[LoongArch] Add isSafeToMove hook to prevent unsafe instruction motion

[heiher]

This patch introduces a new virtual method TargetInstrInfo::isSafeToMove() to allow backends to control whether a machine instruction can be safely moved by optimization passes.

The BranchFolder pass now respects this hook when hoisting common code. By default, all instructions are considered safe to to move.

For LoongArch, isSafeToMove() is overridden to prevent relocation-related instruction sequences (e.g. PC-relative addressing and calls) from being broken by instruction motion. Correspondingly, isSchedulingBoundary() is updated to reuse this logic for consistency.

Fixes #163681

[llvmbot]

@llvm/pr-subscribers-backend-loongarch

Author: hev (heiher)

Changes

This patch introduces a new virtual method TargetInstrInfo::isSafeToMove() to allow backends to control whether a machine instruction can be safely moved by optimization passes.

The BranchFolder pass now respects this hook when hoisting common code. By default, all instructions are considered safe to to move.

For LoongArch, isSafeToMove() is overridden to prevent relocation-related instruction sequences (e.g. PC-relative addressing and calls) from being broken by instruction motion. Correspondingly, isSchedulingBoundary() is updated to reuse this logic for consistency.

Fixes #163681

---

Full diff: https://github.com/llvm/llvm-project/pull/163725.diff

5 Files Affected:

• (modified) llvm/include/llvm/CodeGen/TargetInstrInfo.h (+11)
• (modified) llvm/lib/CodeGen/BranchFolding.cpp (+7-2)
• (modified) llvm/lib/CodeGen/BranchFolding.h (+1-1)
• (modified) llvm/lib/Target/LoongArch/LoongArchInstrInfo.cpp (+27-18)
• (modified) llvm/lib/Target/LoongArch/LoongArchInstrInfo.h (+3)

diff --git a/llvm/include/llvm/CodeGen/TargetInstrInfo.h b/llvm/include/llvm/CodeGen/TargetInstrInfo.h
index 175f205328361..2dcedfb40f3e6 100644
--- a/llvm/include/llvm/CodeGen/TargetInstrInfo.h
+++ b/llvm/include/llvm/CodeGen/TargetInstrInfo.h
@@ -1761,6 +1761,17 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
     return true;
   }
 
+  /// Return true if it's safe to move a machine instruction.
+  /// This allows the backend to prevent certain special instruction
+  /// sequences from being broken by instruction motion in optimization
+  /// passes.
+  /// By default, this returns true for every instruction.
+  virtual bool isSafeToMove(const MachineInstr &MI,
+                            const MachineBasicBlock *MBB,
+                            const MachineFunction &MF) const {
+    return true;
+  }
+
   /// Test if the given instruction should be considered a scheduling boundary.
   /// This primarily includes labels and terminators.
   virtual bool isSchedulingBoundary(const MachineInstr &MI,
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 7292bc2be0df2..8213a91a7750d 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -1818,7 +1818,7 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
 bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
   bool MadeChange = false;
   for (MachineBasicBlock &MBB : llvm::make_early_inc_range(MF))
-    MadeChange |= HoistCommonCodeInSuccs(&MBB);
+    MadeChange |= HoistCommonCodeInSuccs(&MBB, MF);
 
   return MadeChange;
 }
@@ -1948,7 +1948,8 @@ MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
   return PI;
 }
 
-bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
+bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB,
+                                          MachineFunction &MF) {
   MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
   SmallVector<MachineOperand, 4> Cond;
   if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
@@ -1993,6 +1994,10 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
       // Hard to reason about register liveness with predicated instruction.
       break;
 
+    if (!TII->isSafeToMove(*TIB, MBB, MF))
+      // Don't hoist the instruction if it isn't safe to move.
+      break;
+
     bool IsSafe = true;
     for (MachineOperand &MO : TIB->operands()) {
       // Don't attempt to hoist instructions with register masks.
diff --git a/llvm/lib/CodeGen/BranchFolding.h b/llvm/lib/CodeGen/BranchFolding.h
index ff2bbe06c0488..8f0f14367e0eb 100644
--- a/llvm/lib/CodeGen/BranchFolding.h
+++ b/llvm/lib/CodeGen/BranchFolding.h
@@ -196,7 +196,7 @@ class TargetRegisterInfo;
 
     /// If the successors of MBB has common instruction sequence at the start of
     /// the function, move the instructions before MBB terminator if it's legal.
-    bool HoistCommonCodeInSuccs(MachineBasicBlock *MBB);
+    bool HoistCommonCodeInSuccs(MachineBasicBlock *MBB, MachineFunction &MF);
   };
 
 } // end namespace llvm
diff --git a/llvm/lib/Target/LoongArch/LoongArchInstrInfo.cpp b/llvm/lib/Target/LoongArch/LoongArchInstrInfo.cpp
index c89212dae72d9..2b792184087cd 100644
--- a/llvm/lib/Target/LoongArch/LoongArchInstrInfo.cpp
+++ b/llvm/lib/Target/LoongArch/LoongArchInstrInfo.cpp
@@ -378,12 +378,9 @@ bool LoongArchInstrInfo::isBranchOffsetInRange(unsigned BranchOp,
   }
 }
 
-bool LoongArchInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
-                                              const MachineBasicBlock *MBB,
-                                              const MachineFunction &MF) const {
-  if (TargetInstrInfo::isSchedulingBoundary(MI, MBB, MF))
-    return true;
-
+bool LoongArchInstrInfo::isSafeToMove(const MachineInstr &MI,
+                                      const MachineBasicBlock *MBB,
+                                      const MachineFunction &MF) const {
   auto MII = MI.getIterator();
   auto MIE = MBB->end();
 
