[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Reid Kleckner via llvm-dev]

On Tue, Oct 25, 2016 at 1:48 PM, Rafael Espíndola via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> So, for the declaration, do you expect to know the value? If not just
> a declaration to a GlobalVariable should be sufficient.
>

No, the value will be discovered at link time, but we want instruction
selection to treat the symbol as an immediate, not a GlobalAddress that
will receive PIC and GOT treatment.

If we use a declaration as you suggest, this example will compile awkwardly:

@foo = external global i8
define i64 @addfoo(i64 %v) {
  %cast = ptrtoint i8* @foo to i64
  %v1 = add i64 %v, %cast
  ret i64 %v1
}

The ideal code is:
addfoo:
  leaq foo(%rdi), %rax
  retq

Today we select:
addfoo:
        addq    foo at GOTPCREL(%rip), %rdi
        movq    %rdi, %rax
        retq

We could use attributes to try to convince ISel not to form GlobalAddress
nodes, but we might also want to consider just adding a new kind of global.

Chris's proposal of giving this new globalconst pointer type seems like a
reasonable compromise to avoid disturbing the rest of LLVM. The ptrtoint
cast will go away when we build MI.
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