[llvm-branch-commits] [llvm] [LoongArch] Add late branch optimization pass (PR #216261)

[via llvm-branch-commits]

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+//===---- LoongArchLateBranchOpt.cpp - Late Stage Branch 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 file provides LoongArch specific target optimizations, currently it's
+/// limited to convert conditional branches into unconditional branches when
+/// the condition can be statically evaluated.
+///
+//===----------------------------------------------------------------------===//
+
+#include "LoongArchInstrInfo.h"
+#include "LoongArchSubtarget.h"
+
+using namespace llvm;
+
+#define LOONGARCH_LATE_BRANCH_OPT_NAME "LoongArch Late Branch Optimisation Pass"
+
+namespace {
+
+struct LoongArchLateBranchOpt : public MachineFunctionPass {
+  static char ID;
+
+  LoongArchLateBranchOpt() : MachineFunctionPass(ID) {}
+
+  StringRef getPassName() const override {
+    return LOONGARCH_LATE_BRANCH_OPT_NAME;
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  bool runOnMachineFunction(MachineFunction &Fn) override;
+
+private:
+  bool runOnBasicBlock(MachineBasicBlock &MBB) const;
+
+  const LoongArchInstrInfo *TII = nullptr;
+};
+} // namespace
+
+char LoongArchLateBranchOpt::ID = 0;
+INITIALIZE_PASS(LoongArchLateBranchOpt, "loongarch-late-branch-opt",
+                LOONGARCH_LATE_BRANCH_OPT_NAME, false, false)
+
+bool LoongArchLateBranchOpt::runOnBasicBlock(MachineBasicBlock &MBB) const {
+  MachineBasicBlock *TBB, *FBB;
+  SmallVector<MachineOperand, 4> Cond;
+  if (TII->analyzeBranch(MBB, TBB, FBB, Cond, /*AllowModify=*/false))
+    return false;
+
+  if (!TBB || Cond.size() < 1)
+    return false;
+
+  // Try and convert a conditional branch that can be evaluated statically
+  // into an unconditional branch.
+  unsigned Opc = Cond[0].getImm();
+  MachineBasicBlock *Folded;
+  switch (Opc) {
+  case LoongArch::BEQZ:
+  case LoongArch::BNEZ:
+    if (Cond.size() < 2 || !Cond[1].isReg() ||
+        Cond[1].getReg() != LoongArch::R0)
+      return false;
+    Folded = (Opc == LoongArch::BEQZ) ? TBB : FBB;
+    break;
+  case LoongArch::BEQ:
+  case LoongArch::BNE:
+    if (Cond.size() < 3 || !Cond[1].isReg() || !Cond[2].isReg() ||
+        Cond[1].getReg() != Cond[2].getReg())
+      return false;
+    Folded = (Opc == LoongArch::BEQ) ? TBB : FBB;
+    break;
----------------
heiher wrote:

Thanks! That's a good question. Constant conditional comparisons are already folded into `seqeq/setne 0` by [`DAG.FoldSetCC`](https://github.com/llvm/llvm-project/pull/216261/changes#diff-47299c232403370e6e02355713cf717324051be6fbaa48257087ebdf0d46ae37R7317-R7330) before reaching here.

https://github.com/llvm/llvm-project/pull/216261