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

[Craig Topper via llvm-dev]

As a general rule, InstCombine tries not increase the total number of
instructions. If X/Y has another use other than this one, then it still
ends up being calculated. Without the one use check you'd trade 2 fdivs,
for 1 mul (Y * Z), and 2 fdivs ((X*Y)/Z) and the original (X / Y).

~Craig


On Mon, Dec 30, 2019 at 4:07 PM raghesh via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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