[X86] Always return the sret parameter in eax/rax, even on 32-bit

Summary:
This rule was always in the old SysV i386 ABI docs and the new ones that
H.J. Lu has put together, but we never noticed:

  EAX   scratch register; also used to return integer and pointer values
        from functions; also stores the address of a returned struct or union

Fixes PR23491.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9715

llvm-svn: 237175

1 files changed, 15 insertions, 21 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 30cb2238afa..5a7455befc3 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2000,9 +2000,8 @@ X86TargetLowering::LowerReturn(SDValue Chain,
     RetOps.push_back(DAG.getRegister(VA.getLocReg(), VA.getLocVT()));
   }
 
-  // The x86-64 ABIs require that for returning structs by value we copy
+  // All x86 ABIs require that for returning structs by value we copy
   // the sret argument into %rax/%eax (depending on ABI) for the return.
-  // 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.
   //
@@ -2011,8 +2010,6 @@ X86TargetLowering::LowerReturn(SDValue Chain,
   // 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
@@ -2434,24 +2431,21 @@ X86TargetLowering::LowerFormalArguments(SDValue Chain,
     InVals.push_back(ArgValue);
   }
 
-  if (Subtarget->is64Bit() || Subtarget->isTargetKnownWindowsMSVC()) {
-    for (unsigned i = 0, e = ArgLocs.size(); i != e; ++i) {
-      // The x86-64 ABIs require that for returning structs by value we copy
-      // the sret argument into %rax/%eax (depending on ABI) for the return.
-      // Win32 requires us to put the sret argument to %eax as well.
-      // Save the argument into a virtual register so that we can access it
-      // from the return points.
-      if (Ins[i].Flags.isSRet()) {
-        unsigned Reg = FuncInfo->getSRetReturnReg();
-        if (!Reg) {
-          MVT PtrTy = getPointerTy();
-          Reg = MF.getRegInfo().createVirtualRegister(getRegClassFor(PtrTy));
-          FuncInfo->setSRetReturnReg(Reg);
-        }
-        SDValue Copy = DAG.getCopyToReg(DAG.getEntryNode(), dl, Reg, InVals[i]);
-        Chain = DAG.getNode(ISD::TokenFactor, dl, MVT::Other, Copy, Chain);
-        break;
+  for (unsigned i = 0, e = ArgLocs.size(); i != e; ++i) {
+    // All x86 ABIs require that for returning structs by value we copy the
+    // sret argument into %rax/%eax (depending on ABI) for the return. Save
+    // the argument into a virtual register so that we can access it from the
+    // return points.
+    if (Ins[i].Flags.isSRet()) {
+      unsigned Reg = FuncInfo->getSRetReturnReg();
+      if (!Reg) {
+        MVT PtrTy = getPointerTy();
+        Reg = MF.getRegInfo().createVirtualRegister(getRegClassFor(PtrTy));
+        FuncInfo->setSRetReturnReg(Reg);
       }
+      SDValue Copy = DAG.getCopyToReg(DAG.getEntryNode(), dl, Reg, InVals[i]);
+      Chain = DAG.getNode(ISD::TokenFactor, dl, MVT::Other, Copy, Chain);
+      break;
     }
   }