[llvm-dev] Different SelectionDAGs for same CPU

[Josh Sharp via llvm-dev]

Hi Tim,

>That C++ function is probably what looks for an FrameIndex node and
>has been taught that it can be folded into the load.

How do you teach a function that a node can be folded into an instruction?

________________________________
From: Tim Northover <t.p.northover at gmail.com>
Sent: Monday, January 21, 2019 11:52 PM
To: Josh Sharp
Cc: via llvm-dev
Subject: Re: [llvm-dev] Different SelectionDAGs for same CPU

Hi Josh,

On Tue, 22 Jan 2019 at 04:54, Josh Sharp via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> In the first case, node t1 is a separate node whereas in the second case, t1 is inside t4. What difference in implementation could explain this difference in behavior?

The second compiler looks like someone has added extra code to fold a
stack address calculation into the load operation that accesses the
variable.

> Where in the code should I look into?

It could be implemented in a couple of places. Most likely is that
XYZInstrInfo.td (or some related TableGen file) defines a
ComplexPattern that is used by the LDWI instruction definition. That
ComplexPattern tells pattern matching to call a specific function in
XYZISelDAGToDAG.cpp when deciding what to use for the LDWI operands.
That C++ function is probably what looks for an FrameIndex node and
has been taught that it can be folded into the load.

If you just grep the target's code for FrameIndex or frameindex you
should find it pretty quickly though, even if they used some other
method. There don't tend to be many uses of that particular node.

Cheers.

Tim.
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