[LLVMdev] Problem selecting the correct registers for a calling convention

Job Noorman jobnoorman at gmail.com
Tue Jul 2 02:34:23 PDT 2013

Hi Tim,

I finally found some time to work on this issue again and I'm currently trying 
your suggestion: implementing the calling convention in C++.

I have the feeling that isSplit is not enough to deal with my problem since 
there seems to be no way to tell if later arguments belong to the split or 
not. For example, a call to a function f(i32, i16, i16) will look exactly the 
same as one to g(i64):

> Arg: 0, isSplit: 1
> Arg: 1, isSplit: 0
> Arg: 2, isSplit: 0
> Arg: 3, isSplit: 0

The information I need seems to be available in the InputArg class 
(OrigArgIndex and PartOffset) but unfortunately, only its Flags member is 
passed to the calling convention implementation.

Am I missing something here?


On Tuesday 27 November 2012 15:50:38 Tim Northover wrote:
> Hi Job,
> > This issue is basically that I cannot find a way to distinguish two i16
> > arguments from one i32. Is there a way to do this in LLVM? Preferably
> > using
> > tablegen, of course:-)
> I think the property you want is "isSplit" (or, from the TableGen side
> CCIfSplit).
> This gets applied to the first of those i16s that are produced.
> Unfortunately I can't think of much you can do from TableGen to swap
> the registers around (CCIfSplit is useful if the i32 would have to
> start at an even-numbered register, for example). But in the C++
> ISelLowering code you can use that flag to deal with the two registers
> together in a sane way.
> Tim.

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