[llvm] [AArch64][Machine-Combiner] Split loads into lanes of neon vectors into multiple vectors when possible (PR #142941)

[David Green via llvm-commits]

================
@@ -7367,11 +7372,241 @@ static bool getMiscPatterns(MachineInstr &Root,
   return false;
 }
 
+static bool getGatherPattern(MachineInstr &Root,
+                             SmallVectorImpl<unsigned> &Patterns,
+                             unsigned LoadLaneOpCode, unsigned NumLanes) {
+  const MachineRegisterInfo &MRI = Root.getMF()->getRegInfo();
+  const TargetRegisterInfo *TRI =
+      Root.getMF()->getSubtarget().getRegisterInfo();
+
+  // The root of the pattern must load into the last lane of the vector.
+  if (Root.getOperand(2).getImm() != NumLanes - 1)
+    return false;
+
+  // Check that we have load into all lanes except lane 0.
+  // For each load we also want to check that:
+  // 1. It has a single debug use (since we will be replacing the virtual
+  // register)
+  // 2. That the addressing mode only uses a single offset register.
+  auto *CurrInstr = MRI.getUniqueVRegDef(Root.getOperand(1).getReg());
+  auto Range = llvm::seq<unsigned>(1, NumLanes - 1);
+  SmallSet<unsigned, 4> RemainingLanes(Range.begin(), Range.end());
+  while (RemainingLanes.begin() != RemainingLanes.end() &&
+         CurrInstr->getOpcode() == LoadLaneOpCode &&
+         MRI.hasOneNonDBGUse(CurrInstr->getOperand(0).getReg()) &&
+         CurrInstr->getNumOperands() == 4) {
+    RemainingLanes.erase(CurrInstr->getOperand(2).getImm());
+    CurrInstr = MRI.getUniqueVRegDef(CurrInstr->getOperand(1).getReg());
+  }
+
+  if (!RemainingLanes.empty())
+    return false;
+
+  // Match the SUBREG_TO_REG sequence.
+  if (CurrInstr->getOpcode() != TargetOpcode::SUBREG_TO_REG)
+    return false;
+
+  // Verify that the subreg to reg loads an integer into the first lane.
+  auto Lane0LoadReg = CurrInstr->getOperand(2).getReg();
+  unsigned SingleLaneSizeInBits = 128 / NumLanes;
+  if (TRI->getRegSizeInBits(Lane0LoadReg, MRI) != SingleLaneSizeInBits)
+    return false;
+
+  // Verify that it also has a single non debug use.
+  if (!MRI.hasOneNonDBGUse(Lane0LoadReg))
+    return false;
+
+  switch (NumLanes) {
+  case 4:
+    Patterns.push_back(AArch64MachineCombinerPattern::GATHER_i32);
+    break;
+  case 8:
+    Patterns.push_back(AArch64MachineCombinerPattern::GATHER_i16);
+    break;
+  case 16:
+    Patterns.push_back(AArch64MachineCombinerPattern::GATHER_i8);
+    break;
+  default:
+    llvm_unreachable("Got bad number of lanes for gather pattern.");
+  }
+
+  return true;
+}
+
+/// Search for patterns where we use LD1 instructions to load into
+/// separate lanes of an 128 bit Neon register. We can increase MLP
----------------
davemgreen wrote:

MLP == [Memory-level parallelism](https://en.wikipedia.org/wiki/Memory-level_parallelism)? (Not My Little Pony, the first google result that came up). I had thought of this as a way of getting better ILP by avoiding the long insert latencies on the lanewise inserts.

https://github.com/llvm/llvm-project/pull/142941