[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 20 11:29:04 PDT 2020

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:


and would always have the same type as the function. A pltentry would
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.

Thoughts and opinions?


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