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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 means that both time and heap profiling work for parallel
programs. Main internal changes:
- CCCS is no longer a global variable; it is now another
pseudo-register in the StgRegTable struct. Thus every
Capability has its own CCCS.
- There is a new built-in CCS called "IDLE", which records ticks for
Capabilities in the idle state. If you profile a single-threaded
program with +RTS -N2, you'll see about 50% of time in "IDLE".
- There is appropriate locking in rts/Profiling.c to protect the
shared cost-centre-stack data structures.
This patch does enough to get it working, I have cut one big corner:
the cost-centre-stack data structure is still shared amongst all
Capabilities, which means that multiple Capabilities will race when
updating the "allocations" and "entries" fields of a CCS. Not only
does this give unpredictable results, but it runs very slowly due to
cache line bouncing.
It is strongly recommended that you use -fno-prof-count-entries to
disable the "entries" count when profiling parallel programs. (I shall
add a note to this effect to the docs).
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User visible changes
====================
Profilng
--------
Flags renamed (the old ones are still accepted for now):
OLD NEW
--------- ------------
-auto-all -fprof-auto
-auto -fprof-exported
-caf-all -fprof-cafs
New flags:
-fprof-auto Annotates all bindings (not just top-level
ones) with SCCs
-fprof-top Annotates just top-level bindings with SCCs
-fprof-exported Annotates just exported bindings with SCCs
-fprof-no-count-entries Do not maintain entry counts when profiling
(can make profiled code go faster; useful with
heap profiling where entry counts are not used)
Cost-centre stacks have a new semantics, which should in most cases
result in more useful and intuitive profiles. If you find this not to
be the case, please let me know. This is the area where I have been
experimenting most, and the current solution is probably not the
final version, however it does address all the outstanding bugs and
seems to be better than GHC 7.2.
Stack traces
------------
+RTS -xc now gives more information. If the exception originates from
a CAF (as is common, because GHC tends to lift exceptions out to the
top-level), then the RTS walks up the stack and reports the stack in
the enclosing update frame(s).
Result: +RTS -xc is much more useful now - but you still have to
compile for profiling to get it. I've played around a little with
adding 'head []' to GHC itself, and +RTS -xc does pinpoint the problem
quite accurately.
I plan to add more facilities for stack tracing (e.g. in GHCi) in the
future.
Coverage (HPC)
--------------
* derived instances are now coloured yellow if they weren't used
* likewise record field names
* entry counts are more accurate (hpc --fun-entry-count)
* tab width is now correct (markup was previously off in source with
tabs)
Internal changes
================
In Core, the Note constructor has been replaced by
Tick (Tickish b) (Expr b)
which is used to represent all the kinds of source annotation we
support: profiling SCCs, HPC ticks, and GHCi breakpoints.
Depending on the properties of the Tickish, different transformations
apply to Tick. See CoreUtils.mkTick for details.
Tickets
=======
This commit closes the following tickets, test cases to follow:
- Close #2552: not a bug, but the behaviour is now more intuitive
(test is T2552)
- Close #680 (test is T680)
- Close #1531 (test is result001)
- Close #949 (test is T949)
- Close #2466: test case has bitrotted (doesn't compile against current
version of vector-space package)
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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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