[RISCV][VLOPT] Do not optimize VL when isVectorOpUsedAsScalarOp

[michaelmaitland]

This does not have tests, so we will remove this for now and add it back later with tests.

[llvmbot]

@llvm/pr-subscribers-backend-risc-v

Author: Michael Maitland (michaelmaitland)

Changes

This does not have tests, so we will remove this for now and add it back later with tests.

---

Full diff: https://github.com/llvm/llvm-project/pull/120291.diff

1 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp (+3-6)

diff --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
index bd48d730a5704b..a9e5bb6ecd9b8a 100644
--- a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
+++ b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -887,13 +887,10 @@ bool RISCVVLOptimizer::checkUsers(const MachineOperand *&CommonVL,
     // Instructions like reductions may use a vector register as a scalar
     // register. In this case, we should treat it like a scalar register which
     // does not impact the decision on whether to optimize VL.
+    // TODO: Treat it like a scalar register instead of bailing out.
     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() << "    Use this operand as a scalar operand\n");
-      continue;
+      CanReduceVL = false;
+      break;
     }
 
     if (mayReadPastVL(UserMI)) {