[MachineLICM] Fix handling of memoperands

As far as I can tell, the correct interpretation of an empty memoperands list is that we didn't have sufficient room to store information about the MachineInstr, NOT that the MachineInstr doesn't access any particular bit of memory. This appears to be fairly consistent in a number of places, but I'm not 100% sure of this interpretation. I'd really appreciate someone more knowledgeable confirming my reading of the code.

This patch fixes two latent bugs in MachineLICM - given the above assumption - and adds comments to document the meaning and required handling. I don't have test cases; these were noticed by inspection.

Differential Revision: http://reviews.llvm.org/D15730

llvm-svn: 256335

llvm/include/llvm/CodeGen/MachineInstr.h

@@ -92,6 +92,12 @@ class MachineInstr
                                         // information to AsmPrinter.
 
   uint8_t NumMemRefs;                   // Information on memory references.
+  // Note that MemRefs == nullptr,  means 'don't know', not 'no memory access'.
+  // Calling code must treat missing information conservatively.  If the number
+  // of memory operands required to be precise exceeds the maximum value of
+  // NumMemRefs - currently 256 - we remove the operands entirely. Note also
+  // that this is a non-owning reference to a shared copy on write buffer owned
+  // by the MachineFunction and created via MF.allocateMemRefsArray. 
   mmo_iterator MemRefs;
 
   DebugLoc debugLoc;                    // Source line information.
@@ -346,6 +352,9 @@ class MachineInstr
   /// Access to memory operands of the instruction
   mmo_iterator memoperands_begin() const { return MemRefs; }
   mmo_iterator memoperands_end() const { return MemRefs + NumMemRefs; }
+  /// Return true if we don't have any memory operands which described the the
+  /// memory access done by this instruction.  If this is true, calling code
+  /// must be conservative.    
   bool memoperands_empty() const { return NumMemRefs == 0; }
 
   iterator_range<mmo_iterator>  memoperands() {

llvm/lib/CodeGen/MachineLICM.cpp

@@ -330,6 +330,10 @@ bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
 
 /// Return true if instruction stores to the specified frame.
 static bool InstructionStoresToFI(const MachineInstr *MI, int FI) {
+  // If we lost memory operands, conservatively assume that the instruction
+  // writes to all slots. 
+  if (MI->memoperands_empty())
+    return true;
   for (MachineInstr::mmo_iterator o = MI->memoperands_begin(),
          oe = MI->memoperands_end(); o != oe; ++o) {
     if (!(*o)->isStore() || !(*o)->getPseudoValue())
@@ -846,8 +850,14 @@ MachineLICM::calcRegisterCost(const MachineInstr *MI, bool ConsiderSeen,
 
 /// Return true if this machine instruction loads from global offset table or
 /// constant pool.
-static bool isLoadFromGOTOrConstantPool(MachineInstr &MI) {
+static bool mayLoadFromGOTOrConstantPool(MachineInstr &MI) {
   assert (MI.mayLoad() && "Expected MI that loads!");
+
+  // If we lost memory operands, conservatively assume that the instruction
+  // reads from everything.. 
+  if (MI.memoperands_empty())
+    return true;
+
   for (MachineInstr::mmo_iterator I = MI.memoperands_begin(),
          E = MI.memoperands_end(); I != E; ++I) {
     if (const PseudoSourceValue *PSV = (*I)->getPseudoValue()) {
@@ -872,7 +882,7 @@ bool MachineLICM::IsLICMCandidate(MachineInstr &I) {
   // from constant memory are not safe to speculate all the time, for example
   // indexed load from a jump table.
   // Stores and side effects are already checked by isSafeToMove.
-  if (I.mayLoad() && !isLoadFromGOTOrConstantPool(I) &&
+  if (I.mayLoad() && !mayLoadFromGOTOrConstantPool(I) &&
       !IsGuaranteedToExecute(I.getParent()))
     return false;