[LLVMdev] The Trouble with Triples

[Eric Christopher]

On Fri, Jul 31, 2015 at 12:40 PM Eric Christopher <echristo at gmail.com>
wrote:

> On Fri, Jul 31, 2015 at 12:22 PM Owen Anderson <resistor at mac.com> wrote:
>
>>
>> On Jul 31, 2015, at 11:28 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>>
>>
>> On Jul 31, 2015, at 10:56 AM, Eric Christopher <echristo at gmail.com>
>> wrote:
>>
>>
>> On 30 July 2015 at 14:52, Daniel Sanders <Daniel.Sanders at imgtec.com>
>>> wrote:
>>> > We will need a string serialization of the TargetTuple so that we can
>>> store it in the IR and read it back.
>>>
>>> Why does it have to be in the IR? If every tool that deals with IR has
>>> the same options and they mean the same thing I don't see why we'd
>>> need that.
>>>
>>>
>> I agree with the need for serializing information and options into the
>> module.
>>
>>
>>>
>>> > Whether this is in one piece (e.g. 'target tuple = "...") or multiple
>>> pieces (e.g. 'target arch = "..."', 'target endian = "..."') doesn't matter
>>> too much at first but I can see the benefits of the latter being the end
>>> point. If we do choose that multiple pieces then I'd like to pass through
>>> the former first to keep the early steps of the migration to a TargetTuple
>>> as simple and mechanical as possible.
>>>
>>> Assuming we really need it, wouldn't it work if we put in metadata?
>>> That way, it would bloat the IR of targets that really needed it and
>>> not the ones that don't, at the same time as being non-critical to
>>> parsing and validating the IR, so you could safely drop some
>>> information without breaking the file.
>>>
>>>
>> Metadata won't work. Metadata is defined to not affect code correctness
>> and this obviously does.
>>
>>
>>
>> Is it true for Module level Metadata and not only for instruction
>> attached one?
>>
>>
>> There are examples of module-level metadata that impact the correctness
>> of the generated code in the LangRef today:
>>
>> http://llvm.org/docs/LangRef.html#module-flags-metadata
>>
>> http://llvm.org/docs/LangRef.html#objective-c-garbage-collection-module-flags-metadata
>>
>> http://llvm.org/docs/LangRef.html#automatic-linker-flags-module-flags-metadata
>> http://llvm.org/docs/LangRef.html#c-type-width-module-flags-metadata
>>
>
> Wow, those are terrible. I remember the first as it was a hack for dealing
> with LTO and being unable to specify code generation flags. It definitely
> was not meant to affect whether or not correct output happened.
>
> The rest appear to be even worse and should not have gone in. I can
> prepare a patch to remove them if you'd like.
>

In case it wasn't obvious this is me both being sad, mostly joking, and a
little serious about it. I realize that apparently the ship has sailed
here, but these should probably be first class IR citizens rather than
building off of the "named metadata" extension.

That last one definitely should go away in some means.

-eric
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