[LLVMdev] make DataLayout a mandatory part of Module

[Rafael Espíndola]

On 29 January 2014 18:40, Nick Lewycky <nlewycky at google.com> wrote:
> The LLVM Module has an optional target triple and target datalayout. Without
> them, an llvm::DataLayout can't be constructed with meaningful data. The
> benefit to making them optional is to permit optimization that would work
> across all possible DataLayouts, then allow us to commit to a particular one
> at a later point in time, thereby performing more optimization in advance.
>
> This feature is not being used. Instead, every user of LLVM IR in a
> portability system defines one or more standardized datalayouts for their
> platform, and shims to place calls with the outside world. The primary
> reason for this is that independence from DataLayout is not sufficient to
> achieve portability because it doesn't also represent ABI lowering
> constraints. If you have a system that attempts to use LLVM IR in a portable
> fashion and does it without standardizing on a datalayout, please share your
> experience.
>
> The cost to keeping this feature around is that we have to pass around the
> DataLayout object in many places, test for its presence, in some cases write
> different optimizations depending on whether we have DataLayout, and in the
> worst case I can think of, we have two different canonical forms for
> constant expressions depending on whether DL is present. Our canonical IR is
> different with and without datalayout, and we have two canonicalizers
> fighting it out (IR/ConstantFold.cpp and Analysis/ConstantFolding.cpp).
>
> I'm trying to force the issue. Either this is a useful feature to maintain
> in which case I want to see a design on how to defer ABI decisions until a
> later point in time, or else we do not support it and target triple and
> target datalayout become a mandatory part of a valid Module again. I think
> the correct direction is to make them mandatory, but this is a large change
> that warrants debate.

I don't think we can reasonably express all the information needed by
ABIs at the LLVM level. Given that, It would *love* to see DataLayout
become a mandatory part of the IR!

> If we decide that target information should be a mandatory part of a module,
> there's another question about what we should do with existing .bc and .ll
> files that don't have one. Load in a default of the host machine?

For tools that don't link with target (llvm-as and llvm-dis being the
most extreme cases) it would have to be the default "". For opt I
would be ok with "" or the host triple.

Thanks,
Rafael