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

[Eli Friedman via llvm-dev]

Seems fine.  I’m not that attached to my GlobalValue idea.

-Eli

From: Leonard Chan <leonardchan at google.com>
Sent: Thursday, August 27, 2020 10:41 AM
To: Eli Friedman <efriedma at quicinc.com>
Cc: Fāng-ruì Sòng <maskray at google.com>; llvm-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] [RFC][LLVM] New Constant type for representing function PLT entries

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<mailto: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<mailto:efriedma at quicinc.com>> wrote:
> -----Original Message-----
> From: Fāng-ruì Sòng <maskray at google.com<mailto:maskray at google.com>>
> Sent: Friday, August 21, 2020 4:04 PM
> To: Eli Friedman <efriedma at quicinc.com<mailto:efriedma at quicinc.com>>
> Cc: Leonard Chan <leonardchan at google.com<mailto:leonardchan at google.com>>; llvm-dev at lists.llvm.org<mailto: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<mailto:efriedma at quicinc.com>> wrote:
> >
> > > -----Original Message-----
> > > From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto: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<mailto:leonardchan at google.com>>
> > > Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto: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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