[llvm-dev] RFC: __attribute__((ifunc("resolver")) representation in LLVM IR

[Dmitry Polukhin via llvm-dev]

I would like to support __attribute__((target)) later so ifunc won't be opaque
for compiler generated dispatchers.

Thank you all for the feedback!

On Sat, Dec 19, 2015 at 8:49 AM, Eric Christopher via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I think 2 is the only reasonable answer. And to answer Reid's comment:
> yes, it might be nice if we could look through it for compiler generated
> resolver functions - something to keep in mind for the future.
>
> On Fri, Dec 18, 2015, 5:47 PM Reid Kleckner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Are you going to have to teach LLVM how to look through ifuncs, or is it
>> OK if they are totally opaque?
>>
>> For example, with __attribute__((target)), you might have one
>> target-specialized function call another. Is it important that we be able
>> to optimize away the dynamic dispatch there or not?
>>
>> Either way, I think a new IR construct is the way to go.
>>
>> On Fri, Dec 18, 2015 at 11:44 AM, Rafael Espíndola <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I would probably go with 2.
>>>
>>> 3 is particularly undesirable as it make the ifunc really hard to
>>> differentiate from an alias.
>>>
>>> Cheers,
>>> Rafael
>>>
>>>
>>> On 18 December 2015 at 13:44, Dmitry Polukhin via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> > Hi Everyone,
>>> >
>>> > I would like to implement GCC ifunc attribute support in Clang and
>>> LLVM. At
>>> > the moment Clang ignores the attribute with a warning. On LLVM IR level
>>> > there is no support for ifunc. But there is some support for ELF
>>> symbol type
>>> > `@gnu_indirect_function` in ELF reader/writer/target asm parser. This
>>> RFC is
>>> > looking for thoughts and suggestions about representation of ifunc in
>>> LLVM
>>> > IR.
>>> >
>>> > Alternatives for ifunc representation:
>>> >
>>> > 1. Add a boolean flag isIFunc to`GlobalAlias`
>>> > From implementation perspective ifunc is very close to a function
>>> alias that
>>> > points to resolver function as a target. During asm emissions
>>> > `@gnu_indirect_function` symbol attribute is added. In printed LLVM it
>>> could
>>> > look like this:
>>> >
>>> > `@foo = alias ifunc i32 (i32), bitcast (i64 ()* @foo_ifunc to i32
>>> (i32)*)`
>>> >
>>> > Pros:
>>> > - minimal changes LLVM code base
>>> >
>>> > Cons:
>>> > - ifunc is not really an alias, if some code passes through alias and
>>> starts
>>> > using resolver function instead of ifunc, it is always an error
>>> > - in particular in my prototype I had to add weak linkage to inhibit
>>> > optimizations (it prevents them because LLVM assumes that linker could
>>> > change the symbol but it could be fixed without changing linkage)
>>> >
>>> > 2. Extract common parts for alias and ifunc into a base class
>>> > Both alias and ifunc will be derived classes. Similar to first
>>> proposal add
>>> > `@gnu_indirect_function` symbol attribute. In printed LLVM it could
>>> look
>>> > like:
>>> >
>>> > `@foo = ifunc i32 (i32), i64 ()* @foo_ifunc`
>>> >
>>> > Pros:
>>> > - no confusion for existing code
>>> > - cleaner from design perspective
>>> >
>>> > Cons:
>>> > - Add new type of Global i.e. more textual changes
>>> > - Some potential code duplication (need prototyping to estimate)
>>> >
>>> > 3. Emit global asm statement from Clang
>>> > Generate global asm statement like `__asm__ (".type
>>> resolver_alias_name,
>>> > @gnu_indirect_function")` in Clang for alias generated for resolver
>>> > function.
>>> >
>>> > Pros:
>>> > - (almost?) no changes in LLVM
>>> >
>>> > Cons:
>>> > - ifunc is not marked in LLVM IR at all so some hacks are required to
>>> detect
>>> > and inhibit optimizations in LLVM for such alias (it is always wrong
>>> to use
>>> > resolver instead of ifunc they even have different function type)
>>> > - asm statement in general not very reliable mechanism in general,
>>> highly
>>> > depends on expected asm generated by compiler
>>> > - using low-level platform dependent information in Clang
>>> >
>>> > I prototyped first approach, changes in http://reviews.llvm.org/D15525.
>>> But
>>> > got feedback that I should use second approach instead + request to
>>> write
>>> > RFC about this IR extension for broader discussion. I'm convinced that
>>> the
>>> > second approach is cleaner and if there is no better suggestion or push
>>> > back, I'm going to prepare such patch. Third approach mostly added for
>>> > completeness.
>>> >
>>> > References:
>>> > - GCC ifunc documentation
>>> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
>>> > - IFUNC low-level technical details
>>> http://www.airs.com/blog/archives/403
>>> >
>>> > Thanks,
>>> > Dmitry Polukhin
>>> > --
>>> > Software Engineer
>>> > Intel Compiler Team
>>> >
>>> >
>>> > _______________________________________________
>>> > LLVM Developers mailing list
>>> > llvm-dev at lists.llvm.org
>>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >
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>>
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