[AVR] Support most address space casts

All hardware address spaces on AVR can be freely cast between (they keep
the same bit pattern). They just aren't dereferenceable when they're in
a different address space as they really do point to a separate address
space.

This is supported in avr-gcc: https://godbolt.org/z/9Gfvhnhv9

avr-gcc also supports the `__memx` address space which is 24 bits. We
don't support this address space yet but I've added a safeguard just in
case.

Differential Revison: https://reviews.llvm.org/D142107

Author: aykevl

llvm/lib/Target/AVR/AVRTargetMachine.h

@@ -48,6 +48,15 @@ class AVRTargetMachine : public LLVMTargetMachine {
   createMachineFunctionInfo(BumpPtrAllocator &Allocator, const Function &F,
                             const TargetSubtargetInfo *STI) const override;
 
+  bool isNoopAddrSpaceCast(unsigned SrcAs, unsigned DestAs) const override {
+    // While AVR has different address spaces, they are all represented by
+    // 16-bit pointers that can be freely casted between (of course, a pointer
+    // must be cast back to its original address space to be dereferenceable).
+    // To be safe, also check the pointer size in case we implement __memx
+    // pointers.
+    return getPointerSize(SrcAs) == getPointerSize(DestAs);
+  }
+
 private:
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   AVRSubtarget SubTarget;

llvm/test/CodeGen/AVR/addrspacecast.ll

@@ -0,0 +1,65 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=avr | FileCheck %s
+
+@global = external global i8
+@progmem = external addrspace(1) global i8
+
+declare void @func() addrspace(1)
+declare void @doCallback(ptr) addrspace(1)
+
+define ptr @casttodata(ptr addrspace(1) %funcptr) addrspace(1) {
+; CHECK-LABEL: casttodata:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    ret
+  %result = addrspacecast ptr addrspace(1) %funcptr to ptr
+  ret ptr %result
+}
+
+define ptr addrspace(1) @casttofuncptr(ptr %data) addrspace(1) {
+; CHECK-LABEL: casttofuncptr:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    ret
+  %result = addrspacecast ptr %data to ptr addrspace(1)
+  ret ptr addrspace(1) %result
+}
+
+define ptr addrspace(1) @castglobal() addrspace(1) {
+; CHECK-LABEL: castglobal:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    ldi r24, lo8(global)
+; CHECK-NEXT:    ldi r25, hi8(global)
+; CHECK-NEXT:    ret
+  %result = addrspacecast ptr @global to ptr addrspace(1)
+  ret ptr addrspace(1) %result
+}
+
+define ptr @castprogmem() addrspace(1) {
+; CHECK-LABEL: castprogmem:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    ldi r24, lo8(progmem)
+; CHECK-NEXT:    ldi r25, hi8(progmem)
+; CHECK-NEXT:    ret
+  %result = addrspacecast ptr addrspace(1) @progmem to ptr
+  ret ptr %result
+}
+
+define ptr @castfunc() addrspace(1) {
+; CHECK-LABEL: castfunc:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    ldi r24, pm_lo8(func)
+; CHECK-NEXT:    ldi r25, pm_hi8(func)
+; CHECK-NEXT:    ret
+  %result = addrspacecast ptr addrspace(1) @func to ptr
+  ret ptr %result
+}
+
+define void @callCallback() addrspace(1) {
+; CHECK-LABEL: callCallback:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    ldi r24, pm_lo8(func)
+; CHECK-NEXT:    ldi r25, pm_hi8(func)
+; CHECK-NEXT:    rcall doCallback
+; CHECK-NEXT:    ret
+  call addrspace(1) void @doCallback(ptr addrspacecast (ptr addrspace(1) @func to ptr))
+  ret void
+}