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

[Chris Bieneman]

> On Nov 17, 2014, at 9:52 AM, Bob Wilson <bob.wilson at apple.com> wrote:
> 
>> 
>> On Nov 14, 2014, at 2:44 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> 
>> +chrisb
>> 
>>> On 2014-Nov-13, at 16:33, Akira Hatanaka <ahatanak at gmail.com> wrote:
>>> 
>>> I'm working on fixing PR21471, which is about embedding codegen command line options into the bitcode as function or module-level attributes so that they don't get ignored when doing LTO.
>>> 
>>> http://llvm.org/bugs/show_bug.cgi?id=21471
>>> 
>>> I have an initial patch (attached to this email) which enables clang/llvm to recognize one command line option, write it to the IR, and read it out in a backend pass. I'm looking to get feedback from the community on whether I'm headed in the right direction or whether there are alternate ideas before I go all the way on fixing the PR. Specifically, I'd like to know the answers to the following questions:
>>> 
>>> 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?
>>> 2. How to handle cases where two functions in a module have different sets of command line options?
>>> 3. Where should the command line options or module/function attributes be stored once they are read out from the IR?
>>> 
>>> The short description of the approach I took in my patch is that command line options that are important to codegen are collected by cl::ParseCommandLineOptions, written to the bitcode as function or module attributes, and read out directly by the optimization passes that need them. cl::opt options are replaced with CodeGenOpt options (subclass of cl::opt), which are needed only to parse the command line and provide the default value when the corresponding options are not in the bitcode.
>> 
>> I like this approach, since it means the frontend doesn't have to understand
>> options in order to pass them on to the backend.
> 
> I agree. It’s not the approach that was I was expecting, but after reading the patches, I like it.
> 
> If we can get consensus on the approach, I suggest that we stage the work to first adopt Akira’s patches and then we can incrementally convert various options over to use it. There may be complications for some options, so I’d like to see separate patches for each one (possibly with a few similar options combined in one patch).

I’ve been thinking about this over the weekend. If we’re going to take a route like this, it would be better if we marked the options using one of the cl::opt property things (i.e. cl::hidden) rather than using a subclass of cl::opt. That also means moving the handling code either into the parser class, or (my preference) into cl::ParseCommandLine.

Setting a property on the option will be easier to migrate to the new API.

I also think that we need to get rid of the global storage somehow. It feels really nasty storing all the attributes globally when they really should probably be owned by the context or module. At the very least they should be ManagedStatic so that we avoid the static initializers.

-Chris

> 
>> 
>> The static variables should be straightforward to migrate to an LLVMContext
>> once ParseCommandLineOptions stores things there instead of in globals.
>> 
>>> diff --git a/lib/CodeGen/CodeGenOption.cpp b/lib/CodeGen/CodeGenOption.cpp
>>> new file mode 100644
>>> index 0000000..2d74c2f
>>> --- /dev/null
>>> +++ b/lib/CodeGen/CodeGenOption.cpp
>>> @@ -0,0 +1,59 @@
>>> +//===- CodeGen/CodeGenOptions.cpp - Code-gen option.           --*- C++ -*-===//
>>> +//
>>> +//                     The LLVM Compiler Infrastructure
>>> +//
>>> +// This file is distributed under the University of Illinois Open Source
>>> +// License. See LICENSE.TXT for details.
>>> +//
>>> +//===----------------------------------------------------------------------===//
>>> +//
>>> +//===----------------------------------------------------------------------===//
>>> +
>>> +#include "llvm/CodeGen/CodeGenOption.h"
>>> +#include "llvm/IR/Attributes.h"
>>> +#include "llvm/IR/Module.h"
>>> +
>>> +using namespace llvm;
>>> +
>>> +static std::map<std::string, bool> FunctionBoolAttrs;
>>> +static std::map<std::string, bool> ModuleBoolAttrs;
>>> +
>> 
>> @Chris, should these be using ManagedStatic?
>> 
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