llvm dev - Mar 2021 - Understanding TargetLowering::LowerCall

For CallSites that contains a GlobalValue as one of the arguments, where exactly
does the lowering from GlobalAddress to iPTR took place?.


I'm assuming it should be handled in TargetLowering::LowerCall where
target-dependent DAG Nodes got created depending on CCValAssign::isRegLoc() but
from my testing it seems like isRegLoc() is true even for GlobalAddress
arguments.


What am I misunderstanding here?




Zhang
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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.