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

[Dmitry Polukhin via llvm-dev]

>From what I see putting ifunc into separate list is better aligned with
LLVM design so I prepared CL that does it in http://reviews.llvm.org/D15525
Amount of changes is about 3-5x in comparison with using bool flag for
GlobalAlias (first patchset).

Having all global objects in single list does make some sense to me and
iterators could skip objects of uninteresting types. But I think such
change is much bigger and much deeper. So I'm not sure that the result
worse such major change in code base.

On Wed, Dec 23, 2015 at 6:21 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> If changing nothing else I would probably go with another list since
> that is what is done for functions/variables/aliases.
>
> Having said that, more than once I wished we had an easy way to
> iterate all GlobalValues or all GrobalObjects. Any thoughts on the
> tradeoff having a single list with pointer to the start of each sub
> list? Something like
>
> Func1 <-> Func2 <-> Var1 <-> Var2 <-> Alias1 <-> Alias2 <-> IFunc1 <->
> Ifunc2
>
> and keep FirstFunc, FirstVar, FirstAlias and FirstIFunc pointers.
>
> One place where having another list might be a bit annoying is the
> symbol iterator of IRObjectFile.
>
> Cheers,
> Rafael
>
>
> On 22 December 2015 at 08:23, Dmitry Polukhin <dmitry.polukhin at gmail.com>
> wrote:
> > I start prototyping second approach that makes ifunc new type of
> > GlobalValue. And I would like to reconfirm how much we would like to
> share
> > between aliases and ifuncs. For example, should Module have separate
> lists
> > of aliases and ifuncs or they both should be in the same list of indirect
> > symbols? If ifuncs and aliases are in separate lists, common base class
> is
> > less useful and I found myself duplicating aliases code to support ifunc
> > (that is relatively small code but here and there). On the other hand if
> I
> > rename aliases to indirect symbols, all places that work with aliases
> will
> > have to be updated to support this renaming and sometimes check if the
> > object in the list is an alias or ifunc (i.e. it becomes close to first
> > approach + bunch renaming). So I would like to hear community input on
> how
> > separate aliases and ifuncs should be.
> >
> > On Mon, Dec 21, 2015 at 12:19 PM, Dmitry Polukhin
> > <dmitry.polukhin at gmail.com> wrote:
> >>
> >> 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
> >>>>> >
> >>>>> _______________________________________________
> >>>>> LLVM Developers mailing list
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> >>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>>
> >>>>
> >>>> _______________________________________________
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> >>>
> >>>
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> >>>
> >>
> >
>
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