[llvm-dev] Manipulating global address inside GlobalAddress SDNode in (RISCV) LLVM backend

[Reshabh Sharma via llvm-dev]

>
> I don't think there's a real shortage of those, but I confess I'm not
> sure why that's related. You'd need a representation for the LUI and
> ADDI after instruction selection anyway.


Yeah at the end we need a representation for LUI and ADDI. We were trying
to break the 64 bit address from GlobalAddress node into two i32 register.
We will add custom load/store which will generate the address using values
from two registers. We thought LUI and ADDI pair will be good to store the
values in a i32 register.  If we could transform GlobalAddress<0xHighLow>
directly to GlobalAddress<0xLow>, we could use the present RISCVII::MO_HI
and MO_LO as they only exact the 32 high bits. What do you think?
Many thanks for your reply :)

On Tue, Jul 9, 2019 at 9:41 PM Tim Northover <t.p.northover at gmail.com>
wrote:

> On Tue, 9 Jul 2019 at 14:49, Reshabh Sharma via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > For a GlobalAddress node say i64 = GlobalAddress<0xHighLow> we want to
> convert it into i32 = GlobalAddress<0xLow>.
>
> I think you'd have to convert it into a custom RISCVGlobalAddressLow
> and RISCVGlobalAddressHigh pair because the type of GlobalAddress is
> fixed to pointer type in TargetSelectionDAG.td (that's not 100% set in
> stone, but I wouldn't violate it lightly). And that might well be
> functionally equivalent to making an LUI/ADDI pair directly.
>
> > If there is no direct way to do this, we plan to fall back on a backup
> plan to convert the GlobalAddress node into the required LUI and ADDI pair
> but that would require the addition of two new target flag in RISCVII
> namespace.
>
> I don't think there's a real shortage of those, but I confess I'm not
> sure why that's related. You'd need a representation for the LUI and
> ADDI after instruction selection anyway.
>
> Cheers.
>
> Tim.
>
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