[llvm] [RISCV] Improve loop by extract reduction instruction (PR #179215)

[via llvm-commits]

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+//===-------- LoopReduceMotion.cpp - Loop Reduce Motion Optimization ------===//
+//
+// 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 is designed to hoist `ReduceCall` operations out of loops to reduce
+// the number of instructions within the loop body.
+//
+// Below are the target pattern to be matched and the resulting pattern
+// after the transformation.
+//
+// before                    | after
+// ------                    | ------
+// loop:                     | loop:
+//   ...                     |   ...
+//   vc = vecbin va, vb      |   vc = vecbin va, vb
+//   d = reduce_add vc       |   vsum = vadd vsum, vc
+//   sum = add sum, d        |   ...
+//   ...                     |   ...
+// exit:                     | exit:
+//   value = sum             |   d = reduce_add sum
+//   ...                     |   value = d
+//   ...                     |   ...
+//   ret                     |   ret
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Vectorize/LoopReduceMotion.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/Pass.h"
+#include "llvm/Passes/PassBuilder.h"
+#include "llvm/Plugins/PassPlugin.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
+
+#define DEBUG_TYPE "loop-reduce-motion"
+
+using namespace llvm;
+
+class LoopReduceMotion : public FunctionPass {
+  LoopReduceMotionPass Impl;
+
+public:
+  static char ID;
+
+  LoopReduceMotion() : FunctionPass(ID) {}
+
+  StringRef getPassName() const override { return "Loop Reduce Motion Pass"; }
+
+  bool runOnFunction(Function &F) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.addRequired<LoopInfoWrapperPass>();
+    AU.setPreservesCFG();
+  }
+};
+
+char LoopReduceMotion::ID = 0;
+
+PreservedAnalyses LoopReduceMotionPass::run(Function &F,
+                                            FunctionAnalysisManager &FAM) {
+  LoopInfo &LI = FAM.getResult<LoopAnalysis>(F);
+  DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
+  bool Changed = false;
+  for (Loop *L : LI) {
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Anjian-Wen wrote:

I was considering putting this pass in RISCVPassConfig::addIRPasses, I'm not sure where to put it if make it a LoopPass, are there any similar examples I can refer to??

https://github.com/llvm/llvm-project/pull/179215