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This reduces the latency between a context-switch being triggered and
the thread returning to the scheduler, which in turn should reduce the
cost of the GC barrier when there are many cores.
We still retain the old context_switch flag which is checked at the
end of each block of allocation. The idea is that setting HpLim may
fail if the the target thread is modifying HpLim at the same time; the
context_switch flag is a fallback. It also allows us to "context
switch soon" without forcing an immediate switch, which can be costly.
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When returning an unboxed tuple with a single non-void component, we
now use the same calling convention as for returning a value of the
same type as that component. This means that the return convention
for IO now doesn't vary depending on the platform, which make some
parts of the RTS simpler, and fixes a problem I was having with making
the FFI work in unregisterised GHCi (the byte-code compiler makes
some assumptions about calling conventions to keep things simple).
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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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We changed the convention a while ago so that BaseReg is returned to
the scheduler in R1, because BaseReg may change during the run of a
thread, e.g. during a foreign call. A few places got missed,
mostly for very rare events.
Should fix concprog001, although I'm not able to reliably reproduce
the failure.
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We recently discovered that they aren't a win any more, and just cost
code size.
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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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