On 17/10/12 07:48, Geoffrey Mainland wrote:
Thanks for the pointers. I've already rewritten the register allocator,
so I know where the information comes from...it just seems to get lost
when procedures are created. I was hoping there was a function somewhere
that could take a CmmProc and tell me what registers it expected live on
entry, but I don't think such a thing exists. My dumb attempt at writing
such a function is quickly becoming a full abstract interpreter, so I
think my best bet is to change CmmProc to include live register info. If
this turns out to be too painful I may go back to the bitcast idea.
It's not hard to attach this info to a CmmProc when we create it,
because we create the proc at the same time as inserting the copy-in
code for the calling convention. Look at
StgCmmMonad.emitProcWithStackFrame, and you'll need to change
mkCallEntry to return the set of live GlobalRegs. If you get stuck grab
me when I'm back in Cambridge (Friday).
Cheers,
Simon
Cheers,
David
On 16 October 2012 03:45, Geoffrey Mainland <[email protected]> wrote:
Hi all,
I'm working on changing GHC's calling convention to support overlapping
register classes. On x86-64 this will mean that a function can
receive up to
six Float/Double arguments (any mixture) in registers. Right now only
two
Double arguments can be passed in registers on x86-64, even if a
function
takes zero Float arguments---the register classes for Floats and Doubles
cannot overlap even though both are passed in xmm* registers.
This is working fine with the native back-end, but I'm not sure how
to get
it working with the LLVM back-end. Right now LLVM translates a Cmm
procedure
to an LLVM function of a single universal type( that is, all LLVM
functions
that the LLVM back-end produces for Cmm procedures have the same
type) that
takes all STG registers as arguments. A Cmm procedure call passes all
the
STG registers, although the back-end takes advantage of the liveness
information attached to Cmm jumps to pass undefined values for non-live
registers.
Now that register classes can overlap, I need to change this. Since
jumps
have liveness information attached, I can simply pass the live
registers.
But when translating Cmm procedures, I need to know which registers
are live
on entry to the procedure.
The slightly longer-term goal is to add support for passing SSE
vectors in
registers. What I'd like to have ideally is the set of live registers
*and
their Cmm types* at each procedure entry and jump. I envision having a
single STG register class, say X16, for all 128-bit-wide SSE
registers, but
I'd like the Cmm type information when generating LLVM type
signatures so I
can generate the appropriate LLVM types---a 128-bit-wide vector of
32-bit
integers does not have the same type as a 128-bit-wide vector of double
precision floats.
I might be able to do without the Cmm type information by inserting
appropriate casts in the generated LLVM code, but I really need the
set of
live registers on entry to a Cmm procedure. I don't see how to get this
unfortunately. Any hints?
Thanks,
Geoff
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