[llvm] GlobalISel needs fdiv 1 / sqrt(x) to rsq combine (PR #78673)

[Nick Anderson via llvm-commits]

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@@ -33,6 +33,14 @@ def rcp_sqrt_to_rsq : GICombineRule<
          [{ return matchRcpSqrtToRsq(*${rcp}, ${matchinfo}); }]),
   (apply [{ Helper.applyBuildFn(*${rcp}, ${matchinfo}); }])>;
 
+def fdiv_1_by_sqrt_to_rsq : GICombineRule<
+  (defs root:$root),
+  (match (G_FSQRT $sqrt, $x, (MIFlags FmContract)),
+         (G_FCONSTANT $one, $fpimm),
+         (G_FDIV $dst, $one, $sqrt, (MIFlags FmContract)):$root,
+         [{ return ${fpimm}.getFPImm()->isExactlyValue(1.0)
+            || ${fpimm}.getFPImm()->isExactlyValue(-1.0); }]),
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nickleus27 wrote:

> I agree generalizing makes sense if you're going to handle more than 1/sqrt - you can simplify the apply function at least to just emit `y * rsq(x)` and let other combine eliminate a `1*rsq`, or transform `-1*rsq` into `fneg(rsq)`

So should I re-implement this PR to match `y/sqrt(x)` and have the apply function emit `y * rsq(x)`  and a second combine match `1 * rsq` & `-1 * rsq` to `rsq` & `fneg(rsq)` respectively?

https://github.com/llvm/llvm-project/pull/78673