[llvm] [RISCV] Improve instruction selection for most significant bit extraction (PR #151687)

[Piotr Fusik via llvm-commits]

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@@ -1691,6 +1691,20 @@ multiclass SelectCC_GPR_riirr<DAGOperand valty, DAGOperand imm> {
                 valty:$truev, valty:$falsev), []>;
 }
 
+let Predicates = [IsRV32] in {
+def : Pat<(i32 (seteq (i32 (and GPR:$rs1, 0xffffffff80000000)), 0)),
+          (XORI (i32 (SRLI GPR:$rs1, 31)), 1)>;
+def : Pat<(i32 (setlt (i32 GPR:$rs1), 0)), (SRLI GPR:$rs1, 31)>; // compressible
+}
+let Predicates = [IsRV64] in {
+def : Pat<(i64 (seteq (i64 (and GPR:$rs1, 0x8000000000000000)), 0)),
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pfusik wrote:

Sounds like a good first change in tablegen. I can do that next week.

https://github.com/llvm/llvm-project/pull/151687