[SelectionDAG] Handle VSCALE in isKnownNeverZero (#97789)

VSCALE is by definition greater than zero, but this checks it via
getVScaleRange anyway.

The motivation for this is to be able to check if the EVL for a VP
strided load is non-zero in #97394.

I added the tests to the RISC-V backend since the existing X86
known-never-zero.ll test crashed when trying to lower vscale for the
+sse2 RUN line.

Author: lukel97

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5623,6 +5623,15 @@ bool SelectionDAG::isKnownNeverZero(SDValue Op, unsigned Depth) const {
   case ISD::ZERO_EXTEND:
   case ISD::SIGN_EXTEND:
     return isKnownNeverZero(Op.getOperand(0), Depth + 1);
+  case ISD::VSCALE: {
+    const Function &F = getMachineFunction().getFunction();
+    const APInt &Multiplier = Op.getConstantOperandAPInt(0);
+    ConstantRange CR =
+        getVScaleRange(&F, Op.getScalarValueSizeInBits()).multiply(Multiplier);
+    if (!CR.getUnsignedMin().isZero())
+      return true;
+    break;
+  }
   }
 
   return computeKnownBits(Op, Depth).isNonZero();

llvm/test/CodeGen/RISCV/rvv/known-never-zero.ll

@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v -verify-machineinstrs | FileCheck %s
+
+; Use cttz to test if we properly prove never-zero. There is a very
+; simple transform from cttz -> cttz_zero_undef if its operand is
+; known never zero.
+
+; Even without vscale_range, vscale is always guaranteed to be non-zero.
+define i32 @vscale_known_nonzero() {
+; CHECK-LABEL: vscale_known_nonzero:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    addi sp, sp, -16
+; CHECK-NEXT:    .cfi_def_cfa_offset 16
+; CHECK-NEXT:    sd ra, 8(sp) # 8-byte Folded Spill
+; CHECK-NEXT:    .cfi_offset ra, -8
+; CHECK-NEXT:    csrr a0, vlenb
+; CHECK-NEXT:    srli a0, a0, 3
+; CHECK-NEXT:    neg a1, a0
+; CHECK-NEXT:    and a0, a0, a1
+; CHECK-NEXT:    lui a1, 30667
+; CHECK-NEXT:    addiw a1, a1, 1329
+; CHECK-NEXT:    call __muldi3
+; CHECK-NEXT:    srliw a0, a0, 27
+; CHECK-NEXT:    lui a1, %hi(.LCPI0_0)
+; CHECK-NEXT:    addi a1, a1, %lo(.LCPI0_0)
+; CHECK-NEXT:    add a0, a1, a0
+; CHECK-NEXT:    lbu a0, 0(a0)
+; CHECK-NEXT:    ld ra, 8(sp) # 8-byte Folded Reload
+; CHECK-NEXT:    addi sp, sp, 16
+; CHECK-NEXT:    ret
+  %x = call i32 @llvm.vscale()
+  %r = call i32 @llvm.cttz.i32(i32 %x, i1 false)
+  ret i32 %r
+}