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The extra safe points introduced for breakpoints were previously
compiled as normal updatable thunks, but they are guaranteed
single-entry, so we can use non-updatable thunks here. This restores
the tail-call property where it was lost in some cases (although stack
squeezing probably often recovered it), and should improve
performance.
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An AP_STACK now ensures that there is at least AP_STACK_SPLIM words of
stack headroom available after unpacking the payload. Continuations
that require more than AP_STACK_SPLIM words of stack must do their own
stack checks instead of aggregating their stack usage into the parent
frame. I have made this change for the interpreter, but not for
compiled code yet - we should do this in the glorious rewrite of the
code generator.
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* The correct definition of C-- requires that a procedure not
'fall off the end'. The 'never returns' annotation tells us
if a (foreign) call is not going to return.
Validated!
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This patch implements pointer tagging as per our ICFP'07 paper "Faster
laziness using dynamic pointer tagging". It improves performance by
10-15% for most workloads, including GHC itself.
The original patches were by Alexey Rodriguez Yakushev
<mrchebas@gmail.com>, with additions and improvements by me. I've
re-recorded the development as a single patch.
The basic idea is this: we use the low 2 bits of a pointer to a heap
object (3 bits on a 64-bit architecture) to encode some information
about the object pointed to. For a constructor, we encode the "tag"
of the constructor (e.g. True vs. False), for a function closure its
arity. This enables some decisions to be made without dereferencing
the pointer, which speeds up some common operations. In particular it
enables us to avoid costly indirect jumps in many cases.
More information in the commentary:
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/HaskellExecution/PointerTagging
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Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.
The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level. The build system now makes no
pretense at being multi-project, it is just the GHC build system.
No doubt this will break many things, and there will be a period of
instability while we fix the dependencies. A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.
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