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

[Eric Christopher via llvm-dev]

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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