llvm dev - Oct 2016 - RFC: Absolute or "fixed address" symbols as immediate operands

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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On 25 October 2016 at 17:07, Reid Kleckner <rnk at google.com>
wrote:
> 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

You get the code you want with

@foo = external hidden global i8

or

@foo = external protected global i8


Cheers,
Rafael

On Tue, Oct 25, 2016 at 2:48 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
> You get the code you want with
>
> @foo = external hidden global i8
>
> or
>
> @foo = external protected global i8
>
No, that produces this when PIC is on:
        leaq    foo(%rip), %rax
        addq    %rdi, %rax
        retq

We need a way to communicate that foo is really an immediate and not
PC-relative.
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> You get the code you want with
>
> @foo = external hidden global i8
>
> or
>
> @foo = external protected global i8
Sorry, not quiet. What you get is

leaq    foo(%rip), %rax

Since it is still assuming foo is a position in the file.

So yes, we need a way to declare an absolute value. If we can declare
it, we may as well use the same construct to define it.

I guess the only issue then is that the current implementation of
alias becomes even more needlessly generic :-(

Cheers,
Rafael