@@ -429,25 +426,25 @@ bool LoongArchInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
     auto MO2 = Lu32I->getOperand(2).getTargetFlags();
     if (MO0 == LoongArchII::MO_PCREL_HI && MO1 == LoongArchII::MO_PCREL_LO &&
         MO2 == LoongArchII::MO_PCREL64_LO)
-      return true;
+      return false;
     if ((MO0 == LoongArchII::MO_GOT_PC_HI || MO0 == LoongArchII::MO_LD_PC_HI ||
          MO0 == LoongArchII::MO_GD_PC_HI) &&
         MO1 == LoongArchII::MO_GOT_PC_LO && MO2 == LoongArchII::MO_GOT_PC64_LO)
-      return true;
+      return false;
     if (MO0 == LoongArchII::MO_IE_PC_HI && MO1 == LoongArchII::MO_IE_PC_LO &&
         MO2 == LoongArchII::MO_IE_PC64_LO)
-      return true;
+      return false;
     if (MO0 == LoongArchII::MO_DESC_PC_HI &&
         MO1 == LoongArchII::MO_DESC_PC_LO &&
         MO2 == LoongArchII::MO_DESC64_PC_LO)
-      return true;
+      return false;
     break;
   }
   case LoongArch::LU52I_D: {
     auto MO = MI.getOperand(2).getTargetFlags();
     if (MO == LoongArchII::MO_PCREL64_HI || MO == LoongArchII::MO_GOT_PC64_HI ||
         MO == LoongArchII::MO_IE_PC64_HI || MO == LoongArchII::MO_DESC64_PC_HI)
-      return true;
+      return false;
     break;
   }
   default:
@@ -487,7 +484,7 @@ bool LoongArchInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
         auto MO1 = LoongArchII::getDirectFlags(SecondOp->getOperand(2));
         auto MO2 = LoongArchII::getDirectFlags(Ld->getOperand(2));
         if (MO1 == LoongArchII::MO_DESC_PC_LO && MO2 == LoongArchII::MO_DESC_LD)
-          return true;
+          return false;
         break;
       }
       if (SecondOp == MIE ||
@@ -496,34 +493,34 @@ bool LoongArchInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
       auto MO1 = LoongArchII::getDirectFlags(SecondOp->getOperand(2));
       if (MO0 == LoongArchII::MO_PCREL_HI && SecondOp->getOpcode() == AddiOp &&
           MO1 == LoongArchII::MO_PCREL_LO)
-        return true;
+        return false;
       if (MO0 == LoongArchII::MO_GOT_PC_HI && SecondOp->getOpcode() == LdOp &&
           MO1 == LoongArchII::MO_GOT_PC_LO)
-        return true;
+        return false;
       if ((MO0 == LoongArchII::MO_LD_PC_HI ||
            MO0 == LoongArchII::MO_GD_PC_HI) &&
           SecondOp->getOpcode() == AddiOp && MO1 == LoongArchII::MO_GOT_PC_LO)
-        return true;
+        return false;
       break;
     }
     case LoongArch::ADDI_W:
     case LoongArch::ADDI_D: {
       auto MO = LoongArchII::getDirectFlags(MI.getOperand(2));
       if (MO == LoongArchII::MO_PCREL_LO || MO == LoongArchII::MO_GOT_PC_LO)
-        return true;
+        return false;
       break;
     }
     case LoongArch::LD_W:
     case LoongArch::LD_D: {
       auto MO = LoongArchII::getDirectFlags(MI.getOperand(2));
       if (MO == LoongArchII::MO_GOT_PC_LO)
-        return true;
+        return false;
       break;
     }
     case LoongArch::PseudoDESC_CALL: {
       auto MO = LoongArchII::getDirectFlags(MI.getOperand(2));
       if (MO == LoongArchII::MO_DESC_CALL)
-        return true;
+        return false;
       break;
     }
     default:
@@ -531,6 +528,18 @@ bool LoongArchInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
     }
   }
 
+  return true;
+}
+
+bool LoongArchInstrInfo::isSchedulingBoundary(const MachineInstr &MI,
+                                              const MachineBasicBlock *MBB,
+                                              const MachineFunction &MF) const {
+  if (TargetInstrInfo::isSchedulingBoundary(MI, MBB, MF))
+    return true;
+
+  if (!isSafeToMove(MI, MBB, MF))
+    return true;
+
   return false;
 }
 
diff --git a/llvm/lib/Target/LoongArch/LoongArchInstrInfo.h b/llvm/lib/Target/LoongArch/LoongArchInstrInfo.h
index f25958a32bec4..8457d1446ed5e 100644
--- a/llvm/lib/Target/LoongArch/LoongArchInstrInfo.h
+++ b/llvm/lib/Target/LoongArch/LoongArchInstrInfo.h
@@ -64,6 +64,9 @@ class LoongArchInstrInfo : public LoongArchGenInstrInfo {
   bool isBranchOffsetInRange(unsigned BranchOpc,
                              int64_t BrOffset) const override;
 
+  bool isSafeToMove(const MachineInstr &MI, const MachineBasicBlock *MBB,
+                    const MachineFunction &MF) const override;
+
   bool isSchedulingBoundary(const MachineInstr &MI,
                             const MachineBasicBlock *MBB,
                             const MachineFunction &MF) const override;