[llvm] [AArch64] Optimized rdsvl followed by constant mul (PR #162853)

[Kerry McLaughlin via llvm-commits]

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@@ -19579,6 +19579,47 @@ static SDValue performMulCombine(SDNode *N, SelectionDAG &DAG,
        if (ConstValue.sge(1) && ConstValue.sle(16))
          return SDValue();
 
+  // Multiplying an RDSVL value by a constant can sometimes be done cheaper by
+  // folding a power-of-two factor of the constant into the RDSVL immediate and
+  // compensating with an extra shift.
+  //
+  // We rewrite:
+  //   (mul (srl (rdsvl 1), 3), x)
+  // to one of:
+  //   (shl (rdsvl y),  z)   if z > 0
+  //   (srl (rdsvl y), abs(z))   if z < 0
+  // where integers y, z satisfy   x = y * 2^(3 + z)   and   y ∈ [-32, 31].
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kmclaughlin-arm wrote:

Could this be moved to its own function, similar to `performMulVectorExtendCombine`, etc which are called at the beginning of `performMulCombine`?

https://github.com/llvm/llvm-project/pull/162853