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This reverts commit 39b5c1cbd8950755de400933cecca7b8deb4ffcd.
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Signed-off-by: Austin Seipp <austin@well-typed.com>
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This will hopefully help ensure some basic consistency in the forward by
overriding buffer variables. In particular, it sets the wrap length, the
offset to 4, and turns off tabs.
Signed-off-by: Austin Seipp <austin@well-typed.com>
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The main change here is that the Cmm parser now allows high-level cmm
code with argument-passing and function calls. For example:
foo ( gcptr a, bits32 b )
{
if (b > 0) {
// we can make tail calls passing arguments:
jump stg_ap_0_fast(a);
}
return (x,y);
}
More details on the new cmm syntax are in Note [Syntax of .cmm files]
in CmmParse.y.
The old syntax is still more-or-less supported for those occasional
code fragments that really need to explicitly manipulate the stack.
However there are a couple of differences: it is now obligatory to
give a list of live GlobalRegs on every jump, e.g.
jump %ENTRY_CODE(Sp(0)) [R1];
Again, more details in Note [Syntax of .cmm files].
I have rewritten most of the .cmm files in the RTS into the new
syntax, except for AutoApply.cmm which is generated by the genapply
program: this file could be generated in the new syntax instead and
would probably be better off for it, but I ran out of enthusiasm.
Some other changes in this batch:
- The PrimOp calling convention is gone, primops now use the ordinary
NativeNodeCall convention. This means that primops and "foreign
import prim" code must be written in high-level cmm, but they can
now take more than 10 arguments.
- CmmSink now does constant-folding (should fix #7219)
- .cmm files now go through the cmmPipeline, and as a result we
generate better code in many cases. All the object files generated
for the RTS .cmm files are now smaller. Performance should be
better too, but I haven't measured it yet.
- RET_DYN frames are removed from the RTS, lots of code goes away
- we now have some more canned GC points to cover unboxed-tuples with
2-4 pointers, which will reduce code size a little.
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This patch makes two changes to the way stacks are managed:
1. The stack is now stored in a separate object from the TSO.
This means that it is easier to replace the stack object for a thread
when the stack overflows or underflows; we don't have to leave behind
the old TSO as an indirection any more. Consequently, we can remove
ThreadRelocated and deRefTSO(), which were a pain.
This is obviously the right thing, but the last time I tried to do it
it made performance worse. This time I seem to have cracked it.
2. Stacks are now represented as a chain of chunks, rather than
a single monolithic object.
The big advantage here is that individual chunks are marked clean or
dirty according to whether they contain pointers to the young
generation, and the GC can avoid traversing clean stack chunks during
a young-generation collection. This means that programs with deep
stacks will see a big saving in GC overhead when using the default GC
settings.
A secondary advantage is that there is much less copying involved as
the stack grows. Programs that quickly grow a deep stack will see big
improvements.
In some ways the implementation is simpler, as nothing special needs
to be done to reclaim stack as the stack shrinks (the GC just recovers
the dead stack chunks). On the other hand, we have to manage stack
underflow between chunks, so there's a new stack frame
(UNDERFLOW_FRAME), and we now have separate TSO and STACK objects.
The total amount of code is probably about the same as before.
There are new RTS flags:
-ki<size> Sets the initial thread stack size (default 1k) Egs: -ki4k -ki2m
-kc<size> Sets the stack chunk size (default 32k)
-kb<size> Sets the stack chunk buffer size (default 1k)
-ki was previously called just -k, and the old name is still accepted
for backwards compatibility. These new options are documented.
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These are no longer used: once upon a time they used to have different
layout from IND and IND_PERM respectively, but that is no longer the
case since we changed the remembered set to be an array of addresses
instead of a linked list of closures.
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This replaces the global blackhole_queue with a clever scheme that
enables us to queue up blocked threads on the closure that they are
blocked on, while still avoiding atomic instructions in the common
case.
