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

[Hal Finkel via llvm-dev]

Hi, Leonard,

What are the motivating use cases?

  -Hal

On 8/20/20 1:29 PM, Leonard Chan via llvm-dev wrote:
>
> Hi all,
>
>
> We would like to propose a new Constant type in LLVM for representing 
> entries in the Procedure Linkage Table (PLT).
>
>
> The PLT is a data structure used for dispatching position-independent 
> function calls to appropriate functions where the address of the 
> function is not known statically. Right now, if a call is made to a 
> function, it may be lowered to a direct call to the function itself or 
> the PLT entry for that function. LLVM has checks that dictate if the 
> function call should be a direct reference or PLT entry, but we don’t 
> have a way in IR to semantically represent that the PLT entry should 
> be used.
>
>
> The proposed constant would be analogous to BlockAddress, but instead 
> represent the PLT entry for functions. The usage could look something 
> like:
>
>
> pltentry(@function)
>
>
> and would always have the same type as the function. A pltentrywould 
> operate exactly like a function, but the main difference is that it’s 
> lowered to the PLT entry (function at plt) on targets that support PLT 
> relocations. The linker can then decide if it should be relaxed into a 
> direct reference or remain a PLT entry.
>
>
> I have a very rough WIP implementation at 
> https://reviews.llvm.org/D77248 <https://reviews.llvm.org/D77248>.
>
>
> Thoughts and opinions?
>
>
> Thanks,
>
> Leonard
>
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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