[AMDGPU] SILoadStoreOptimizer: break lists on instructions with side effects

[foad]

Summary

This just helps to keep the lists shorter and faster to sort. NFCI.

[piotr]

LGTM

llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp

for (; MBBI != E; ++MBBI) {
     if ((getInstClass(MBBI->getOpcode(), *TII) != InstClass) ||
         (getInstSubclass(MBBI->getOpcode(), *TII) != InstSubclass)) {
       // This is not a matching instruction, but we can keep looking as
       // long as one of these conditions are met:
       // 1. It is safe to move I down past MBBI.
       // 2. It is safe to move MBBI down past the instruction that I will
       //    be merged into.
 
-      if (MBBI->hasUnmodeledSideEffects()) {
-        // We can't re-order this instruction with respect to other memory
-        // operations, so we fail both conditions mentioned above.
-        return false;
-      }
-
       if (MBBI->mayLoadOrStore() &&
           (!memAccessesCanBeReordered(*CI.I, *MBBI, AA) ||
            !canMoveInstsAcrossMemOp(*MBBI, InstsToMove, AA))) {
         // We fail condition #1, but we may still be able to satisfy condition
         // #2.  Add this instruction to the move list and then we will check
         // if condition #2 holds once we have selected the matching instruction.
         InstsToMove.push_back(&*MBBI);
         addDefsUsesToList(*MBBI, RegDefsToMove, PhysRegUsesToMove);
SILoadStoreOptimizer::collectMergeableInsts(
   for (; BlockI != End; ++BlockI) {
     MachineInstr &MI = *BlockI;
 
     // We run this before checking if an address is mergeable, because it can produce
     // better code even if the instructions aren't mergeable.
     if (promoteConstantOffsetToImm(MI, Visited, AnchorList))
       Modified = true;
 
-    // Don't combine if volatile. We also won't be able to merge across this, so
-    // break the search. We can look after this barrier for separate merges.
-    if (MI.hasOrderedMemoryRef()) {
-      LLVM_DEBUG(dbgs() << "Breaking search on memory fence: " << MI);
+    // Treat volatile accesses, ordered accesses and unmodeled side effects as
+    // barriers. We can look after this barrier for separate merges.
+    if (MI.hasOrderedMemoryRef() || MI.hasUnmodeledSideEffects()) {
+      LLVM_DEBUG(dbgs() << "Breaking search on barrier: " << MI);
 
       // Search will resume after this instruction in a separate merge list.
       ++BlockI;
       break;
     }
 
     const InstClassEnum InstClass = getInstClass(MI.getOpcode(), *TII);
     if (InstClass == UNKNOWN)