[COFF, ARM64] Decide when to mark struct returns as SRet

Summary:
Refer the MS ARM64 ABI Convention for the behavior for struct returns:
https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions#return-values

Reviewers: mstorsjo, compnerd, rnk, javed.absar, yinma, efriedma

Reviewed By: rnk, efriedma

Subscribers: haripul, TomTan, yinma, efriedma, kristof.beyls, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D49464

llvm-svn: 338050

clang/include/clang/CodeGen/CGFunctionInfo.h

@@ -96,6 +96,7 @@ private:
   bool InReg : 1;           // isDirect() || isExtend() || isIndirect()
   bool CanBeFlattened: 1;   // isDirect()
   bool SignExt : 1;         // isExtend()
+  bool SuppressSRet : 1;    // isIndirect()
 
   bool canHavePaddingType() const {
     return isDirect() || isExtend() || isIndirect() || isExpand();
@@ -111,13 +112,14 @@ private:
   }
 
   ABIArgInfo(Kind K)
-      : TheKind(K), PaddingInReg(false), InReg(false) {
+      : TheKind(K), PaddingInReg(false), InReg(false), SuppressSRet(false) {
   }
 
 public:
   ABIArgInfo()
       : TypeData(nullptr), PaddingType(nullptr), DirectOffset(0),
-        TheKind(Direct), PaddingInReg(false), InReg(false) {}
+        TheKind(Direct), PaddingInReg(false), InReg(false),
+        SuppressSRet(false) {}
 
   static ABIArgInfo getDirect(llvm::Type *T = nullptr, unsigned Offset = 0,
                               llvm::Type *Padding = nullptr,
@@ -406,6 +408,16 @@ public:
     CanBeFlattened = Flatten;
   }
 
+  bool getSuppressSRet() const {
+    assert(isIndirect() && "Invalid kind!");
+    return SuppressSRet;
+  }
+
+  void setSuppressSRet(bool Suppress) {
+    assert(isIndirect() && "Invalid kind!");
+    SuppressSRet = Suppress;
+  }
+
   void dump() const;
 };
 

clang/lib/CodeGen/CGCall.cpp

@@ -1988,6 +1988,7 @@ void CodeGenModule::ConstructAttributeList(
   // Attach attributes to sret.
   if (IRFunctionArgs.hasSRetArg()) {
     llvm::AttrBuilder SRETAttrs;
+    if (!RetAI.getSuppressSRet())
       SRETAttrs.addAttribute(llvm::Attribute::StructRet);
     hasUsedSRet = true;
     if (RetAI.getInReg())

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -1060,10 +1060,22 @@ bool MicrosoftCXXABI::classifyReturnType(CGFunctionInfo &FI) const {
     // the second parameter.
     FI.getReturnInfo() = ABIArgInfo::getIndirect(Align, /*ByVal=*/false);
     FI.getReturnInfo().setSRetAfterThis(FI.isInstanceMethod());
+
+    // aarch64-windows requires that instance methods use X1 for the return
+    // address. So for aarch64-windows we do not mark the
+    // return as SRet.
+    FI.getReturnInfo().setSuppressSRet(CGM.getTarget().getTriple().getArch() ==
+                                       llvm::Triple::aarch64);
     return true;
   } else if (!RD->isPOD()) {
     // If it's a free function, non-POD types are returned indirectly.
     FI.getReturnInfo() = ABIArgInfo::getIndirect(Align, /*ByVal=*/false);
+
+    // aarch64-windows requires that non-POD, non-instance returns use X0 for
+    // the return address. So for aarch64-windows we do not mark the return as
+    // SRet.
+    FI.getReturnInfo().setSuppressSRet(CGM.getTarget().getTriple().getArch() ==
+                                       llvm::Triple::aarch64);
     return true;
   }
 

clang/test/CodeGen/arm64-microsoft-arguments.cpp

@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -triple aarch64-windows -ffreestanding -emit-llvm -O0 \
+// RUN: -x c++ -o - %s | FileCheck %s
+
+struct pod { int a, b, c, d, e; };
+
+struct non_pod {
+  int a;
+  non_pod() {}
+};
+
+struct pod s;
+struct non_pod t;
+
+struct pod bar() { return s; }
+struct non_pod foo() { return t; }
+// CHECK: define {{.*}} void @{{.*}}bar{{.*}}(%struct.pod* noalias sret %agg.result)
+// CHECK: define {{.*}} void @{{.*}}foo{{.*}}(%struct.non_pod* noalias %agg.result)
+
+
+// Check instance methods.
+struct pod2 { int x; };
+struct Baz { pod2 baz(); };
+
+int qux() { return Baz().baz().x; }
+// CHECK: declare {{.*}} void @{{.*}}baz@Baz{{.*}}(%struct.Baz*, %struct.pod2*)