Advantages:
- gets rid of a locked global data structure and some tricky GC code
(replacing it with some per-thread data structures and different
tricky GC code :)
- wakeups are more prompt: parallel/concurrent performance should
benefit. I haven't seen anything dramatic in the parallel
benchmarks so far, but a couple of threading benchmarks do improve
a bit.
- waking up a thread blocked on a blackhole is now O(1) (e.g. if
it is the target of throwTo).
- less sharing and better separation of Capabilities: communication
is done with messages, the data structures are strictly owned by a
Capability and cannot be modified except by sending messages.
- this change will utlimately enable us to do more intelligent
scheduling when threads block on each other. This is what started
off the whole thing, but it isn't done yet (#3838).
I'll be documenting all this on the wiki in due course.
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At the moment, this just saves a memory reference in the GC inner loop
(worth a percent or two of GC time). Later, it will hopefully let me
experiment with partial steps, and simplifying the generation/step
infrastructure.
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This has no effect with static libraries, but when the RTS is in a
shared library it does two things:
- it prevents the function from being exposed by the shared library
- internal calls to the function can use the faster non-PLT calls,
because the function cannot be overriden at link time.
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This turns out to be quite vital for parallel programs:
- The way we discover which threads to traverse is by finding
dirty threads via the remembered sets (aka mutable lists).
- A dirty thread will be on the remembered set of the capability
that was running it, and we really want to traverse that thread's
stack using the GC thread for the capability, because it is in
that CPU's cache. If we get this wrong, we get penalised badly by
the memory system.
Previously we had per-capability mutable lists but they were
aggregated before GC and traversed by just one of the GC threads.
This resulted in very poor performance particularly for parallel
programs with deep stacks.
Now we keep per-capability remembered sets throughout GC, which also
removes a lock (recordMutableGen_sync).
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There was already a check to avoid updating an IND, but it was
originally there to avoid a bug which doesn't exist now. Furthermore
the test and update are not atomic, so another thread could be
updating this thunk while we are. We have to just go ahead and update
anyway - it might waste a little work, but this is a very rare case.
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The following changes restore ticky-ticky profiling to functionality
from its formerly bit-rotted state. Sort of. (It got bit-rotted as part
of the switch to the C-- back-end.)
The way that ticky-ticky is supposed to work is documented in Section 5.7
of the GHC manual (though the manual doesn't mention that it hasn't worked
since sometime around 6.0, alas). Changes from this are as follows (which
I'll document on the wiki):
* In the past, you had to build all of the libraries with way=t in order to
use ticky-ticky, because it entailed a different closure layout. No longer.
You still need to do make way=t in rts/ in order to build the ticky RTS,
but you should now be able to mix ticky and non-ticky modules.
* Some of the counters that worked in the past aren't implemented yet.
I was originally just trying to get entry counts to work, so those should
be correct. The list of counters was never documented in the first place,
so I hope it's not too much of a disaster that some don't appear anymore.
Someday, someone (perhaps me) should document all the counters and what
they do. For now, all of the counters are either accurate (or at least as
accurate as they always were), zero, or missing from the ticky profiling
report altogether.
This hasn't been particularly well-tested, but these changes shouldn't
affect anything except when compiling with -fticky-ticky (famous last
words...)
Implementation details:
I got rid of StgTicky.h, which in the past had the macros and declarations
for all of the ticky counters. Now, those macros are defined in Cmm.h.
StgTicky.h was still there for inclusion in C code. Now, any remaining C
code simply cannot call the ticky macros -- or rather, they do call those
macros, but from the perspective of C code, they're defined as no-ops.
(This shouldn't be too big a problem.)
I added a new file TickyCounter.h that has all the declarations for ticky
counters, as well as dummy macros for use in C code. Someday, these
declarations should really be automatically generated, since they need
to be kept consistent with the macros defined in Cmm.h.
Other changes include getting rid of the header that was getting added to
closures before, and getting rid of various code having to do with eager
blackholing and permanent indirections (the changes under compiler/
and rts/Updates.*).
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This affects -debug only, avoids crash with test conc012.
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So that we can build the RTS with the NCG.
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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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