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

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.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: atanasyan, cfe-commits

Differential Revision: http://reviews.llvm.org/D4811

llvm-svn: 217147

1 files changed, 6 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index fb3b931ce95..d33d6a09e01 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -5548,7 +5548,12 @@ MipsABIInfo::returnAggregateInRegs(QualType RetTy, uint64_t Size) const {
 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()) {