[llvm] [GISel] funnel shift combiner port from SelectionDAG ISel to GlobalISel (PR #135132)

[Axel Sorenson via llvm-commits]

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@@ -1033,6 +1033,24 @@ def funnel_shift_overshift: GICombineRule<
   (apply [{ Helper.applyFunnelShiftConstantModulo(*${root}); }])
 >;
 
+// Transform: fshl x, ?, y | shl x, y -> fshl x, ?, y
+def funnel_shift_or_shift_to_funnel_shift_left: GICombineRule<
+  (defs root:$root), 
+  (match (G_FSHL $out1, $x, $_, $y),
+         (G_SHL $out2, $x, $y),
+         (G_OR $root, $out1, $out2)),
+  (apply (G_FSHL $root, $x, $_, $y))
+>;
+
+// Transform: fshr ?, x, y | srl x, y -> fshr ?, x, y
+def funnel_shift_or_shift_to_funnel_shift_right: GICombineRule<
+  (defs root:$root), 
+  (match (G_FSHR $out1, $_, $x, $y),
+         (G_LSHR $out2, $x, $y),
+         (G_OR $root, $out1, $out2)),
----------------
axelcool1234 wrote:

> Missing hasOneUse checks?

The original SelectionDAG ISel didn't have a check for this. I do see how it'd be problematic for the `G_OR` and `G_LSHR` statements to be removed if later code relied on them. How did the SelectionDAG ISel combiner not case this problem?

I also couldn't manage to find any Tablegen code utilizing `hasOneUse`.  If you have any resources or examples, I'd be immensely grateful!

https://github.com/llvm/llvm-project/pull/135132