Index: lib/Target/X86/X86InstrInfo.cpp
===================================================================
--- lib/Target/X86/X86InstrInfo.cpp
+++ lib/Target/X86/X86InstrInfo.cpp
@@ -5413,7 +5413,16 @@
   MIB->addOperand(TailCall.getOperand(0)); // Destination.
   MIB.addImm(0); // Stack offset (not used).
   MIB->addOperand(BranchCond[0]); // Condition.
-  MIB.copyImplicitOps(TailCall); // Regmask and (imp-used) parameters.
+
+  // Pick up implicitly used registers (call parameters).
+  for (const MachineOperand &MO : TailCall.implicit_operands()) {
+    if (MO.isReg())
+      MIB->addOperand(MO);
+  }
+
+  // Note: We must not copy the regmask from TailCall, as the conditional tail
+  // call does not clobber any registers unless the branch is taken, in which
+  // case control leaves the function so it doesn't matter.
 
   I->eraseFromParent();
 }
Index: test/CodeGen/X86/conditional-tailcall.ll
===================================================================
--- test/CodeGen/X86/conditional-tailcall.ll
+++ test/CodeGen/X86/conditional-tailcall.ll
@@ -1,5 +1,5 @@
-; RUN: llc < %s -mtriple=i686-linux -show-mc-encoding | FileCheck %s
-; RUN: llc < %s -mtriple=x86_64-linux -show-mc-encoding | FileCheck %s
+; RUN: llc < %s -mtriple=i686-linux -show-mc-encoding | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK32
+; RUN: llc < %s -mtriple=x86_64-linux -show-mc-encoding | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK64
 
 declare void @foo()
 declare void @bar()
@@ -51,3 +51,90 @@
 ; CHECK-NOT: xor
 ; CHECK: ret
 }
+
+
+
+%"class.std::basic_string" = type { %"struct.std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Alloc_hider" }
+%"struct.std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Alloc_hider" = type { i8* }
+declare zeroext i1 @_Z20isValidIntegerSuffixN9__gnu_cxx17__normal_iteratorIPKcSsEES3_(i8*, i8*)
+
+define zeroext i1 @pr31257(%"class.std::basic_string"* nocapture readonly dereferenceable(8) %s) minsize {
+; CHECK-LABEL: pr31257
+entry:
+  %_M_p.i.i = getelementptr inbounds %"class.std::basic_string", %"class.std::basic_string"* %s, i64 0, i32 0, i32 0
+  %0 = load i8*, i8** %_M_p.i.i, align 8
+  %arrayidx.i.i.i54 = getelementptr inbounds i8, i8* %0, i64 -24
+  %_M_length.i.i55 = bitcast i8* %arrayidx.i.i.i54 to i64*
+  %1 = load i64, i64* %_M_length.i.i55, align 8
+  %add.ptr.i56 = getelementptr inbounds i8, i8* %0, i64 %1
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %it.sroa.0.0 = phi i8* [ %0, %entry ], [ %incdec.ptr.i, %for.inc ]
+  %state.0 = phi i32 [ 0, %entry ], [ %state.1, %for.inc ]
+  %cmp.i = icmp eq i8* %it.sroa.0.0, %add.ptr.i56
+  br i1 %cmp.i, label %5, label %for.body
+
+for.body:                                         ; preds = %for.cond
+  switch i32 %state.0, label %for.inc [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb14
+    i32 2, label %sw.bb22
+  ]
+
+sw.bb:                                            ; preds = %for.body
+  %2 = load i8, i8* %it.sroa.0.0, align 1
+  switch i8 %2, label %if.else [
+    i8 43, label %for.inc
+    i8 45, label %for.inc
+  ]
+
+if.else:                                          ; preds = %sw.bb
+  %conv9 = zext i8 %2 to i32
+  %isdigittmp45 = add nsw i32 %conv9, -48
+  %isdigit46 = icmp ult i32 %isdigittmp45, 10
+  br i1 %isdigit46, label %for.inc, label %cleanup.thread.loopexit
+
+sw.bb14:                                          ; preds = %for.body
+  %3 = load i8, i8* %it.sroa.0.0, align 1
+  %conv16 = zext i8 %3 to i32
+  %isdigittmp43 = add nsw i32 %conv16, -48
+  %isdigit44 = icmp ult i32 %isdigittmp43, 10
+  br i1 %isdigit44, label %for.inc, label %cleanup.thread.loopexit
+
+sw.bb22:                                          ; preds = %for.body
+  %4 = load i8, i8* %it.sroa.0.0, align 1
+  %conv24 = zext i8 %4 to i32
+  %isdigittmp = add nsw i32 %conv24, -48
+  %isdigit = icmp ult i32 %isdigittmp, 10
+  br i1 %isdigit, label %for.inc, label %if.else28
+
+; Make sure Machine Copy Propagation doesn't delete the mov to %ecx becaue it
+; thinks the conditional tail call clobbers it.
+; CHECK64-LABEL: .LBB2_11:
+; CHECK64:       movzbl  (%rdi), %ecx
+; CHECK64-NEXT:  addl    $-48, %ecx
+; CHECK64-NEXT:  cmpl    $10, %ecx
+; CHECK64-NEXT:  movl    %r9d, %ecx
+; CHECK64-NEXT:  jae     _Z20isValidIntegerSuffixN9__gnu_cxx17__normal_iteratorIPKcSsEE
+
+if.else28:                                        ; preds = %sw.bb22
+  %call34 = tail call zeroext i1 @_Z20isValidIntegerSuffixN9__gnu_cxx17__normal_iteratorIPKcSsEES3_(i8* nonnull %it.sroa.0.0, i8* %add.ptr.i56)
+  br label %cleanup.thread
+
+for.inc:                                          ; preds = %sw.bb, %sw.bb, %sw.bb22, %sw.bb14, %if.else, %for.body
+  %state.1 = phi i32 [ %state.0, %for.body ], [ 1, %sw.bb ], [ 2, %if.else ], [ 2, %sw.bb14 ], [ 2, %sw.bb22 ], [ 1, %sw.bb ]
+  %incdec.ptr.i = getelementptr inbounds i8, i8* %it.sroa.0.0, i64 1
+  br label %for.cond
+
+; <label>:5:                                      ; preds = %for.cond
+  %cmp37 = icmp eq i32 %state.0, 2
+  br label %cleanup.thread
+
+cleanup.thread.loopexit:                          ; preds = %if.else, %sw.bb14
+  br label %cleanup.thread
+
+cleanup.thread:                                   ; preds = %cleanup.thread.loopexit, %if.else28, %5
+  %6 = phi i1 [ %cmp37, %5 ], [ %call34, %if.else28 ], [ false, %cleanup.thread.loopexit ]
+  ret i1 %6
+}
Index: test/CodeGen/X86/tail-call-conditional.mir
===================================================================
--- test/CodeGen/X86/tail-call-conditional.mir
+++ test/CodeGen/X86/tail-call-conditional.mir
@@ -48,7 +48,7 @@
   ; CHECK-NEXT: %rdi = COPY %rsi
   ; CHECK-NEXT: %rsi = COPY %rax
   ; CHECK-NEXT: CMP64ri8 %rax, 9, implicit-def %eflags
-  ; CHECK-NEXT: TCRETURNdi64cc @f1, 0, 3, csr_64, implicit %rsp, implicit %eflags, implicit %rsp, implicit %rdi, implicit %rsi
+  ; CHECK-NEXT: TCRETURNdi64cc @f1, 0, 3, implicit %rsp, implicit %eflags, implicit %rsp, implicit %rdi, implicit %rsi
 
   bb.1:
     successors: %bb.2, %bb.3