[llvm] [RISCV] Introduce VLOptimizer pass (PR #108640)

[Pengcheng Wang via llvm-commits]

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@@ -0,0 +1,1569 @@
+//===-------------- RISCVVLOptimizer.cpp - VL Optimizer -------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+//
+// This pass reduces the VL where possible at the MI level, before VSETVLI
+// instructions are inserted.
+//
+// The purpose of this optimization is to make the VL argument, for instructions
+// that have a VL argument, as small as possible. This is implemented by
+// visiting each instruction in reverse order and checking that if it has a VL
+// argument, whether the VL can be reduced.
+//
+//===---------------------------------------------------------------------===//
+
+#include "RISCV.h"
+#include "RISCVMachineFunctionInfo.h"
+#include "RISCVSubtarget.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/InitializePasses.h"
+
+#include <algorithm>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "riscv-vl-optimizer"
+
+namespace {
+
+class RISCVVLOptimizer : public MachineFunctionPass {
+  const MachineRegisterInfo *MRI;
+  const MachineDominatorTree *MDT;
+
+public:
+  static char ID;
+
+  RISCVVLOptimizer() : MachineFunctionPass(ID) {}
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    AU.addRequired<MachineDominatorTreeWrapperPass>();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  StringRef getPassName() const override { return "RISC-V VL Optimizer"; }
----------------
wangpc-pp wrote:

We usually add a macro `PASS_NAME` so we can reuse it when initializing pass?
```
#define PASS_NAME "RISC-V VL Optimizer"
```

https://github.com/llvm/llvm-project/pull/108640