[all-commits] [llvm/llvm-project] a97f62: [llvm][IR] Add dso_local_equivalent Constant

[Leonard Chan via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: a97f62837f59620a60b3ed9f29568440c7d0553a
      https://github.com/llvm/llvm-project/commit/a97f62837f59620a60b3ed9f29568440c7d0553a
  Author: Leonard Chan <leonardchan at google.com>
  Date:   2020-11-19 (Thu, 19 Nov 2020)

  Changed paths:
    M llvm/docs/LangRef.rst
    M llvm/include/llvm/Analysis/ConstantFolding.h
    M llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
    M llvm/include/llvm/IR/Constants.h
    M llvm/include/llvm/IR/Value.def
    M llvm/include/llvm/Target/TargetLoweringObjectFile.h
    M llvm/lib/Analysis/ConstantFolding.cpp
    M llvm/lib/AsmParser/LLLexer.cpp
    M llvm/lib/AsmParser/LLParser.cpp
    M llvm/lib/AsmParser/LLToken.h
    M llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
    M llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
    M llvm/lib/IR/AsmWriter.cpp
    M llvm/lib/IR/Constants.cpp
    M llvm/lib/IR/Core.cpp
    M llvm/lib/IR/LLVMContextImpl.h
    A llvm/test/CodeGen/X86/dso_local_equivalent.ll
    M llvm/test/CodeGen/X86/relptr-rodata.ll

  Log Message:
  -----------
  [llvm][IR] Add dso_local_equivalent Constant

The `dso_local_equivalent` constant is a wrapper for functions that represents a
value which is functionally equivalent to the global passed to this. That is, if
this accepts a function, calling this constant should have the same effects as
calling the function directly. This could be a direct reference to the function,
the `@plt` modifier on X86/AArch64, a thunk, or anything that's equivalent to the
resolved function as a call target.

When lowered, the returned address must have a constant offset at link time from
some other symbol defined within the same binary. The address of this value is
also insignificant. The name is leveraged from `dso_local` where use of a function
or variable is resolved to a symbol in the same linkage unit.

In this patch:
- Addition of `dso_local_equivalent` and handling it
- Update Constant::needsRelocation() to strip constant inbound GEPs and take
  advantage of `dso_local_equivalent` for relative references

This is useful for the [Relative VTables C++ ABI](https://reviews.llvm.org/D72959)
which makes vtables readonly. This works by replacing the dynamic relocations for
function pointers in them with static relocations that represent the offset between
the vtable and virtual functions. If a function is externally defined,
`dso_local_equivalent` can be used as a generic wrapper for the function to still
allow for this static offset calculation to be done.

See [RFC](http://lists.llvm.org/pipermail/llvm-dev/2020-August/144469.html) for more details.

Differential Revision: https://reviews.llvm.org/D77248