llvm dev - Dec 2015 - RFC: __attribute__((ifunc("resolver")) representation in LLVM IR

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
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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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
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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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
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>