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[InstCombine] Update contributor guide for commuted tests (NFC) #155574

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[InstCombine] Update contributor guide for commuted tests (NFC) #155574

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19 changes: 3 additions & 16 deletions llvm/docs/InstCombineContributorGuide.md
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Expand Up @@ -96,22 +96,9 @@ may be omitted.
If the transform involves commutative operations, add tests with commuted
(swapped) operands.

Make sure that the operand order stays intact in the CHECK lines of your
pre-commited tests. You should not see something like this:

```llvm
; CHECK-NEXT: [[OR:%.*]] = or i8 [[X]], [[Y]]
; ...
%or = or i8 %y, %x
```

If this happens, you may need to change one of the operands to have higher
complexity (include the "thwart" comment in that case):

```llvm
%y2 = mul i8 %y, %y ; thwart complexity-based canonicalization
%or = or i8 %y, %x
```
As an exception, it is not necessary to test commutation if one of the operands
is a constant: In this case, the constant operand is always canonicalized to
the right.

### Add vector tests

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