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

[Thorsten Schütt via llvm-commits]

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@@ -1857,6 +1857,26 @@ 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),
+   (apply [{ Helper.replaceInstWithUndef(*${Aext}); }])>;
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tschuett wrote:

` Helper.matchAnyExplicitUseIsUndef` checks in C++, if there is an undef on the anyext. It will fail 99.999999999999% of the time. Now it is a correct pattern and the combiner takes care of the matching.

I said in the summary `Is there a limit on the number of patterns?`.

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