[llvm] [GIsel] Add combiner for constant-folding G_BITCAST (PR #193060)

[Alex MacLean via llvm-commits]

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@@ -1390,6 +1394,23 @@ def constant_fold_cast_op : GICombineRule<
    [{ return Helper.matchConstantFoldCastOp(*${d}, ${matchinfo}); }]),
   (apply [{ Helper.replaceInstWithConstant(*${d}, ${matchinfo}); }])>;
 
+
+def constant_fold_bitcast_of_const : BuildFnCombineRule<
+  (match (G_CONSTANT $src, $imm),
+         (G_BITCAST $d, $src):$root,
+   [{ return Helper.matchConstantFoldBitcast(
+        ${d}, ${imm}.getCImm()->getValue(), ${matchinfo}); }])>;
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AlexMaclean wrote:

I'm not sure I understand. I don't think this case is as simple. We need C++ logic in the match to ensure we don't match on bitcasts to vectors. We also need a C++ apply to construct the new constant.

https://github.com/llvm/llvm-project/pull/193060