[AMDGPU] SILoadStoreOptimizer: rewrite checkAndPrepareMerge. NFCI.

[foad]

Summary

Separate the function clearly into:

• Checks that can be done on CI and Paired before the loop.
• The loop over all instructions between CI and Paired.
• Checks that must be done on InstsToMove after the loop.

Previously these were mostly done inside the loop in a very
confusing way.

[piotr]

LGTM

llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp

949–953

Not related to your change, but the mention of "DS" is a left-over from the times this pass only handled ds_read/ds_write. Can you please remove it before committing (no need to resubmit)?

llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp

bool SILoadStoreOptimizer::checkAndPrepareMerge(
     CombineInfo &CI, CombineInfo &Paired,
     SmallVectorImpl<MachineInstr *> &InstsToMove) {
   // If another instruction has already been merged into CI, it may now be a
   // type that we can't do any further merging into.
   if (CI.InstClass == UNKNOWN || Paired.InstClass == UNKNOWN)
     return false;
   assert(CI.InstClass == Paired.InstClass);
 
+  if (getInstSubclass(CI.I->getOpcode(), *TII) !=
+      getInstSubclass(Paired.I->getOpcode(), *TII))
+    return false;
+
   // Check both offsets (or masks for MIMG) can be combined and fit in the
   // reduced range.
   if (CI.InstClass == MIMG) {
     if (!dmasksCanBeCombined(CI, *TII, Paired))
       return false;
   } else {
     if (!widthsFit(*STM, CI, Paired) || !offsetsCanBeCombined(CI, *STM, Paired))
       return false;
   }
 
-  const unsigned Opc = CI.I->getOpcode();
-  const unsigned InstSubclass = getInstSubclass(Opc, *TII);
-
   DenseSet<Register> RegDefsToMove;
   DenseSet<Register> PhysRegUsesToMove;
   addDefsUsesToList(*CI.I, RegDefsToMove, PhysRegUsesToMove);
 
-  MachineBasicBlock::iterator E = std::next(Paired.I);
-  MachineBasicBlock::iterator MBBI = std::next(CI.I);
   MachineBasicBlock::iterator MBBE = CI.I->getParent()->end();
-  for (; MBBI != E; ++MBBI) {
-
+  for (MachineBasicBlock::iterator MBBI = CI.I; ++MBBI != Paired.I;) {
     if (MBBI == MBBE) {
       // CombineInfo::Order is a hint on the instruction ordering within the
       // basic block. This hint suggests that CI precedes Paired, which is
       // true most of the time. However, moveInstsAfter() processing a
       // previous list may have changed this order in a situation when it
       // moves an instruction which exists in some other merge list.
       // In this case it must be dependent.
       return false;
     }
 
-    if ((getInstClass(MBBI->getOpcode(), *TII) != CI.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.
+    // Keep going as long as one of these conditions are met:
 
-      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);
-        continue;
-      }
 
-      // When we match I with another DS instruction we will be moving I down
-      // to the location of the matched instruction any uses of I will need to
-      // be moved down as well.
-      addToListsIfDependent(*MBBI, RegDefsToMove, PhysRegUsesToMove,
-                            InstsToMove);
-      continue;
-    }
+    // When we match I with another load/store instruction we will be moving I
+    // down to the location of the matched instruction any uses of I will need
+    // to be moved down as well.
+    addToListsIfDependent(*MBBI, RegDefsToMove, PhysRegUsesToMove, InstsToMove);
 
-    // Handle a case like
-    //   DS_WRITE_B32 addr, v, idx0
-    //   w = DS_READ_B32 addr, idx0
-    //   DS_WRITE_B32 addr, f(w), idx1
-    // where the DS_READ_B32 ends up in InstsToMove and therefore prevents
-    // merging of the two writes.
-    if (addToListsIfDependent(*MBBI, RegDefsToMove, PhysRegUsesToMove,
-                              InstsToMove))
-      continue;
+  // If Paired depends on any of the instructions we plan to move, give up.
+  if (addToListsIfDependent(*Paired.I, RegDefsToMove, PhysRegUsesToMove,
+    return false;
 
-    if (&*MBBI == &*Paired.I) {
-      // We need to go through the list of instructions that we plan to
-      // move and make sure they are all safe to move down past the merged
-      // instruction.
-      if (canMoveInstsAcrossMemOp(*MBBI, InstsToMove, AA)) {
+  if (!canMoveInstsAcrossMemOp(*Paired.I, InstsToMove, AA))
+    return false;
 
-        // Call offsetsCanBeCombined with modify = true so that the offsets are
-        // correct for the new instruction.  This should return true, because
-        // this function should only be called on CombineInfo objects that
-        // have already been confirmed to be mergeable.
-        if (CI.InstClass == DS_READ || CI.InstClass == DS_WRITE)
-          offsetsCanBeCombined(CI, *STM, Paired, true);
-        return true;
-      }
-      return false;
-    }
-
-    // We've found a load/store that we couldn't merge for some reason.
-    // We could potentially keep looking, but we'd need to make sure that
-    // it was safe to move I and also all the instruction in InstsToMove
-    // down past this instruction.
-    // check if we can move I across MBBI and if we can move all I's users
-    if (!memAccessesCanBeReordered(*CI.I, *MBBI, AA) ||
-        !canMoveInstsAcrossMemOp(*MBBI, InstsToMove, AA))
-      break;
-  }
-  return false;
-}
 
 unsigned SILoadStoreOptimizer::read2Opcode(unsigned EltSize) const {
   if (STM->ldsRequiresM0Init())
     return (EltSize == 4) ? AMDGPU::DS_READ2_B32 : AMDGPU::DS_READ2_B64;
   return (EltSize == 4) ? AMDGPU::DS_READ2_B32_gfx9 : AMDGPU::DS_READ2_B64_gfx9;
 }
 
 unsigned SILoadStoreOptimizer::read2ST64Opcode(unsigned EltSize) const {