Index: llvm/trunk/lib/CodeGen/MachineInstr.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MachineInstr.cpp
+++ llvm/trunk/lib/CodeGen/MachineInstr.cpp
@@ -581,10 +581,8 @@
   if (BasePtr == nullptr)
     return false;
 
-  return isDereferenceableAndAlignedPointer(BasePtr, 1,
-                                            APInt(DL.getPointerSize(),
-                                                  Offset + Size),
-                                            DL);
+  return isDereferenceableAndAlignedPointer(
+      BasePtr, 1, APInt(DL.getPointerSizeInBits(), Offset + Size), DL);
 }
 
 /// getConstantPool - Return a MachinePointerInfo record that refers to the
Index: llvm/trunk/test/CodeGen/Mips/pr33978.ll
===================================================================
--- llvm/trunk/test/CodeGen/Mips/pr33978.ll
+++ llvm/trunk/test/CodeGen/Mips/pr33978.ll
@@ -0,0 +1,20 @@
+; RUN: llc -march=mips -mcpu=mips32r2 < %s -o /dev/null
+
+; Test that SelectionDAG does not crash during DAGCombine when two pointers
+; to the stack match with differing bases and offsets when expanding memcpy.
+; This could result in one of the pointers being considered dereferenceable
+; and other not.
+
+define void @foo(i8*) {
+start:
+  %a = alloca [22 x i8]
+  %b = alloca [22 x i8]
+  %c = bitcast [22 x i8]* %a to i8*
+  %d = getelementptr inbounds [22 x i8], [22 x i8]* %b, i32 0, i32 2
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %c, i8* %d, i32 20, i32 1, i1 false)
+  %e = getelementptr inbounds [22 x i8], [22 x i8]* %b, i32 0, i32 6
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %0, i8* %e, i32 12, i32 1, i1 false)
+  ret void
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i32(i8*, i8*, i32, i32, i1)