[llvm] [GISel] Fold bitreverse(shl/srl(bitreverse(x),y)) -> srl/shl(x,y) (PR #91355)

[Thorsten Schütt via llvm-commits]

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@@ -323,6 +323,24 @@ def reduce_shl_of_extend : GICombineRule<
          [{ return Helper.matchCombineShlOfExtend(*${mi}, ${matchinfo}); }]),
   (apply [{ Helper.applyCombineShlOfExtend(*${mi}, ${matchinfo}); }])>;
 
+// Combine bitreverse(shl (bitreverse x), y)) -> (lshr x, y)
+def bitreverse_shl : GICombineRule<
+  (defs root:$d, build_fn_matchinfo:$matchinfo),
+  (match (G_BITREVERSE $rev, $val),
+         (G_SHL $src, $rev, $amt),
+         (G_BITREVERSE $d, $src):$mi,
+         [{ return Helper.matchBitreverseShift(*${mi}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${mi}, ${matchinfo}); }])>;
+
+// Combine bitreverse(lshr (bitreverse x), y)) -> (shl x, y)
+def bitreverse_lshr : GICombineRule<
+  (defs root:$d, build_fn_matchinfo:$matchinfo),
+  (match (G_BITREVERSE $rev, $val),
+         (G_LSHR $src, $rev, $amt),
+         (G_BITREVERSE $d, $src):$mi,
+         [{ return Helper.matchBitreverseShift(*${mi}, ${matchinfo}); }]),
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tschuett wrote:

Instead of passing in a MachineInstr and reconstructing the pattern, you could pass in MachineOperands, e.g, d, val, amt. Then you can take the registers of the MOs, do the legality check, and build the shift.

https://github.com/llvm/llvm-project/pull/91355