[llvm] [AArch64] Implement latency costs for loads (PR #201567)

[John Brawn via llvm-commits]

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@@ -5055,8 +5054,59 @@ InstructionCost AArch64TTIImpl::getMemoryOpCost(unsigned Opcode, Type *Ty,
   if (CostKind == TTI::TCK_CodeSize || CostKind == TTI::TCK_SizeAndLatency)
     return LT.first;
 
-  if (CostKind != TTI::TCK_RecipThroughput)
-    return 1;
+  if (CostKind == TTI::TCK_Latency) {
+    // Latency doesn't make much sense for stores, so just return 1
+    if (Opcode == Instruction::Store)
+      return 1;
+    // We expect the load to become LT.first loads of type LT.second. The
+    // latency will be the latency of the last load plus the time it gets to get
+    // there, which will be the amount of other loads before that (i.e. total
+    // loads - 1). We get the latency from the SchedModel, and assume that the
+    // loads become the variant with unsigned integer offset.
+    unsigned Inst = 0;
+    if (LT.second.isScalableVector() ||
+        ST->useSVEForFixedLengthVectors(LT.second)) {
+      Inst = AArch64::LDR_ZXI;
+    } else if (LT.second.isVector() || LT.second.isFloatingPoint()) {
+      switch (LT.second.getSizeInBits()) {
+      case 8:
+        Inst = AArch64::LDRBui;
+        break;
+      case 16:
+        Inst = AArch64::LDRHui;
+        break;
+      case 32:
+        Inst = AArch64::LDRSui;
+        break;
+      case 64:
+        Inst = AArch64::LDRDui;
+        break;
+      case 128:
+        Inst = AArch64::LDRQui;
+        break;
+      default:
+        llvm_unreachable("Unexpected float or vector type");
+      }
+    } else {
+      switch (LT.second.getSizeInBits()) {
+      case 8:
+        Inst = AArch64::LDRBBui;
+        break;
+      case 16:
+        Inst = AArch64::LDRHHui;
+        break;
+      case 32:
+        Inst = AArch64::LDRWui;
+        break;
+      case 64:
+        Inst = AArch64::LDRXui;
+        break;
+      default:
+        llvm_unreachable("Unexpected integer type");
+      }
+    }
+    return (LT.first - 1) + SchedModel.computeInstrLatency(Inst);
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john-brawn-arm wrote:

I've adjusted this to use a latency of 4 for the generic cpu, which means the selectopt behaviour doesn't change.

https://github.com/llvm/llvm-project/pull/201567