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

[Pierre van Houtryve via llvm-commits]

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@@ -33,6 +33,11 @@ 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:$root, build_fn_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_FSQRT, G_FDIV):$root,
+         [{ return matchFDivSqrt(*${root}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
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Pierre-vh wrote:

We can't create intrinsics in MIR patterns yet (I always forget to work on it haha), so that needs to stay in C++.
You will need to create your own apply function with the contents of the BuildFn you have below.

 -  You won't need matchdata, you can directly pass `${x}.getReg()` - `${x}` is a substitution that'll be replaced with a reference to the operand `$x` matched 
 - You will need to pass the `G_FDIV` inst so you can delete it. You can do that by adding a name to that pattern (just like the wip_match_opcode earlier was named $root). It'll be a `MachineInstr *`.


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