[llvm] [RISCV] Add a MIR pass to reassociate shXadd, add, and slli to form more shXadd. (PR #87544)

[Craig Topper via llvm-commits]

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@@ -0,0 +1,166 @@
+//===- RISCVOptZba.cpp - MI Zba instruction optimizations -----------------===//
+//
+// 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 attempts to reassociate expressions like
+//   (sh3add Z, (add X, (slli Y, 6)))
+// To
+//   (sh3add (sh3add Y, Z), X)
+//
+// This reduces number of instructions needing by spreading the shift amount
+// across to 2 sh3adds. This can be generalized to sh1add/sh2add and other
+// shift amounts.
+//
+// TODO: We can also support slli.uw by using shXadd.uw or the inner shXadd.
+//
+//===---------------------------------------------------------------------===//
+
+#include "RISCV.h"
+#include "RISCVSubtarget.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "riscv-opt-zba"
+#define RISCV_OPT_ZBA_NAME "RISC-V Optimize Zba"
+
+namespace {
+
+class RISCVOptZba : public MachineFunctionPass {
+public:
+  static char ID;
+
+  RISCVOptZba() : MachineFunctionPass(ID) {}
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  StringRef getPassName() const override { return RISCV_OPT_ZBA_NAME; }
+};
+
+} // end anonymous namespace
+
+char RISCVOptZba::ID = 0;
+INITIALIZE_PASS(RISCVOptZba, DEBUG_TYPE, RISCV_OPT_ZBA_NAME, false, false)
+
+FunctionPass *llvm::createRISCVOptZbaPass() { return new RISCVOptZba(); }
+
+static bool findShift(Register Reg, const MachineBasicBlock &MBB,
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topperc wrote:

Yes. I originally had it returning a bunch of output parameters instead of the Shift instruction. Then I realized I needed the shift instruction in order to erase it.

https://github.com/llvm/llvm-project/pull/87544