llvm dev - Sep 2021 - PatternMatch Commutatively?

Hi:


I was trying to match the following pattern:


```
...
%8 = load i64, i64* %3, align 8
%9 = xor i64 %8, -1
...
%11 = or i64 %9, %10
...

```


which is roughly (~X | Y).


Given Or is commutative, I was under the assumption that trying to match (X |
~Y) would match (X | ~Y) and (~X | Y), but from testing with LLVM 11 it seems I
had to match the pattern twice:


```
if (match(bo, m_Or(m_Value(X), m_Not(m_Value(Y))))) {
  // X | (~Y)
  ....
} else if (match(bo, m_Or(m_Not(m_Value(X)), m_Value(Y)))) {
  // (~X) | Y
  ....
}

```


and only the second code path is used.






Going through IR/PatternMatch.h , it seems that while the
template BinaryOp_match has an optional argument Commutable,
it defaults to false and none of m_Or, m_Xor or m_And seem to set it to true.


What am I misunderstanding here?




Zhang
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You want `m_c_Or()`

On Thu, Sep 23, 2021 at 12:49 PM Zhang via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi:
>
> I was trying to match the following pattern:
>
> ```
> ...
> %8 = load i64, i64* %3, align 8
> %9 = xor i64 %8, -1
> ...
> %11 = or i64 %9, %10
> ...
> ```
>
> which is roughly (~X | Y).
>
> Given Or is commutative, I was under the assumption that trying to match (X
| ~Y) would match (X | ~Y) and (~X | Y), but from testing with LLVM 11 it seems
I had to match the pattern twice:
>
> ```
> if (match(bo, m_Or(m_Value(X), m_Not(m_Value(Y))))) {
>   // X | (~Y)
>   ....
> } else if (match(bo, m_Or(m_Not(m_Value(X)), m_Value(Y)))) {
>   // (~X) | Y
>   ....
> }
> ```
>
> and only the second code path is used.
>
>
>
> Going through IR/PatternMatch.h , it seems that while the template
BinaryOp_match has an optional argument Commutable, it defaults to false and
none of m_Or, m_Xor or m_And seem to set it to true.
>
> What am I misunderstanding here?
>
>
> Zhang
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