X86 ABI fix for return values > 24 bytes.

The return value's address must be returned in %rax.
i.e. the callee needs to copy the sret argument (%rdi)
into the return value (%rax).

This probably won't manifest as a bug when the caller is LLVM-compiled
code. But it is an ABI guarantee and tools expect it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228321 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86ISelLowering.cpp

@@ -2107,14 +2107,15 @@ X86TargetLowering::LowerReturn(SDValue Chain,
   // Win32 requires us to put the sret argument to %eax as well.
   // We saved the argument into a virtual register in the entry block,
   // so now we copy the value out and into %rax/%eax.
-  if (DAG.getMachineFunction().getFunction()->hasStructRetAttr() &&
-      (Subtarget->is64Bit() || Subtarget->isTargetKnownWindowsMSVC())) {
-    MachineFunction &MF = DAG.getMachineFunction();
-    X86MachineFunctionInfo *FuncInfo = MF.getInfo<X86MachineFunctionInfo>();
-    unsigned Reg = FuncInfo->getSRetReturnReg();
-    assert(Reg &&
-           "SRetReturnReg should have been set in LowerFormalArguments().");
-    SDValue Val = DAG.getCopyFromReg(Chain, dl, Reg, getPointerTy());
+  //
+  // Checking Function.hasStructRetAttr() here is insufficient because the IR
+  // may not have an explicit sret argument. If FuncInfo.CanLowerReturn is
+  // false, then an sret argument may be implicitly inserted in the SelDAG. In
+  // either case FuncInfo->setSRetReturnReg() will have been called.
+  if (unsigned SRetReg = FuncInfo->getSRetReturnReg()) {
+    assert((Subtarget->is64Bit() || Subtarget->isTargetKnownWindowsMSVC()) &&
+           "No need for an sret register");
+    SDValue Val = DAG.getCopyFromReg(Chain, dl, SRetReg, getPointerTy());
 
     unsigned RetValReg
         = (Subtarget->is64Bit() && !Subtarget->isTarget64BitILP32()) ?

test/CodeGen/X86/sret-implicit.ll

@@ -0,0 +1,10 @@
+; RUN: llc -mtriple=x86_64-apple-darwin8 < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-pc-linux < %s | FileCheck %s
+
+; CHECK-LABEL: return32
+; CHECK-DAG: movq	$0, (%rdi)
+; CHECK-DAG: movq	%rdi, %rax
+; CHECK: retq
+define i256 @return32() {
+  ret i256 0
+}

test/CodeGen/X86/vselect.ll

@@ -275,6 +275,7 @@ define <16 x double> @select_illegal(<16 x double> %a, <16 x double> %b) {
 ; CHECK-NEXT:    movaps %xmm2, 32(%rdi)
 ; CHECK-NEXT:    movaps %xmm1, 16(%rdi)
 ; CHECK-NEXT:    movaps %xmm0, (%rdi)
+; CHECK-NEXT:    movq %rdi, %rax
 ; CHECK-NEXT:    retq
   %sel = select <16 x i1> <i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 false, i1 false, i1 false, i1 false, i1 false, i1 false, i1 false, i1 false>, <16 x double> %a, <16 x double> %b
   ret <16 x double> %sel