Changes between Initial Version and Version 1 of Performance

Timestamp:

10/31/06 14:22:13 (7 years ago)

Author:

igloo

Comment:

From HaWiki?

Performance

@@ +1 @@
+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)