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

[Sanjay Patel via llvm-dev]

A couple more general comments:
1. There shouldn't be any correctness issues removing one-use checks (the
transform should be safe independently of use-counts).
2. Ideally, you can remove a m_OneUse() from the code and run 'make check'
or similar, and you will see a regression test failure because we have a
'negative' test to cover that pattern. That should make it clear that the
one-use check is there intentionally and what the effect of removing it is.
We've gotten better about including those kinds of regression tests over
time, but I don't know what percentage of all instcombine transforms
actually have that test coverage.

On Mon, Dec 30, 2019 at 7:12 PM Craig Topper via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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