Faster laziness using dynamic pointer tagging

In the light of evidence that Haskell programs compiled by GHC exhibit large numbers of mispredicted branches on modern processors, we re-examine the "tagless" aspect of the STG-machine that GHC uses as its evaluation model. We propose two tagging strategies: a simple strategy called semitagging that seeks to avoid one common source of unpredictable indirect jumps, and a more complex strategy called dynamic pointertagging that uses the spare low bits in a pointer to encode information about the pointed-to object. Both of these strategies have been implemented and exhaustively measured in the context of a production compiler, GHC, and the paper contains detailed descriptions of the implementations. Our measurements demonstrate significant performance improvements (14% for dynamic pointer-tagging with only a 2% increase in code size), and we further demonstrate that much of the improvement can be attributed to the elimination of mispredicted branch instructions. As part of our...

• Type: Conference
• Year: 2007
• Where: ICFP
• Authors: Simon Marlow, Alexey Rodriguez Yakushev, Simon L. Peyton Jones
• Keywords: GHC Exhibit, ICFP 2007, Programming Languages, Spare Low Bits, Unpredictable Indirect Jumps