[mips] Handle transparent unions correctly.

[dsanders]

Summary

This fixes MultiSource/Applications/lemon on big-endian N32 by correcting the
handling of the argument to wait(). glibc defines it as a transparent union of
void* and int*. Such unions are passed according to the rules of the first
member so the argument must be passed as if it were a void* (sign extended from
i32 to i64) and not as a union (shifted to the upper bits of an i64).

wait() already behaves correctly on big-endian O32 and N64 since the union is
already the same size as an argument slot.

[atanasyan]

LGTM (with a nit).

lib/CodeGen/TargetInfo.cpp

5706

I would move this call to the first line of the MipsABIInfo::classifyArgumentType function to be sure that all subsequent actions deal with updated Ty.

lib/CodeGen/TargetInfo.cpp

MipsABIInfo::classifyArgumentType(QualType Ty, uint64_t &Offset) const {
   uint64_t TySize = getContext().getTypeSize(Ty);
   uint64_t Align = getContext().getTypeAlign(Ty) / 8;
 
   Align = std::min(std::max(Align, (uint64_t)MinABIStackAlignInBytes),
                    (uint64_t)StackAlignInBytes);
   unsigned CurrOffset = llvm::RoundUpToAlignment(Offset, Align);
   Offset = CurrOffset + llvm::RoundUpToAlignment(TySize, Align * 8) / 8;
 
+  Ty = useFirstFieldIfTransparentUnion(Ty);
+
   if (isAggregateTypeForABI(Ty) || Ty->isVectorType()) {
     // Ignore empty aggregates.
     if (TySize == 0)
       return ABIArgInfo::getIgnore();
 
     if (CGCXXABI::RecordArgABI RAA = getRecordArgABI(Ty, getCXXABI())) {
       Offset = OrigOffset + MinABIStackAlignInBytes;
       return ABIArgInfo::getIndirect(0, RAA == CGCXXABI::RAA_DirectInMemory);

test/CodeGen/mips-transparent-union.c

+// RUN: %clang_cc1 -triple mips64-linux-gnu -S -o - -emit-llvm %s | FileCheck %s
+//
+// Transparent unions are passed according to the calling convention rules of
+// the first member. In this case, it is as if it were a void pointer so we
+// do not have the inreg attribute we would normally have for unions.
+//
+// This comes up in glibc's wait() function and matters for the big-endian N32
+// case where pointers are promoted to i64 and a non-transparent union would be
+// passed in the upper 32-bits of an i64.
+
+union either_pointer {
+  void *void_ptr;
+  int *int_ptr;
+} __attribute__((transparent_union));
+
+extern void foo(union either_pointer p);
+
+int data;
+
+void bar(void) {
+  return foo(&data);
+}
+
+// CHECK-LABEL: define void @bar()
+// CHECK:         call void @foo(i8* %{{[0-9]+}})
+
+// CHECK: declare void @foo(i8*)