[ms-cxxabi] Add a new calling convention that swaps 'this' and 'sret'

MSVC always places the 'this' parameter for a method first.  The
implicit 'sret' pointer for methods always comes second.  We already
implement this for __thiscall by putting sret parameters on the stack,
but __cdecl methods require putting both parameters on the stack in
opposite order.

Using a special calling convention allows frontends to keep the sret
parameter first, which avoids breaking lots of assumptions in LLVM and
Clang.

Fixes PR15768 with the corresponding change in Clang.

Reviewers: ributzka, majnemer

Differential Revision: http://llvm-reviews.chandlerc.com/D2663

llvm-svn: 200561

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86CallingConv.h b/llvm/lib/Target/X86/X86CallingConv.h
index e76f9fda2db..040da3535e8 100644
--- a/llvm/lib/Target/X86/X86CallingConv.h
+++ b/llvm/lib/Target/X86/X86CallingConv.h
@@ -29,6 +29,33 @@ inline bool CC_X86_AnyReg_Error(unsigned &, MVT &, MVT &,
   return false;
 }
 
+inline bool CC_X86_CDeclMethod_SRet(unsigned &ValNo, MVT &ValVT, MVT &LocVT,
+                                    CCValAssign::LocInfo &LocInfo,
+                                    ISD::ArgFlagsTy &ArgFlags, CCState &State) {
+  // Swap the order of the first two parameters if the first parameter is sret.
+  if (ArgFlags.isSRet()) {
+    assert(ValNo == 0);
+    assert(ValVT == MVT::i32);
+    State.AllocateStack(8, 4);
+    State.addLoc(CCValAssign::getCustomMem(ValNo, ValVT, 4, LocVT, LocInfo));
+
+    // Indicate that we need to swap the order of the first and second
+    // parameters by "allocating" register zero.  There are no register
+    // parameters with cdecl methods, so we can use this to communicate to the
+    // next call.
+    State.AllocateReg(1);
+    return true;
+  } else if (ValNo == 1 && State.isAllocated(1)) {
+    assert(ValVT == MVT::i32 && "non-i32-sized this param unsupported");
+    // Stack was already allocated while processing sret.
+    State.addLoc(CCValAssign::getCustomMem(ValNo, ValVT, 0, LocVT, LocInfo));
+    return true;
+  }
+
+  // All other args use the C calling convention.
+  return false;
+}
+
 } // End llvm namespace
 
 #endif