[llvm-dev] Duplicating node in SelectionDAG?

[Ryan Taylor via llvm-dev]

Matt,

  I would have suspecting the same but it's not matching that.

For example:

c = a * b; // this matches mem, mem, reg just fine

c = a * a; // this does not match

Maybe because of the chaining up from the TargetGlobalAddress? but the
first example has that also for a and b, it's basically the exact same DAG
for those two examples except in a * a there is only one chain leading to
the ld (and one ld) and the mul's operand 1 and 2 go to the one load.

It would seem to me it's not matching since it expects two load nodes and
there is only one.

Thanks.

On Tue, Sep 29, 2015 at 4:00 PM, Matt Arsenault <Matthew.Arsenault at amd.com>
wrote:

> On 09/29/2015 12:25 PM, Ryan Taylor via llvm-dev wrote:
>
> It appears that it's impossible to duplicate a node in the dag. For
> example, there is some code:
>
> Correct, nodes are uniqued and CSEd
>
>
> b = a * a; // a is a  global int
>
> A LD node is generated for A and it goes into both Operand 0 and 1 of the
> MUL node. The issue is I'm trying to match a pattern of:
>
> set dstReg:$dstD  (OpNode (srcAType (load addr32:$srcA)), (srcBType (load
> addr32:$srcB)))
>
> so basically a mem, mem, reg operation.
>
> The issue is this pattern won't match in the above example because there
> is only one LD generated for 'a'. I tried to duplicate the LD in the dag
> but it doesn't show up, it always reduces it to only one LD no matter what,
> even if I have multiple loads in the IR also.
>
> #1. Is it possible to duplicate an exact copy of a node in the dag?
>
> No
>
> #2. How would I go about matching this pattern in table gen?
>
> I would be surprised that patterns that happen to match the same node
> multiple times as operands wouldn't work as is. Maybe there is some
> complexity because these nodes have chains added? Have you looked at the
> generated matching code for why it isn't selecting? If all else fails you
> should be able to use a ComplexPattern
>
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