[llvm-dev] Understanding TargetLowering::LowerCall

[Zhang via llvm-dev]

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

[Tim Northover via llvm-dev]

On Mon, 22 Mar 2021 at 07:36, Zhang via llvm-dev
<llvm...@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.
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[jeniffer lesley via llvm-dev]

Hi Tim, 

I had similar problem a couple of weeks ago but I somehow circumvented it. However, I dont understand much from the description. Can you please provide some example of backend which does this? probably what functions in X86 backend handle this lowering? If I look at the code I will be able to understand more clearly.

[Tim Northover via llvm-dev]

On Mon, 22 Mar 2021 at 11:48, jeniffer lesley <jeniffe...@gmail.com> wrote:
> I had similar problem a couple of weeks ago but I somehow circumvented it. However, I dont understand much from the description. Can you please provide some example of backend which does this? probably what functions in X86 backend handle this lowering? If I look at the code I will be able to understand more clearly.

On X86 it's X86TargetLowering::LowerGlobalOrExternal.

[Zhang via llvm-dev]

Thanks Tim.

One more thing I'm still not quite clear is how to differ GlobalAddress from registers when selection instructions.

Say the Initial DAG is :

```

load<(dereferenceable load 4 from @gv)> t0, TargetGlobalAddress:i64<i32* @gv> 0, undef:i64

```

And in our ISA we have:

```

LOAD_RM reg1,reg2 which basically means reg1 = *reg2

```

However when I use [(set (i32 GP32:$dst),(load GP64:$reg))] as the pattern for this ISA. the aforementioned DAGNode got matched as well.

How do I prevent such nodes from being matched since in our ISA GlobalAddress requires custom lowering

 

 

------------------ Original ------------------

From:  "Tim Northover"<t.p.no...@gmail.com>;

Date:  Mon, Mar 22, 2021 05:57 PM

To:  "Zhang"<ad...@mayuyu.io>;

Cc:  "llvm-dev"<llvm...@lists.llvm.org>;

Subject:  Re: [llvm-dev] Understanding TargetLowering::LowerCall

[Tim Northover via llvm-dev]

On Tue, 23 Mar 2021 at 03:58, Zhang <ad...@mayuyu.io> wrote:
> Say the Initial DAG is :
> ```
> load<(dereferenceable load 4 from @gv)> t0, TargetGlobalAddress:i64<i32* @gv> 0, undef:i64
> ```

The initial DAG shouldn't have a *Target*GlobalAddress. That's an
already-selected machine node and so it won't be further processed by
instruction selection. So you probably need to find whatever code you
have that's turning the original GlobalAddress into a
TargetGlobalAddress and modify that to actually insert the correct
instructions too.

Cheers.

Tim