Use less byval on 32-bit Windows x86 for classes with bases

This comes up in V8, which has a Handle template class that wraps a
typed pointer, and is frequently passed by value. The pointer is stored
in the base, HandleBase. This change allows us to pass the struct as a
pointer instead of using byval. This avoids creating tons of temporary
allocas that we copy from during call lowering.

Eventually, it would be good to use FCAs here instead.

llvm-svn: 291917

clang/lib/CodeGen/TargetInfo.cpp

@@ -1197,6 +1197,39 @@ static bool is32Or64BitBasicType(QualType Ty, ASTContext &Context) {
   return Size == 32 || Size == 64;
 }
 
+static bool addFieldSizes(ASTContext &Context, const RecordDecl *RD,
+                          uint64_t &Size) {
+  for (const auto *FD : RD->fields()) {
+    // Scalar arguments on the stack get 4 byte alignment on x86. If the
+    // argument is smaller than 32-bits, expanding the struct will create
+    // alignment padding.
+    if (!is32Or64BitBasicType(FD->getType(), Context))
+      return false;
+
+    // FIXME: Reject bit-fields wholesale; there are two problems, we don't know
+    // how to expand them yet, and the predicate for telling if a bitfield still
+    // counts as "basic" is more complicated than what we were doing previously.
+    if (FD->isBitField())
+      return false;
+
+    Size += Context.getTypeSize(FD->getType());
+  }
+  return true;
+}
+
+static bool addBaseAndFieldSizes(ASTContext &Context, const CXXRecordDecl *RD,
+                                 uint64_t &Size) {
+  // Don't do this if there are any non-empty bases.
+  for (const CXXBaseSpecifier &Base : RD->bases()) {
+    if (!addBaseAndFieldSizes(Context, Base.getType()->getAsCXXRecordDecl(),
+                              Size))
+      return false;
+  }
+  if (!addFieldSizes(Context, RD, Size))
+    return false;
+  return true;
+}
+
 /// Test whether an argument type which is to be passed indirectly (on the
 /// stack) would have the equivalent layout if it was expanded into separate
 /// arguments. If so, we prefer to do the latter to avoid inhibiting
@@ -1207,8 +1240,9 @@ bool X86_32ABIInfo::canExpandIndirectArgument(QualType Ty) const {
   if (!RT)
     return false;
   const RecordDecl *RD = RT->getDecl();
+  uint64_t Size = 0;
   if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
-    if (!IsWin32StructABI ) {
+    if (!IsWin32StructABI) {
       // On non-Windows, we have to conservatively match our old bitcode
       // prototypes in order to be ABI-compatible at the bitcode level.
       if (!CXXRD->isCLike())
@@ -1217,30 +1251,12 @@ bool X86_32ABIInfo::canExpandIndirectArgument(QualType Ty) const {
       // Don't do this for dynamic classes.
       if (CXXRD->isDynamicClass())
         return false;
-      // Don't do this if there are any non-empty bases.
-      for (const CXXBaseSpecifier &Base : CXXRD->bases()) {
-        if (!isEmptyRecord(getContext(), Base.getType(), /*AllowArrays=*/true))
-          return false;
-      }
     }
-  }
-
-  uint64_t Size = 0;
-
-  for (const auto *FD : RD->fields()) {
-    // Scalar arguments on the stack get 4 byte alignment on x86. If the
-    // argument is smaller than 32-bits, expanding the struct will create
-    // alignment padding.
-    if (!is32Or64BitBasicType(FD->getType(), getContext()))
+    if (!addBaseAndFieldSizes(getContext(), CXXRD, Size))
       return false;
-
-    // FIXME: Reject bit-fields wholesale; there are two problems, we don't know
-    // how to expand them yet, and the predicate for telling if a bitfield still
-    // counts as "basic" is more complicated than what we were doing previously.
-    if (FD->isBitField())
+  } else {
+    if (!addFieldSizes(getContext(), RD, Size))
       return false;
-
-    Size += getContext().getTypeSize(FD->getType());
   }
 
   // We can do this if there was no alignment padding.

clang/test/CodeGenCXX/microsoft-abi-sret-and-byval.cpp

@@ -64,6 +64,10 @@ struct BigWithDtor {
   int a, b, c, d, e, f;
 };
 
+struct BaseNoByval : Small {
+  int bb;
+};
+
 // WIN32: declare void @"{{.*take_bools_and_chars.*}}"
 // WIN32:       (<{ i8, [3 x i8], i8, [3 x i8], %struct.SmallWithDtor,
 // WIN32:           i8, [3 x i8], i8, [3 x i8], i32, i8, [3 x i8] }>* inalloca)
@@ -127,6 +131,12 @@ void medium_arg(Medium s) {}
 // WIN64: define void @"\01?medium_arg@@YAXUMedium@@@Z"(i64 %s.coerce)
 // WOA: define arm_aapcs_vfpcc void @"\01?medium_arg@@YAXUMedium@@@Z"([2 x i32] %s.coerce)
 
+void base_no_byval_arg(BaseNoByval s) {}
+// LINUX-LABEL: define void @_Z17base_no_byval_arg11BaseNoByval(%struct.BaseNoByval* byval align 4 %s)
+// WIN32: define void @"\01?base_no_byval_arg@@YAXUBaseNoByval@@@Z"(i32 %s.0, i32 %s.1)
+// WIN64: define void @"\01?base_no_byval_arg@@YAXUBaseNoByval@@@Z"(i64 %s.coerce)
+// WOA: define arm_aapcs_vfpcc void @"\01?base_no_byval_arg@@YAXUBaseNoByval@@@Z"([2 x i32] %s.coerce)
+
 void small_arg_with_ctor(SmallWithCtor s) {}
 // LINUX-LABEL: define void @_Z19small_arg_with_ctor13SmallWithCtor(%struct.SmallWithCtor* byval align 4 %s)
 // WIN32: define void @"\01?small_arg_with_ctor@@YAXUSmallWithCtor@@@Z"(i32 %s.0)