[mips] Zero-sized structs cannot be ignored in MipsABIInfo::classifyReturnType() for O32

[dsanders]

Summary

They are returned indirectly which causes the other arguments to move to
the next argument slot.

With this, utils/ABITest does not discover any failing cases in the first
500 attempts on big/little endian for O32. Previously some of these failed.
Also tested N32/N64 little endian (big endian has other known issues) with
no issues.

[dsanders]

No longer affects N32/N64.
Test also covers N32.

[atanasyan]

LGTM

lib/CodeGen/TargetInfo.cpp

MipsABIInfo::returnAggregateInRegs(QualType RetTy, uint64_t Size) const {
 
   CoerceToIntArgs(Size, RTList);
   return llvm::StructType::get(getVMContext(), RTList);
 }
 
 ABIArgInfo MipsABIInfo::classifyReturnType(QualType RetTy) const {
   uint64_t Size = getContext().getTypeSize(RetTy);
 
-  if (RetTy->isVoidType() || Size == 0)
+  if (RetTy->isVoidType())
+    return ABIArgInfo::getIgnore();
+
+  // O32 doesn't treat zero-sized structs differently from other structs.
+  // However, N32/N64 ignores zero sized return values.
+  if (!IsO32 && Size == 0)
     return ABIArgInfo::getIgnore();
 
   if (isAggregateTypeForABI(RetTy) || RetTy->isVectorType()) {
     if (Size <= 128) {
       if (RetTy->isAnyComplexType())
         return ABIArgInfo::getDirect();
 
       // O32 returns integer vectors in registers.

test/CodeGen/mips-zero-sized-struct.c

+// RUN: %clang -target mips-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck -check-prefix=O32 %s
+// RUN: %clang -target mipsel-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck -check-prefix=O32 %s
+// RUN: %clang -target mips64-unknown-linux-gnu -S -emit-llvm -o - %s -mabi=n32 | FileCheck -check-prefix=N32 %s
+// RUN: %clang -target mips64el-unknown-linux-gnu -S -emit-llvm -o - %s -mabi=n32 | FileCheck -check-prefix=N32 %s
+// RUN: %clang -target mips64-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck -check-prefix=N64 %s
+// RUN: %clang -target mips64el-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck -check-prefix=N64 %s
+
+// O32: define void @fn28(%struct.T2* noalias sret %agg.result, i8 signext %arg0)
+// N32: define void @fn28(i8 signext %arg0)
+// N64: define void @fn28(i8 signext %arg0)
+
+typedef struct T2 {  } T2;
+T2 T2_retval;
+T2 fn28(char arg0) {
+  return T2_retval;
+}