[llvm-dev] Any significance for m_OneUse in (X / Y) / Z => X / (Y * Z) ??

[raghesh via llvm-dev]

Dear All,

The InstCombine pass performs the following transformation.

   Z / (X / Y) => (Y * Z) / X

This is performed only when operand Op1 ( (X/Y) in this case) has only one
use in future. The code snippet is shown below.

    if (match(Op1, m_OneUse(m_FDiv(m_Value(X), m_Value(Y)))) &&
        (!isa<Constant>(Y) || !isa<Constant>(Op0))) {
      // Z / (X / Y) => (Y * Z) / X
      Value *YZ = Builder.CreateFMulFMF(Y, Op0, &I);
      return BinaryOperator::CreateFDivFMF(YZ, X, &I);
    }

It would be great if someone explains if there is any issue
(correctness/performance-wise) if we avoid the m_OueUse check. What if we
perform the transformation even if there are multiple uses?

There are similar transformations which perform the m_OueUse check. We may
avoid those too if there is no particular reason for the check.

Regards,
------------------------------
Raghesh Aloor
AMD India Pvt. Ltd.
Bengaluru.
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