[LLVMdev] [cfe-dev] [RFC] Encoding Compile Flags into the IR

[John McCall]

On Apr 30, 2012, at 6:11 PM, Jim Grosbach wrote:
> On Apr 30, 2012, at 4:12 PM, Bill Wendling wrote:
>> On Apr 30, 2012, at 1:00 PM, dag at cray.com wrote:
>>> Bill Wendling <wendling at apple.com> writes:
>>> 
>>>> Link-Time Optimization has a problem. We need to preserve some of the
>>>> flags with which the modules were compiled so that the semantics of
>>>> the resulting program are correct. For example, a module compiled with
>>>> `-msoft-float' should use library calls for floating point. And that's
>>>> only the tip of the proverbial iceberg.
>>> 
>>> This is an important missing feature.
>>> 
>>>> - Named metadata would be used for those options which affect code
>>>> generation for the functions, but which doesn't prevent two modules
>>>> from being merged together. For example, `-fomit-frame-pointer'
>>>> applies to individual functions, but it doesn't prevent a module
>>>> compiled with `-fno-omit-frame-pointer' from being merged with one
>>>> compiled with `-fomit-frame-pointer'. We would use a named metadata
>>>> flag:
>>> 
>>> Doesn't this violate the "no semantics" requirement of metadata?  What
>>> happens if the metadata gets dropped?
>>> 
>> Named metadata cannot be stripped by normal methods.
>> 
> 
> Do we elsewhere use named metadata for any information that is required for correctness?

IIRC, the ARC frontend uses it to tell the optimizer what assembly
(if any) to add after calls to objc_retainAutoreleasedReturnValue.

John.