This page collects information about the performance of GHC and GHC-compiled code. We're interested in programs that GHC compiles slowly, programs that run slowly (especially small ones), and ideas for improving performance of either GHC itself or GHC-compiled code.

Performance of GHC

The Todo list for GHC performance:

• Emit the interface immediately after typechecking in non-optimised compilation
• Turn on the NCG by default for -O (possibly improve x87 float code first?)
• Stringbuffer could use mmap()
• Improve performance of generating output (SimonMarlow?: done).
• Profile & do a space-leak sweep (SimonMarlow?: done, fixed one space leak, space usage not great but could be worse).
• Improve binary interface representation (SimonMarlow?: some improvement here).

Half-baked ideas:

• glom the interfaces for a package into a single uber-interface with one string table.
• Classic loop optimizations that are not invariant hoisting. (Jan-Willem Maessen)

Modules that GHC compiles slowly

Simon PJ found a case of non-linearity in the simplifier; we are currently testing a patch that hopefully fixes all of the following:

• Language.Haskell.Syntax
• large constant structures taking non-linear time to compile
• large Happy-generated grammars
• WashNGo-2.4.6/WASH/HTMLMonad98.hs from WASH/CGI (SimonMarlow?: investigating)
• dsrun011 from the test suite (SimonMarlow?: fixed)
• record with lots of fields (20+), deriving Show, with -O (SimonMarlow?: fixed)

Performance of compiled code

Programs that run too slowly

• Various of the shootout benchmarks (Todo: expand).

Performance Todo list

Here we list some problems that we know about, and/or ideas for improving things.

• x86_64: use more registers (the NCG needs to save/restore volatile regs over C calls, apart from that we're there).
• via-C produces code with too many jumps, especially with later versions of GCC. GCC commons up identical basic blocks containing a single jump instruction.
• GC has been reported to behave non-linearly  http://www.haskell.org//pipermail/glasgow-haskell-users/2005-April/008301.html (SimonMarlow?: turned out to be caused by swapping)
• Space leak: a function that is statically known to not use some or all of its arguments should have a function descriptor that reflects the unused pointers as non-pointers, so that a PAP applied to unused args doesn't retain them.
• Push heap check into a case branch if that means we can avoid a heap check in a tail-recursive loop.
• Make ticky-ticky profiling work again.
• Make the NCG handle loops, and perform some simple loop optimisations.
• Better optimisation pass sequences? (c.f. Laszlo Nemeth's research  http://www.tcs.informatik.uni-muenchen.de/~hwloidl/TFP04/Abstracts/17.html)
• Can we do anything about Double alignment?
• better GC for chains of THUNK_SELECTORs (see comments in GC.c)
• Int64 reported to be slow  http://www.haskell.org//pipermail/glasgow-haskell-users/2005-June/008574.html (Lemmih: Fixed. It's now only three times slower to use Int64 over Int in that example. (patch not commited yet)).
• System.Random could be optimized more.

A few slightly larger projects:

• parallel GC for multi-processors
• Allow strict enumerations to be "unboxed"

Some more open-ended ideas:

• Strict or unlifted types, or strictness annotations on functions
• How about an anti-strictness annotation? Value is always re-evaluated when used?
• Can GC performance be improved at all? Perhaps: a combination of depth-first and breadth-first traversal instead of just breadth-first. (SimonMarlow?: attempted, no joy. But managed to find some other opportunities for improvement in the GC while I was there.)
• improve compacting GC performance (ask SimonMarlow? for ideas)
• Whole program optimisation infrastructure. Dump intermediate representation into a special section of the .o files (or use another file) and upon linking the program/library run another optimisation pass over the entire lot before finally generating c/cmm. Only enabled with -O2/-O3 or something.

Size of compiled code

List modules or programs that produce code that is too large:

The Todo list for code size:

• Implement dynamic libraries on other platforms: x86/Linux, x86_64/Linux, x86/*BSD, x86_64/*BSD.
• Find common code fragments and share them
• Top level CSE of lambdas module alpha equivalence. This come up from time to time when inlining occurs. It doesn't look like it gets done, but I could be missing the pass. (Jan-Willem Maessen)