wiki:Commentary/Compiler/UnpackPrimitiveFields

Version 1 (modified by tibbe, 2 years ago) (diff)

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Unpacking primitive fields

This page describes a proposal to automatically unpack (strict) primitive fields. A primitive fields is a field that when unpacked has a pointer-sized representation. Examples include Int, Word, Float, and newtypes thereof.

Goals and non-goals

This proposal is about changing the default behavior of GHC, not changing expressiveness. Users can still use UNPACK and NOUNPACK to explicitly control the memory representation of fields.

There are two goals:

  1. Reduce the amount of boilerplate experienced programmers have to write: As of Feb 18th 2012, the bytestring, text, and containers packages had 46 fields that matched the definition of primitive given above. 43 of these had an explicit UNPACK pragma (and the remaining 3 could have had one without changing the performance of the program.)
  1. To provide better defaults for beginner and intermediate level Haskellers. Not unpacking e.g. Int fields can have a large, negative effect on performance and many beginner and intermediate level Haskellers are bitten by this.

Detailed design

Benchmarks