[llvm-dev] Capturing x86 %rax in prologue to use in body

[John Reagan via llvm-dev]

As part of our OpenVMS-specific changes, we're adding an arg-count to
all of our calls.

We've done this as part of callLowering to put the # of slots used into
%ah and leaving %al to hold the standard info about vector registers. 
That part is working.

However, we're struggling somewhat on the code in the prologue to
capture the %ah value to then be obtained by our "arg_count" compiler
builtin.  We found where the 1st parameter is put into a virtual
register in the event that it has to be passed back in %rax for
functions returning large structures.  We certainly could do the same
with %ah, but what IR would we generate to fetch that value in the body
of a routine?

We haven't created our own private OpenVMS calling convention but just
riding along on the standard convention.  Another thought would be to
create a new CC but that seems overkill just for this.

Anybody have suggestions on where we should be poking with our
screwdriver?  Do we have to write a new intrinsic to obtain that saved
value?

Thanks for any advice

John


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