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The problem with unreachable code is that it might refer to undefined
registers. This happens accidentally: a block can be orphaned by an
optimisation, for example when the result of a comparsion becomes
known.
The register allocator panics when it finds an undefined register,
because they shouldn't occur in generated code. So we need to also
discard unreachable code to prevent this panic being triggered by
optimisations.
The register alloator already does a strongly-connected component
analysis, so it ought to be easy to make it discard unreachable code
as part of that traversal. It turns out that we need a different
variant of the scc algorithm to do that (see Digraph), however the new
variant also generates slightly better code by putting the blocks
within a loop in a better order for register allocation.
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This removes the OldCmm data type and the CmmCvt pass that converts
new Cmm to OldCmm. The backends (NCGs, LLVM and C) have all been
converted to consume new Cmm.
The main difference between the two data types is that conditional
branches in new Cmm have both true/false successors, whereas in OldCmm
the false case was a fallthrough. To generate slightly better code we
occasionally need to invert a conditional to ensure that the
branch-not-taken becomes a fallthrough; this was previously done in
CmmCvt, and it is now done in CmmContFlowOpt.
We could go further and use the Hoopl Block representation for native
code, which would mean that we could use Hoopl's postorderDfs and
analyses for native code, but for now I've left it as is, using the
old ListGraph representation for native code.
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This squashes the "out of spill slots" panic that occasionally happens
on x86, by adding instructions to bump and retreat the C stack pointer
as necessary. The panic has become more common since the new codegen,
because we lump code into larger blocks, and the register allocator
isn't very good at reusing stack slots for spilling (see Note [extra
spill slots]).
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Proc-point splitting is only required by backends that do not support
having proc-points within a code block (that is, everything except the
native backend, i.e. LLVM and C).
Not doing proc-point splitting saves some compilation time, and might
produce slightly better code in some cases.
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CmmTop -> CmmDecl
CmmPgm -> CmmGroup
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I introduced this to support explicitly recording the info table label
in RawCmm for another patch I am working on, but it turned out to lead
to significant simplification in those parts of the compiler that
consume RawCmm.
Now, instead of lots of tests for null [CmmStatic] we have a simple
test of a Maybe, and have reduced the number of guys that need to know
how to convert entry->info labels by a TON. There are only 3 callers
of that function now!
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I observed that the [CmmStatics] within CmmData uses the list in a very stylised way.
The first item in the list is almost invariably a CmmDataLabel. Many parts of the
compiler pattern match on this list and fail if this is not true.
This patch makes the invariant explicit by introducing a structured type CmmStatics
that holds the label and the list of remaining [CmmStatic].
There is one wrinkle: the x86 backend sometimes wants to output an alignment directive just
before the label. However, this can be easily fixed up by parameterising the native codegen
over the type of CmmStatics (though the GenCmmTop parameterisation) and using a pair
(Alignment, CmmStatics) there instead.
As a result, I think we will be able to remove CmmAlign and CmmDataLabel from the CmmStatic
data type, thus nuking a lot of code and failing pattern matches. This change will come as part
of my next patch.
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This changes the new code generator to make use of the Hoopl package
for dataflow analysis. Hoopl is a new boot package, and is maintained
in a separate upstream git repository (as usual, GHC has its own
lagging darcs mirror in http://darcs.haskell.org/packages/hoopl).
During this merge I squashed recent history into one patch. I tried
to rebase, but the history had some internal conflicts of its own
which made rebase extremely confusing, so I gave up. The history I
squashed was:
- Update new codegen to work with latest Hoopl
- Add some notes on new code gen to cmm-notes
- Enable Hoopl lag package.
- Add SPJ note to cmm-notes
- Improve GC calls on new code generator.
Work in this branch was done by:
- Milan Straka <fox@ucw.cz>
- John Dias <dias@cs.tufts.edu>
- David Terei <davidterei@gmail.com>
Edward Z. Yang <ezyang@mit.edu> merged in further changes from GHC HEAD
and fixed a few bugs.
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- nativeGen/Instruction defines a type class for a generic
instruction set. Each of the instruction sets we have,
X86, PPC and SPARC are instances of it.
- The register alloctors use this type class when they need
info about a certain register or instruction, such as
regUsage, mkSpillInstr, mkJumpInstr, patchRegs..
- nativeGen/Platform defines some data types enumerating
the architectures and operating systems supported by the
native code generator.
- DynFlags now keeps track of the current build platform, and
the PositionIndependentCode module uses this to decide what
to do instead of relying of #ifdefs.
- It's not totally retargetable yet. Some info info about the
build target is still hardwired, but I've tried to contain
most of it to a single module, TargetRegs.
- Moved the SPILL and RELOAD instructions into LiveInstr.
- Reg and RegClass now have their own modules, and are shared
across all architectures.
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