RISCV: Remove faulty assert that ignored subregister uses (#141658)

This was asserting the raw virtual register class was a scalar
class, instead of computing the net result of the register class
plus the subregister index on the operand. The machine verifier
should be checking this was a valid combination in the first place,
so just drop the assert.

Author: arsenm

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1298,12 +1298,7 @@ RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
   // Instructions like reductions may use a vector register as a scalar
   // register. In this case, we should treat it as only reading the first lane.
   if (isVectorOpUsedAsScalarOp(UserOp)) {
-    [[maybe_unused]] Register R = UserOp.getReg();
-    [[maybe_unused]] const TargetRegisterClass *RC = MRI->getRegClass(R);
-    assert(RISCV::VRRegClass.hasSubClassEq(RC) &&
-           "Expect LMUL 1 register class for vector as scalar operands!");
     LLVM_DEBUG(dbgs() << "    Used this operand as a scalar operand\n");
-
     return MachineOperand::CreateImm(1);
   }
 

llvm/test/CodeGen/RISCV/rvv/vl-optimizer-subreg-assert.mir

@@ -0,0 +1,32 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=riscv64 -mattr=+v -run-pass=riscv-vl-optimizer -o - %s | FileCheck %s
+# Check that there is no assert on subregister uses.
+
+---
+name: vl_optimizer_subreg_assert
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $v8m2
+
+    ; CHECK-LABEL: name: vl_optimizer_subreg_assert
+    ; CHECK: liveins: $v8m2
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:vrm8 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF1:%[0-9]+]]:vmv0 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF2:%[0-9]+]]:vrm8 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[PseudoVMERGE_VVM_M8_:%[0-9]+]]:vrm8nov0 = PseudoVMERGE_VVM_M8 $noreg, killed [[DEF2]], [[DEF]], [[DEF1]], -1, 6 /* e64 */
+    ; CHECK-NEXT: [[PseudoVREDMAXU_VS_M8_E64_:%[0-9]+]]:vr = PseudoVREDMAXU_VS_M8_E64 $noreg, [[PseudoVMERGE_VVM_M8_]], [[PseudoVMERGE_VVM_M8_]].sub_vrm1_0, -1, 6 /* e64 */, 1 /* ta, mu */
+    ; CHECK-NEXT: [[PseudoVMV_X_S:%[0-9]+]]:gpr = PseudoVMV_X_S killed [[PseudoVREDMAXU_VS_M8_E64_]], 6 /* e64 */
+    ; CHECK-NEXT: $x10 = COPY [[PseudoVMV_X_S]]
+    ; CHECK-NEXT: PseudoRET implicit $x10
+    %0:vrm8 = IMPLICIT_DEF
+    %1:vmv0 = IMPLICIT_DEF
+    %2:vrm8 = IMPLICIT_DEF
+    %3:vrm8nov0 = PseudoVMERGE_VVM_M8 $noreg, killed %2, %0, %1, -1, 6 /* e64 */
+    %4:vr = PseudoVREDMAXU_VS_M8_E64 $noreg, %3, %3.sub_vrm1_0, -1, 6 /* e64 */, 1 /* ta, mu */
+    %5:gpr = PseudoVMV_X_S killed %4, 6 /* e64 */
+    $x10 = COPY %5
+    PseudoRET implicit $x10
+
+...