diff --git a/llvm/include/llvm/MC/MCParser/MCAsmParser.h b/llvm/include/llvm/MC/MCParser/MCAsmParser.h
--- a/llvm/include/llvm/MC/MCParser/MCAsmParser.h
+++ b/llvm/include/llvm/MC/MCParser/MCAsmParser.h
@@ -168,6 +168,8 @@
   virtual void setParsingInlineAsm(bool V) = 0;
   virtual bool isParsingInlineAsm() = 0;
 
+  virtual bool isParsingMasm() const { return false; }
+
   /// Parse MS-style inline assembly.
   virtual bool parseMSInlineAsm(
       void *AsmLoc, std::string &AsmString, unsigned &NumOutputs,
diff --git a/llvm/lib/MC/MCParser/MasmParser.cpp b/llvm/lib/MC/MCParser/MasmParser.cpp
--- a/llvm/lib/MC/MCParser/MasmParser.cpp
+++ b/llvm/lib/MC/MCParser/MasmParser.cpp
@@ -248,6 +248,8 @@
   }
   bool isParsingInlineAsm() override { return ParsingInlineAsm; }
 
+  bool isParsingMasm() const override { return true; }
+
   bool parseMSInlineAsm(void *AsmLoc, std::string &AsmString,
                         unsigned &NumOutputs, unsigned &NumInputs,
                         SmallVectorImpl<std::pair<void *,bool>> &OpDecls,
diff --git a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
--- a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -1607,7 +1607,8 @@
   } else {
     BaseReg = BaseReg ? BaseReg : 1;
     return X86Operand::CreateMem(getPointerWidth(), SegReg, Disp, BaseReg,
-                                 IndexReg, Scale, Start, End, Size, Identifier,
+                                 IndexReg, Scale, Start, End, Size,
+                                 /*DefaultBaseReg=*/X86::NoRegister, Identifier,
                                  Decl, FrontendSize);
   }
 }
@@ -2171,6 +2172,14 @@
     return CreateMemForInlineAsm(RegNo, Disp, BaseReg, IndexReg,
                                  Scale, Start, End, Size, SM.getSymName(),
                                  SM.getIdentifierInfo());
+
+  // When parsing x64 MASM, all memory operands default to RIP-relative when
+  // interpreted as non-absolute references.
+  if (Parser.isParsingMasm() && is64BitMode())
+    return X86Operand::CreateMem(getPointerWidth(), RegNo, Disp, BaseReg,
+                                 IndexReg, Scale, Start, End, Size,
+                                 /*DefaultBaseReg=*/X86::RIP);
+
   if (!(BaseReg || IndexReg || RegNo))
     return X86Operand::CreateMem(getPointerWidth(), Disp, Start, End, Size);
   return X86Operand::CreateMem(getPointerWidth(), RegNo, Disp,
diff --git a/llvm/lib/Target/X86/AsmParser/X86Operand.h b/llvm/lib/Target/X86/AsmParser/X86Operand.h
--- a/llvm/lib/Target/X86/AsmParser/X86Operand.h
+++ b/llvm/lib/Target/X86/AsmParser/X86Operand.h
@@ -60,6 +60,7 @@
     unsigned SegReg;
     const MCExpr *Disp;
     unsigned BaseReg;
+    unsigned DefaultBaseReg;
     unsigned IndexReg;
     unsigned Scale;
     unsigned Size;
@@ -184,6 +185,10 @@
     assert(Kind == Memory && "Invalid access!");
     return Mem.BaseReg;
   }
+  unsigned getMemDefaultBaseReg() const {
+    assert(Kind == Memory && "Invalid access!");
+    return Mem.DefaultBaseReg;
+  }
   unsigned getMemIndexReg() const {
     assert(Kind == Memory && "Invalid access!");
     return Mem.IndexReg;
@@ -540,7 +545,10 @@
 
   void addMemOperands(MCInst &Inst, unsigned N) const {
     assert((N == 5) && "Invalid number of operands!");
-    Inst.addOperand(MCOperand::createReg(getMemBaseReg()));
+    if (getMemBaseReg())
+      Inst.addOperand(MCOperand::createReg(getMemBaseReg()));
+    else
+      Inst.addOperand(MCOperand::createReg(getMemDefaultBaseReg()));
     Inst.addOperand(MCOperand::createImm(getMemScale()));
     Inst.addOperand(MCOperand::createReg(getMemIndexReg()));
     addExpr(Inst, getMemDisp());
@@ -633,6 +641,7 @@
     Res->Mem.SegReg   = 0;
     Res->Mem.Disp     = Disp;
     Res->Mem.BaseReg  = 0;
+    Res->Mem.DefaultBaseReg = 0;
     Res->Mem.IndexReg = 0;
     Res->Mem.Scale    = 1;
     Res->Mem.Size     = Size;
@@ -648,11 +657,14 @@
   static std::unique_ptr<X86Operand>
   CreateMem(unsigned ModeSize, unsigned SegReg, const MCExpr *Disp,
             unsigned BaseReg, unsigned IndexReg, unsigned Scale, SMLoc StartLoc,
-            SMLoc EndLoc, unsigned Size = 0, StringRef SymName = StringRef(),
-            void *OpDecl = nullptr, unsigned FrontendSize = 0) {
+            SMLoc EndLoc, unsigned Size = 0,
+            unsigned DefaultBaseReg = X86::NoRegister,
+            StringRef SymName = StringRef(), void *OpDecl = nullptr,
+            unsigned FrontendSize = 0) {
     // We should never just have a displacement, that should be parsed as an
     // absolute memory operand.
-    assert((SegReg || BaseReg || IndexReg) && "Invalid memory operand!");
+    assert((SegReg || BaseReg || IndexReg || DefaultBaseReg) &&
+           "Invalid memory operand!");
 
     // The scale should always be one of {1,2,4,8}.
     assert(((Scale == 1 || Scale == 2 || Scale == 4 || Scale == 8)) &&
@@ -661,6 +673,7 @@
     Res->Mem.SegReg   = SegReg;
     Res->Mem.Disp     = Disp;
     Res->Mem.BaseReg  = BaseReg;
+    Res->Mem.DefaultBaseReg = DefaultBaseReg;
     Res->Mem.IndexReg = IndexReg;
     Res->Mem.Scale    = Scale;
     Res->Mem.Size     = Size;