[llvm] [RISCV][VLOPT] Add support for checkUsers when UserMI is a Single-Width Integer Reduction (PR #120345)

[Michael Maitland via llvm-commits]

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@@ -886,11 +903,30 @@ 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.
+    // does not impact the decision on whether to optimize VL. But if there is
+    // another user of MI and it has VL=0, we need to be sure not to reduce the
+    // VL of MI to zero when the VLOp of UserOp is may be non-zero.
     if (isVectorOpUsedAsScalarOp(UserOp)) {
-      CanReduceVL = false;
-      break;
+      [[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");
+      const MCInstrDesc &Desc = UserMI.getDesc();
+      unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
+      const MachineOperand &VLOp = UserMI.getOperand(VLOpNum);
+      if ((VLOp.isReg() && VLOp.getReg() != RISCV::X0) ||
+          (VLOp.isImm() && VLOp.getImm() != 0)) {
+        if (!CommonVL) {
+          CommonVL = &VLOp;
+          continue;
+        }
+        if (!CommonVL->isIdenticalTo(VLOp)) {
+          CanReduceVL = false;
+          break;
+        }
+        continue;
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michaelmaitland wrote:

This should go outside this if statement I think

https://github.com/llvm/llvm-project/pull/120345