (less dictionary selection etc), but one closure gets one word bigger. I'll accept these changes in exchange for more robust performance. Some ghci.debugger output wobbled around (order of bindings being displayed). I have no idea why; but I accepted the changes.
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