[RISCV] Teach computeKnownBitsForTargetNode that VLENB will never be more than 65536/8.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -6452,6 +6452,8 @@ void RISCVTargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
   case RISCVISD::READ_VLENB:
     // We assume VLENB is at least 16 bytes.
     Known.Zero.setLowBits(4);
+    // We assume VLENB is no more than 65536 / 8 bytes.
+    Known.Zero.setBitsFrom(14);
     break;
   case ISD::INTRINSIC_W_CHAIN: {
     unsigned IntNo = Op.getConstantOperandVal(1);

llvm/test/CodeGen/RISCV/rvv/rvv-vscale.i64.ll

@@ -50,9 +50,9 @@ define i64 @vscale_uimmpow2xlen() nounwind {
 ;
 ; RV32-LABEL: vscale_uimmpow2xlen:
 ; RV32:       # %bb.0: # %entry
-; RV32-NEXT:    csrr a1, vlenb
-; RV32-NEXT:    slli a0, a1, 3
-; RV32-NEXT:    srli a1, a1, 29
+; RV32-NEXT:    csrr a0, vlenb
+; RV32-NEXT:    slli a0, a0, 3
+; RV32-NEXT:    mv a1, zero
 ; RV32-NEXT:    ret
 entry:
   %0 = call i64 @llvm.vscale.i64()