llvm dev - Oct 2020 - Single use rule for chained node in pattern matcher

Hi,

I'm trying to write a pattern for a truncating strict​*floating-point
instruction that broadcast the result in the resulting register. However the
pattern is not recognized by the pattern matcher because the any_fpround node is
used more than once due to the broadcasting. I'm not quite sure I understand
the reason behind that single use rule [1] (introduced by commit [2]) but all
the uses are within the same pattern, as arguments of the same node even. Is
there a way to relax the single use rule to allow uses in the same node? If not,
how would you suggest I go about implementing pattern matching for this
instruction?

[1]
https://reviews.llvm.org/source/llvm-github/browse/master/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp$3214-3228
[2] https://reviews.llvm.org/rGf8695c1ee9bb8b18a4e1991591fa92383d427435

Thanks in advance for your help. Best regards,

Thomas
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Single-use is an easy condition to check, easier than analyzing the consequences
of folding a sub-DAG on a case by case basis.

In general, chain edges can appear. Consider this case (with arrows being
chains):

A --> C
 \   /
   B

The chains are A->C, B->C, assume B has multiple uses, but A and C are
single-use.  If ABC gets matched to T, B must remain due to the extra uses, and
we end up with T having a cyclic chain edges with B.

--
Krzysztof Parzyszek  mailto:kparzysz at quicinc.com   AI tools development

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Thomas
Preud'homme via llvm-dev
Sent: Monday, October 19, 2020 4:40 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Cc: Chris Lattner <clattner at llvm.org>
Subject: [EXT] [llvm-dev] Single use rule for chained node in pattern matcher

Hi,

I'm trying to write a pattern for a truncating strict​*floating-point
instruction that broadcast the result in the resulting register. However the
pattern is not recognized by the pattern matcher because the any_fpround node is
used more than once due to the broadcasting. I'm not quite sure I understand
the reason behind that single use rule [1] (introduced by commit [2]) but all
the uses are within the same pattern, as arguments of the same node even. Is
there a way to relax the single use rule to allow uses in the same node? If not,
how would you suggest I go about implementing pattern matching for this
instruction?

[1]
https://reviews.llvm.org/source/llvm-github/browse/master/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp$3214-3228
[2] https://reviews.llvm.org/rGf8695c1ee9bb8b18a4e1991591fa92383d427435

Thanks in advance for your help. Best regards,

Thomas

Hi Krzysztof,

Thanks for the explanation. How about adding the requirement that not only all
those uses are by the same node but also that the node is within the sub-DAG
being matched?

Best regards,

Thomas
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Krzysztof
Parzyszek via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 19 October 2020 16:53
To: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Single use rule for chained node in pattern matcher

Single-use is an easy condition to check, easier than analyzing the consequences
of folding a sub-DAG on a case by case basis.

In general, chain edges can appear. Consider this case (with arrows being
chains):

A --> C
 \   /
   B

The chains are A->C, B->C, assume B has multiple uses, but A and C are
single-use.  If ABC gets matched to T, B must remain due to the extra uses, and
we end up with T having a cyclic chain edges with B.

--
Krzysztof Parzyszek  mailto:kparzysz at quicinc.com   AI tools development

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Thomas
Preud'homme via llvm-dev
Sent: Monday, October 19, 2020 4:40 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Cc: Chris Lattner <clattner at llvm.org>
Subject: [EXT] [llvm-dev] Single use rule for chained node in pattern matcher

Hi,

I'm trying to write a pattern for a truncating strict​*floating-point
instruction that broadcast the result in the resulting register. However the
pattern is not recognized by the pattern matcher because the any_fpround node is
used more than once due to the broadcasting. I'm not quite sure I understand
the reason behind that single use rule [1] (introduced by commit [2]) but all
the uses are within the same pattern, as arguments of the same node even. Is
there a way to relax the single use rule to allow uses in the same node? If not,
how would you suggest I go about implementing pattern matching for this
instruction?

[1]
https://reviews.llvm.org/source/llvm-github/browse/master/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp$3214-3228
[2] https://reviews.llvm.org/rGf8695c1ee9bb8b18a4e1991591fa92383d427435

Thanks in advance for your help. Best regards,

Thomas
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