[CodeGen] Limit mem operands checks count for reasonable compilation speed.

Author: ivafanas

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -2057,6 +2057,21 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
   /// overhead or too rigid restriction.
   virtual unsigned getMemOperandAACheckLimit() const { return 16; }
 
+  /// Return the maximum number of memory operands to check instruction for
+  /// memory-related properties.
+  ///
+  /// After MIR transformations like tail merging etc. memory operands are
+  /// united for the merged result instructions. Compiler might ends up with
+  /// thousands of memory operands for each instruction for tricky CFGs like
+  /// for switch construction.
+  ///
+  /// Even linear algorithms on instructions with thousands of memory operands
+  /// leads to significant compilation slowdown.
+  ///
+  /// Heuristic is designed to limit checks count for algorithms where
+  /// conservative answer like "I don't know" is possible.
+  virtual unsigned getMemOperandLinearCheckLimit() const { return 16; }
+
   /// Return an array that contains the ids of the target indices (used for the
   /// TargetIndex machine operand) and their names.
   ///

llvm/lib/CodeGen/MachineInstr.cpp

@@ -1581,6 +1581,12 @@ bool MachineInstr::hasOrderedMemoryRef() const {
   if (memoperands_empty())
     return true;
 
+  // Conservatively skip analysis if there are too many memory operands. Keep
+  // compilation time reasonable.
+  const TargetInstrInfo *TII = getMF()->getSubtarget().getInstrInfo();
+  if (getNumMemOperands() > TII->getMemOperandLinearCheckLimit())
+    return true;
+
   // Check if any of our memory operands are ordered.
   return llvm::any_of(memoperands(), [](const MachineMemOperand *MMO) {
     return !MMO->isUnordered();
@@ -1600,7 +1606,15 @@ bool MachineInstr::isDereferenceableInvariantLoad() const {
   if (memoperands_empty())
     return false;
 
-  const MachineFrameInfo &MFI = getParent()->getParent()->getFrameInfo();
+  const MachineFunction &MF = *getMF();
+
+  // Conservatively skip analysis if there are too many memory operands. Keep
+  // compilation time reasonable.
+  const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
+  if (getNumMemOperands() > TII->getMemOperandLinearCheckLimit())
+    return false;
+
+  const MachineFrameInfo &MFI = MF.getFrameInfo();
 
   for (MachineMemOperand *MMO : memoperands()) {
     if (!MMO->isUnordered())