diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2544,6 +2544,10 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
                     MF.getFunction()->hasStructRetAttr(), CLI.RetTy,
                     Outs, OutVals, Ins, DAG);
 
+    if (!isTailCall && CLI.CS && CLI.CS->isMustTailCall())
+      report_fatal_error("failed to perform tail call elimination on a call "
+                         "site marked musttail");
+
     // Sibcalls are automatically detected tailcalls which do not require
     // ABI changes.
     if (!MF.getTarget().Options.GuaranteedTailCallOpt && isTailCall)

diff --git a/llvm/test/Bitcode/tailcall.ll b/llvm/test/Bitcode/tailcall.ll
@@ -0,0 +1,17 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+; Check that musttail and tail roundtrip.
+
+declare cc8191 void @t1_callee()
+define cc8191 void @t1() {
+; CHECK: tail call cc8191 void @t1_callee()
+  tail call cc8191 void @t1_callee()
+  ret void
+}
+
+declare cc8191 void @t2_callee()
+define cc8191 void @t2() {
+; CHECK: musttail call cc8191 void @t2_callee()
+  musttail call cc8191 void @t2_callee()
+  ret void
+}
diff --git a/llvm/test/CodeGen/X86/musttail.ll b/llvm/test/CodeGen/X86/musttail.ll
@@ -0,0 +1,23 @@
+; RUN: llc -march=x86 < %s | FileCheck %s
+
+; FIXME: Eliminate this tail call at -O0, since musttail is a correctness
+; requirement.
+; RUN: not llc -march=x86 -O0 < %s
+
+declare void @t1_callee(i8*)
+define void @t1(i32* %a) {
+; CHECK-LABEL: t1:
+; CHECK: jmp {{_?}}t1_callee
+  %b = bitcast i32* %a to i8*
+  musttail call void @t1_callee(i8* %b)
+  ret void
+}
+
+declare i8* @t2_callee()
+define i32* @t2() {
+; CHECK-LABEL: t2:
+; CHECK: jmp {{_?}}t2_callee
+  %v = musttail call i8* @t2_callee()
+  %w = bitcast i8* %v to i32*
+  ret i32* %w
+}
diff --git a/llvm/test/Verifier/musttail-invalid.ll b/llvm/test/Verifier/musttail-invalid.ll
@@ -0,0 +1,75 @@
+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+; Each musttail call should fail to validate.
+
+declare x86_stdcallcc void @cc_mismatch_callee()
+define void @cc_mismatch() {
+; CHECK: mismatched calling conv
+  musttail call x86_stdcallcc void @cc_mismatch_callee()
+  ret void
+}
+
+declare void @more_parms_callee(i32)
+define void @more_parms() {
+; CHECK: mismatched parameter counts
+  musttail call void @more_parms_callee(i32 0)
+  ret void
+}
+
+declare void @mismatched_intty_callee(i8)
+define void @mismatched_intty(i32) {
+; CHECK: mismatched parameter types
+  musttail call void @mismatched_intty_callee(i8 0)
+  ret void
+}
+
+declare void @mismatched_vararg_callee(i8*, ...)
+define void @mismatched_vararg(i8*) {
+; CHECK: mismatched varargs
+  musttail call void (i8*, ...)* @mismatched_vararg_callee(i8* null)
+  ret void
+}
+
+; We would make this an implicit sret parameter, which would disturb the
+; tail call.
+declare { i32, i32, i32 } @mismatched_retty_callee(i32)
+define void @mismatched_retty(i32) {
+; CHECK: mismatched return types
+  musttail call { i32, i32, i32 } @mismatched_retty_callee(i32 0)
+  ret void
+}
+
+declare void @mismatched_byval_callee({ i32 }*)
+define void @mismatched_byval({ i32 }* byval %a) {
+; CHECK: mismatched ABI impacting function attributes
+  musttail call void @mismatched_byval_callee({ i32 }* %a)
+  ret void
+}
+
+declare void @mismatched_inreg_callee(i32 inreg)
+define void @mismatched_inreg(i32 %a) {
+; CHECK: mismatched ABI impacting function attributes
+  musttail call void @mismatched_inreg_callee(i32 inreg %a)
+  ret void
+}
+
+declare void @mismatched_sret_callee(i32* sret)
+define void @mismatched_sret(i32* %a) {
+; CHECK: mismatched ABI impacting function attributes
+  musttail call void @mismatched_sret_callee(i32* sret %a)
+  ret void
+}
+
+declare i32 @not_tail_pos_callee()
+define i32 @not_tail_pos() {
+; CHECK: musttail call must be precede a ret with an optional bitcast
+  %v = musttail call i32 @not_tail_pos_callee()
+  %w = add i32 %v, 1
+  ret i32 %w
+}
+
+define void @inline_asm() {
+; CHECK: cannot use musttail call with inline asm
+  musttail call void asm "ret", ""()
+  ret void
+}
diff --git a/llvm/test/Verifier/musttail-valid.ll b/llvm/test/Verifier/musttail-valid.ll
@@ -0,0 +1,16 @@
+; RUN: llvm-as %s -o /dev/null
+
+; Should assemble without error.
+
+declare void @similar_param_ptrty_callee(i8*)
+define void @similar_param_ptrty(i32*) {
+  musttail call void @similar_param_ptrty_callee(i8* null)
+  ret void
+}
+
+declare i8* @similar_ret_ptrty_callee()
+define i32* @similar_ret_ptrty() {
+  %v = musttail call i8* @similar_ret_ptrty_callee()
+  %w = bitcast i8* %v to i32*
+  ret i32* %w
+}