[llvm] [GlobalISel] Combine [a, s, z]ext of undef into 0 or undef (PR #117439)

[Amara Emerson via llvm-commits]

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@@ -1857,6 +1857,27 @@ class integer_of_opcode<Instruction castOpcode> : GICombineRule <
 
 def integer_of_truncate : integer_of_opcode<G_TRUNC>;
 
+def anyext_undef: GICombineRule<
+   (defs root:$root),
+   (match (G_IMPLICIT_DEF $undef),
+          (G_ANYEXT $root, $undef):$Aext,
+   [{ return Helper.isUndefLegalOrBeforeLegalizer(MRI.getType(${Aext}->getOperand(0).getReg())); }]),
+   (apply [{ Helper.replaceInstWithUndef(*${Aext}); }])>;
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aemerson wrote:

Sure ok, we have a tiny amount of support. We still have 99% to go.

https://github.com/llvm/llvm-project/pull/117439