[PATCH] D85709: [InstSimplify] Implement Instruction simplification for X/sqrt(X) to sqrt(X).

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D85709#2209979 <https://reviews.llvm.org/D85709#2209979>, @cameron.mcinally wrote:

> I'm fairly sure this transform is a performance loss. For a target like Skylake Server, a SQRT(x) can take up to 20 cycles. But a RSQRT(x) is about 6 cycles and a MUL(y) is 4 cycles. We'd be better off with a X*RSQRT(X).

That is up to backends to decide. InstSimplify/InstCombine (and a few others) are canonicalization, target-independent passes.
A single `sqrt(x)` is more canonical IR than `x/sqrt(x)`, because it's less instructions and `x` has less uses.


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https://reviews.llvm.org/D85709