[AVR] Disable post increment load from program memory space

[benshi001]

Summary

We temporarily only allow post increment load/store from/to data memory,
and disable post increment load from program memory.

Updates https://github.com/llvm/llvm-project/issues/59914

llvm/lib/Target/AVR/AVRISelLowering.cpp

bool AVRTargetLowering::getPostIndexedAddressParts(SDNode *N, SDNode *Op,
   if (const ConstantSDNode *RHS = dyn_cast<ConstantSDNode>(Op->getOperand(1))) {
     int RHSC = RHS->getSExtValue();
     if (Op->getOpcode() == ISD::SUB)
       RHSC = -RHSC;
     if ((VT == MVT::i16 && RHSC != 2) || (VT == MVT::i8 && RHSC != 1)) {
       return false;
     }
 
+    // FIXME: We temporarily disable post-increment load from program memory,
+    //        due to bug https://github.com/llvm/llvm-project/issues/59914.
+    if (const LoadSDNode *LD = dyn_cast<LoadSDNode>(N))
+      if (AVR::isProgramMemoryAccess(LD))
+        return false;
+
     Base = Op->getOperand(0);
     Offset = DAG.getConstant(RHSC, DL, MVT::i8);
     AM = ISD::POST_INC;
 
     return true;
   }
 
   return false;

llvm/test/CodeGen/AVR/load.ll

while.body:                                       ; preds = %entry, %while.body
   %0 = load i16, i16* %incdec.ptr
   %add = add nsw i16 %0, %r.05
   %tobool = icmp eq i16 %dec, 0
   br i1 %tobool, label %while.end, label %while.body
 while.end:                                        ; preds = %while.body, %entry
   %r.0.lcssa = phi i16 [ 0, %entry ], [ %add, %while.body ]
   ret i16 %r.0.lcssa
 }
+
+define ptr addrspace(1) @load16_postinc_progmem(ptr addrspace(1) readonly %0) {
+; CHECK-LABEL: load16_postinc_progmem:
+; CHECK:         movw r30, [[REG0:r[0-9]+]]
+; CHECK:         lpm  [[REG1:r[0-9]+]], Z+
+; CHECK:         lpm  [[REG1:r[0-9]+]], Z
+; CHECK:         call foo
+; CHECK:         adiw [[REG0:r[0-9]+]], 2
+  %2 = load i16, ptr addrspace(1) %0, align 1
+  tail call addrspace(1) void @foo(i16 %2)
+  %3 = getelementptr inbounds i16, ptr addrspace(1) %0, i16 1
+  ret ptr addrspace(1) %3
+}
+
+declare void @foo(i16)