[PATCH] D86726: [InstCombine]: Transform 1.0/sqrt(X) * X to X/sqrt(X) and X * 1.0/sqrt(X) to X/sqrt(X)

[Noel Grandin via Phabricator via llvm-commits]

grandinj added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp:552
+
+    // X * 1.0/sqrt(X) = X/sqrt(X).
+    if (match(Op1, (m_FDiv(m_SpecificFP(1.0), m_Value(Y)))) &&
----------------
venkataramanan.kumar.llvm wrote:
> grandinj wrote:
> > Just a drive-by commentator: I am surprised there is not an existing transform which does
> >    1 * X ==> X
> >    X * 1 ==> X
> >    X / 1 ==> X
> Yes they are available. 
> 
> This one is specifically targeting the pattern where one operand  for multiplication  is X and the other operand is Divide.  The dividend is 1.0 and divisor is sqrt(x).  
> 
> x * 1/sqrt(x) ==> x/sqrt(x)  later on we try to fold it to sqrt(x) under associative math option. 
If they are available, then surely it is not necessary to do

    X * 1.0 / sqrt(X) ==> X/sqrt(X).

Surely by the time it gets there, that would already have been folded to

    X / sqrt(X)

?


Repository:
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  https://reviews.llvm.org/D86726/new/

https://reviews.llvm.org/D86726