[clang] Fix wrong ABI of AVRTiny.

A scalar which exceeds 4 bytes should be returned via a stack slot,
on an AVRTiny device.

Reviewed By: aykevl

Differential Revision: https://reviews.llvm.org/D138125

Author: benshi001

clang/lib/CodeGen/TargetInfo.cpp

@@ -8339,15 +8339,16 @@ class AVRABIInfo : public DefaultABIInfo {
       : DefaultABIInfo(CGT), ParamRegs(NPR), RetRegs(NRR) {}
 
   ABIArgInfo classifyReturnType(QualType Ty, bool &LargeRet) const {
-    if (isAggregateTypeForABI(Ty)) {
-      // On AVR, a return struct with size less than or equals to 8 bytes is
-      // returned directly via registers R18-R25. On AVRTiny, a return struct
-      // with size less than or equals to 4 bytes is returned directly via
-      // registers R22-R25.
-      if (getContext().getTypeSize(Ty) <= RetRegs * 8)
-        return ABIArgInfo::getDirect();
-      // A return struct with larger size is returned via a stack
-      // slot, along with a pointer to it as the function's implicit argument.
+    // On AVR, a return struct with size less than or equals to 8 bytes is
+    // returned directly via registers R18-R25. On AVRTiny, a return struct
+    // with size less than or equals to 4 bytes is returned directly via
+    // registers R22-R25.
+    if (isAggregateTypeForABI(Ty) &&
+        getContext().getTypeSize(Ty) <= RetRegs * 8)
+      return ABIArgInfo::getDirect();
+    // A return value (struct or scalar) with larger size is returned via a
+    // stack slot, along with a pointer as the function's implicit argument.
+    if (getContext().getTypeSize(Ty) > RetRegs * 8) {
       LargeRet = true;
       return getNaturalAlignIndirect(Ty);
     }

clang/test/CodeGen/avr/struct.c renamed to clang/test/CodeGen/avr/return-value.c

@@ -33,16 +33,22 @@ struct s04 foo04(int a, int b) {
   return a0;
 }
 
+long long fooi64(void) {
+  return 0xaa5533;
+}
+
 // AVR: %struct.s10 = type { i16, i16, i16, i16, i16 }
 // AVR: %struct.s06 = type { i16, i16, i16 }
 // AVR: %struct.s04 = type { i16, i16 }
 // AVR: define{{.*}} void @foo10(ptr {{.*}}, i16 noundef %a, i16 noundef %b, i16 noundef %c)
 // AVR: define{{.*}} %struct.s06 @foo06(i16 noundef %a, i16 noundef %b, i16 noundef %c)
 // AVR: define{{.*}} %struct.s04 @foo04(i16 noundef %a, i16 noundef %b)
+// AVR: define{{.*}} i64 @fooi64()
 
 // TINY: %struct.s10 = type { i16, i16, i16, i16, i16 }
 // TINY: %struct.s06 = type { i16, i16, i16 }
 // TINY: %struct.s04 = type { i16, i16 }
 // TINY: define{{.*}} void @foo10(ptr {{.*}}, i16 noundef %a, i16 noundef %b, i16 noundef %c)
 // TINY: define{{.*}} void @foo06(ptr {{.*}}, i16 noundef %a, i16 noundef %b, i16 noundef %c)
 // TINY: define{{.*}} %struct.s04 @foo04(i16 noundef %a, i16 noundef %b)
+// TINY: define{{.*}} void @fooi64(ptr {{.*}})