[llvm-dev] [RFC] A proposal for byval in a world with opaque pointers

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Antoine Pitrou via llvm-dev" <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Sent: Tuesday, January 19, 2016 5:54:27 PM
> Subject: Re: [llvm-dev] [RFC] A proposal for byval in a world with opaque	pointers
> 
> On Wed, 20 Jan 2016 01:26:16 +0200
> "Eddy B. via llvm-dev" <llvm-dev at lists.llvm.org> wrote:
> > 2016-01-20 1:11 GMT+02:00 Antoine Pitrou via llvm-dev <
> > llvm-dev at lists.llvm.org>:
> > 
> > > On Wed, 20 Jan 2016 00:47:56 +0200
> > > "Eddy B. via llvm-dev" <llvm-dev at lists.llvm.org> wrote:
> > > >
> > > > I would love to know your thoughts on this, and more
> > > > specifically:
> > > > Which of the 3 (byval(T), byval(N) and byval + dereferenceable
> > > > + align)
> > > > do you think would provide the easiest transition path for
> > > > front-ends?
> > >
> > > We (llvmlite and Numba) don't use byval specifically but, were we
> > > to use
> > > it, I think byval(T) would by far be the easiest for us.  By
> > > contrast,
> > > computing ABI sizes is a bit of a PITA due to llvmlite's
> > > architecture
> > > (llvmlite's IR generation side doesn't call into LLVM at all, it
> > > generates textual IR in pure Python).
> > >
> > If you have the datalayout string, the ABI sizes become almost
> > trivial to
> > compute, the algorithms for structs are simple and I'm not aware of
> > any
> > special-cases that would complicate a front-end's computation.
> 
> For us the datalayout string is more of an implementation detail on
> LLVM's side, our front-end is not normally aware of it.  As for the
> algorithm, it may be trivial but is there an outline or specification
> of it somewhere? :-)
> 

The format is documented http://llvm.org/docs/LangRef.html#data-layout but you're not generally expected to parse the string directly. Instead, you'd use the DataLayout class (include/llvm/IR/DataLayout.h) which contains functions like 'uint64_t getTypeSizeInBits(Type *Ty) const;'

 -Hal

> Regards
> 
> Antoine.
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory