Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -8825,8 +8825,10 @@
           auto *R = cast<RegisterSDNode>(AsmNodeOperands[CurOp+1]);
           Register TiedReg = R->getReg();
           MVT RegVT = R->getSimpleValueType(0);
-          const TargetRegisterClass *RC = TiedReg.isVirtual() ?
-            MRI.getRegClass(TiedReg) : TRI.getMinimalPhysRegClass(TiedReg);
+          const TargetRegisterClass *RC =
+              TiedReg.isVirtual()     ? MRI.getRegClass(TiedReg)
+              : RegVT != MVT::Untyped ? TLI.getRegClassFor(RegVT)
+                                      : TRI.getMinimalPhysRegClass(TiedReg);
           unsigned NumRegs = InlineAsm::getNumOperandRegisters(OpFlag);
           for (unsigned i = 0; i != NumRegs; ++i)
             Regs.push_back(MRI.createVirtualRegister(RC));
Index: llvm/test/CodeGen/X86/20210831-inlineasm.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/20210831-inlineasm.ll
@@ -0,0 +1,20 @@
+; RUN: llc < %s -O0 -mtriple=x86_64-unknown-linux-gnu
+; https://bugs.llvm.org/show_bug.cgi?id=51699
+
+%"[]u8" = type { i8*, i64 }
+%std.mem.Allocator = type { void ({ %"[]u8", i16 }*, %std.builtin.StackTrace*, %std.mem.Allocator*, i64, i29, i29, i64)*, void ({ i64, i16 }*, %std.builtin.StackTrace*, %std.mem.Allocator*, %"[]u8"*, i29, i64, i29, i64)* }
+%std.builtin.StackTrace = type { i64, %"[]usize" }
+%"[]usize" = type { i64*, i64 }
+
+define  void @fun(%"[]u8"* %0) #0 {
+Entry:
+  %1 = alloca [6 x i64], align 8
+  br label %ErrRetContinue
+
+ErrRetContinue:                                   ; preds = %Entry
+  %2 = call i64 asm sideeffect "rolq $$3,  %rdi ; rolq $$13, %rdi\0Arolq $$61, %rdi ; rolq $$51, %rdi\0Axchgq %rbx,%rbx\0A", "={rdx},{rax},0,~{cc},~{memory}"(i64 undef, i64 0)
+  %3 = call fastcc i64 undef(%std.mem.Allocator* undef, %"[]u8"* %0, i29 undef, i64 0, i29 0, i64 undef)
+  ret void
+}
+
+attributes #0 = { sspstrong }