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

[Jay Foad 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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jayfoad wrote:

> Could we do -1/sqrt(x) to fneg(rsq(x))?

Yes, especially since fneg is cheap since it can be folded into lots of other (AMDGPU) instructions. But it does make me wonder again, if you're going to generalise, why not generalise to any value? I.e. y/sqrt(x) -> y*rsq(x)

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