[LLVMdev] [RFC] Encoding Compile Flags into the IR

[Bill Wendling]

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?
> 
The "Module-level Flags" is implemented using named metadata. Removing those flags would result in incorrect semantics for the Objective-C runtime. So there is a precedent.

-bw