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

[Daniel Sanders]

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.

http://reviews.llvm.org/D4811

Files:
  lib/CodeGen/TargetInfo.cpp
  test/CodeGen/mips-zero-sized-struct.c

Index: lib/CodeGen/TargetInfo.cpp
===================================================================
--- lib/CodeGen/TargetInfo.cpp
+++ lib/CodeGen/TargetInfo.cpp
@@ -5519,7 +5519,7 @@
 ABIArgInfo MipsABIInfo::classifyReturnType(QualType RetTy) const {
   uint64_t Size = getContext().getTypeSize(RetTy);
 
-  if (RetTy->isVoidType() || Size == 0)
+  if (RetTy->isVoidType())
     return ABIArgInfo::getIgnore();
 
   if (isAggregateTypeForABI(RetTy) || RetTy->isVectorType()) {
Index: test/CodeGen/mips-zero-sized-struct.c
===================================================================
--- /dev/null
+++ test/CodeGen/mips-zero-sized-struct.c
@@ -0,0 +1,12 @@
+// RUN: %clang -target mips-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang -target mipsel-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang -target mips64-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang -target mips64el-unknown-linux-gnu -S -emit-llvm -o - %s | FileCheck %s
+
+// CHECK: define void @fn28(%struct.T2* noalias sret %agg.result, i8 signext %arg0)
+
+typedef struct T2 {  } T2;
+T2 T2_retval;
+T2 fn28(char arg0) {
+  return T2_retval;
+}
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