[llvm-dev] [RFC][LLVM] New Constant type for representing function PLT entries

Leonard Chan via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 27 10:40:58 PDT 2020

Does anyone have more comments? Otherwise, I'll go ahead and update my
patch to reflect my update.

On Mon, Aug 24, 2020 at 6:47 PM Leonard Chan <leonardchan at google.com> wrote:

> Thanks for the responses! I’m going to see if I can summarize the concerns
> and ideas people have (for my own clarity) and see where we can go on from
> there. Folks seem to be on board with the idea of introducing some new IR
> entity that (after linking) *could* be a reference into the PLT, but some
> kinks need to be worked out first:
> *Naming* (Thanks for clarifications maskray at . I mixed up some terminology
> and concepts.): Because the PLT is primarily the concern of the linker, the
> naming probably shouldn’t be directly tied to “PLT”. The initial proposal
> was for something that matched the @plt modifier on x86, so that’s what
> inspired the naming. The intended behavior of this IR level change is that
> at least on x86 or aarch64, the resolved constant could be lowered to
> something that has the `@plt` syntax, but I suppose other targets could
> have their own meaning for “the address of this function is insignificant.”
> *Abstraction*: The IR representation of this probably shouldn’t be too
> strictly mapped to object file representations. It’s useful to have an IR
> pattern that can be mapped to relocations on different binary formats, but
> we don’t want to introduce a state where we have new Constants for
> individual relocations. The IR-entity should remain abstract enough that
> it’s not tied to a specific relocation, but it can still be lowered
> appropriately by different backends.
> As an update to the proposal, instead of `pltentry(@func)`, we can call it
> something like `unnamedfunc(@func)` and everywhere it’s used, it means:
> “The value used here is functionally equivalent to the original function,
> but may not be a reference to the original function. The address of this
> value is insignificant.” This is leveraged from `unnamed_addr` where the
> address of a global variable is insignificant, but this would instead be
> tied to instances where the function is used rather than be attached to the
> function declaration/definition. `unnamedfunc(@func)` could be lowered to a
> direct reference (func), the @plt modifier on x86/aarch64 (func at plt), a
> thunk, or anything that’s equivalent to the resolved function at runtime.
> Implementation-wise, I imagine we don’t want this as a subclass of
> GlobalValue. As Peter suggested, this may not eventually lower to a symbol.
> If it were a GlobalValue, that would also imply linkage types and
> visibility would also apply to it which might not make sense. A GlobalValue
> also seems to imply a module-level entity when this would primarily be used
> on individual locations where a function would normally be used.
> Is there anything else that should be addressed? Hopefully this addresses
> some concerns.
> On Fri, Aug 21, 2020 at 6:14 PM Eli Friedman <efriedma at quicinc.com> wrote:
>> > -----Original Message-----
>> > From: Fāng-ruì Sòng <maskray at google.com>
>> > Sent: Friday, August 21, 2020 4:04 PM
>> > To: Eli Friedman <efriedma at quicinc.com>
>> > Cc: Leonard Chan <leonardchan at google.com>; llvm-dev at lists.llvm.org
>> > Subject: [EXT] Re: [llvm-dev] [RFC][LLVM] New Constant type for
>> > representing function PLT entries
>> >
>> > On Fri, Aug 21, 2020 at 3:32 PM Eli Friedman <efriedma at quicinc.com>
>> wrote:
>> > >
>> > > > -----Original Message-----
>> > > > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
>> Fangrui
>> > > > Song via llvm-dev
>> > > > Sent: Thursday, August 20, 2020 10:18 PM
>> > > > To: Leonard Chan <leonardchan at google.com>
>> > > > Cc: llvm-dev <llvm-dev at lists.llvm.org>
>> > > > Subject: [EXT] Re: [llvm-dev] [RFC][LLVM] New Constant type for
>> > > > representing function PLT entries
>> > > >
>> > > Maybe it would make sense to introduce a GlobalValue to represent
>> this,
>> > > along the lines of GlobalIFunc?  I guess the end result isn't a lot
>> different
>> > > from the original proposal: you still end up with a Constant that
>> represents
>> > > the PLT entry.  But I think it would fit more smoothly into existing
>> > > optimizations that understand GlobalValues.  And it would make it
>> clear that
>> > > importing one from another IR module might be a non-trivial
>> operation.  (I
>> > > don't think we actually do cross-DSO optimizations at the moment, but
>> it's
>> > > not outside the realm of possibility.)
>> > >
>> > A new subclass of GlobalIndirectSymbol? Looks fine. Do you think the
>> > name "plt" should be moved from the name of the subclass to an
>> > argument of the syntax (like alias/ifunc)?
>> The IR concept could be generalized so it doesn't actually depend on the
>> object format having a PLT.  For example, it could be used to refer to an
>> import thunk on Windows.  So I'm not sure we want to name the IR concept
>> after a "PLT".
>> LLVM IR currently has the dso_local concept, so maybe we want to leverage
>> that in the name. Maybe something like GlobalDSOLocalFunc?  (Not sure I'm
>> really happy with that exact name, but hopefully the idea makes sense.)
>> -Eli
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