[LLVMdev] [RFC] Embedding command line options in bitcode (PR21471)

[Bob Wilson]

> On Nov 19, 2014, at 6:06 PM, Chandler Carruth <chandlerc at google.com> wrote:
> 
> So I am deeply concerned about the direction this is taking. I'm trying to catch up on the thread, but I think Chris already highlighted my issue:
> 
> On Fri, Nov 14, 2014 at 4:57 PM, Chris Bieneman <cbieneman at apple.com <mailto:cbieneman at apple.com>> wrote:
>>> 1. How do we make sure we continue to be able to use the command line options we've been using for llc and other tools?
> 
> In discussions about the new cl::opt API I believe the general idea was that most of the options expressed using cl::opt are actually only relevant as debug options, so I think one big part of this work is really going to be identifying a subset of the current options which are actually relevant to expose in the IR.
> 
> I think this is critical.
> 
> The whole idea of cl::opt API is for *debugging* options. IE, not supported, expected variations on how passes behave. Those should always be controlled (at the LLVM API layer) through constructors and parameters, not through a side-layer.
> 
> There are parts of LLVM currently abusing the cl::opt mechanism to control fundamental functionality, but we should *absolutely* not bake any part of that or support for that into the IR! We should go find and fix those places to use reasonable APIs. Once we have that, I am very supportive of getting a good system for transmitting these options in bitcode and such in order to better support LTO. However, I think that in essentially every case there are going to be two options:
> 
> 1) Turn these options into function attributes because they can reasonably live as function attributes and different variations can co-exist within a module.

I have heard several people express strong opinions that even when those options are represented as function attributes, we will want a mechanism to override those with llc command line options for experimentation and debugging. If so, they will still need to exist as cl::opt (or some other equivalent). Are you suggesting otherwise?

> 
> 2) Keep the options as module-level options, but insist that they match for all modules being merged in LTO.
> 
> 3) (very rare) have clean, well specified merge semantics to merge different options from different modules in LTO. I think these only come up quite rarely. The only really good example I know of would be something like "library link dependencies" where it is a list that we clearly just take the union to merge.
> 
> 
>>> 3. Where should the command line options or module/function attributes be stored once they are read out from the IR?
> 
> My suggestion would be the OptionStore that I proposed here: http://reviews.llvm.org/D6207 <http://reviews.llvm.org/D6207>
> 
> Not to knock the option store (i quite like it), but I think that should be reserved for the cl::opt-style (but with your new API which is way better) debugging options, and never touch the IR.
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