[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:

Ah, this exists in the artifact combiner: https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h#L270-L281. I was a bit confused because I had run into this case before and it was getting folded.

If you're adding it to CombinerHelper, could it be removed from the artifact combiner? Not sure if that will lead to an infinite loop though. I think this can then also be introduced for anyext, sext, zext in that case.

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