[llvm] [GlobalIsel] Combine trunc of binop (PR #107721)

[Dhruv Chawla via llvm-commits]

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@@ -1867,6 +1867,33 @@ class buildvector_of_opcode<Instruction castOpcode> : GICombineRule <
 
 def buildvector_of_truncate : buildvector_of_opcode<G_TRUNC>;
 
+// narrow binop.
+// trunc (binop X, C) --> binop (trunc X, trunc C)
+class narrow_binop_opcode<Instruction binopOpcode> : GICombineRule <
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (G_CONSTANT $const, $imm),
+         (binopOpcode $binop, $x, $const):$Binop,
+         (G_TRUNC $root, $binop):$Trunc,
+         [{ return Helper.matchNarrowBinop(*${Trunc}, *${Binop}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${Trunc}, ${matchinfo}); }])>;
+
+def narrow_binop_add : narrow_binop_opcode<G_ADD>;
+def narrow_binop_sub : narrow_binop_opcode<G_SUB>;
+def narrow_binop_mul : narrow_binop_opcode<G_MUL>;
+def narrow_binop_and : narrow_binop_opcode<G_AND>;
+def narrow_binop_or  : narrow_binop_opcode<G_OR>;
+def narrow_binop_xor : narrow_binop_opcode<G_XOR>;
+
+// Cast of integer.
+class integer_of_opcode<Instruction castOpcode> : GICombineRule <
+  (defs root:$root, apint_matchinfo:$matchinfo),
+  (match (G_CONSTANT $int, $imm),
+         (castOpcode $root, $int):$Cast,
+         [{ return Helper.matchCastOfInteger(*${Cast}, ${matchinfo}); }]),
+  (apply [{ Helper.replaceInstWithConstant(*${Cast}, ${matchinfo}); }])>;
+
+def integer_of_truncate : integer_of_opcode<G_TRUNC>;
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dc03-work wrote:

Does this really not already exist as a fold?

https://github.com/llvm/llvm-project/pull/107721