[SelectionDAG] Fix calling convention in expansion of ?MULO.

The SMULO/UMULO DAG nodes, when not directly supported by the target,
expand to a multiplication twice as wide. In case that the resulting
type is not legal, an __mul?i3 intrinsic is used. Since the type is
not legal, the legalizer cannot directly call the intrinsic with
the wide arguments; instead, it "pre-lowers" them by splitting them
in halves.

The "pre-lowering" code in essence made assumptions about
the calling convention, specifically that i(N*2) values will be
split into two iN values and passed in consecutive registers in
little-endian order. This, naturally, breaks on a big-endian system,
such as our OR1K out-of-tree backend.

Thanks to James Miller <[email protected]> for help in debugging.

Differential Revision: https://reviews.llvm.org/D25223

llvm-svn: 283203

Author: whitequark

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -3470,8 +3470,18 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
       // pre-lowered to the correct types. This all depends upon WideVT not
       // being a legal type for the architecture and thus has to be split to
       // two arguments.
-      SDValue Args[] = { LHS, HiLHS, RHS, HiRHS };
-      SDValue Ret = ExpandLibCall(LC, WideVT, Args, 4, isSigned, dl);
+      SDValue Ret;
+      if(DAG.getDataLayout().isLittleEndian()) {
+        // Halves of WideVT are packed into registers in different order
+        // depending on platform endianness. This is usually handled by
+        // the C calling convention, but we can't defer to it in
+        // the legalizer.
+        SDValue Args[] = { LHS, HiLHS, RHS, HiRHS };
+        Ret = ExpandLibCall(LC, WideVT, Args, 4, isSigned, dl);
+      } else {
+        SDValue Args[] = { HiLHS, LHS, HiRHS, RHS };
+        Ret = ExpandLibCall(LC, WideVT, Args, 4, isSigned, dl);
+      }
       BottomHalf = DAG.getNode(ISD::EXTRACT_ELEMENT, dl, VT, Ret,
                                DAG.getIntPtrConstant(0, dl));
       TopHalf = DAG.getNode(ISD::EXTRACT_ELEMENT, dl, VT, Ret,