[llvm-dev] Understanding TargetLowering::LowerCall

[Tim Northover via llvm-dev]

On Mon, 22 Mar 2021 at 07:36, Zhang via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> For CallSites that contains a GlobalValue as one of the arguments, where exactly does the lowering from GlobalAddress to iPTR took place?.

It's generally no different for a callsite than any other GlobalValue
user. The generic SelectionDAGBuilder code will create a GlobalAddress
SDNode with iPTR type because the type of a GlobalValue is actually a
pointer to the type of the underlying object.

The call lowering code will get this iPTR node, and may well decide to
pass it to the callee in a register.

As for the GlobalAddress node, that goes through normal lowering to a
target-specific instruction sequence to materialize the address of the
global object. Often this is via custom C++ in LegalizeDAG (i.e.
`setOperationAction(ISD::GlobalValue, MVT::i64, Custom)`).

Cheers.

Tim.