[X86][KCFI] Do not emit a type prefix for nocf_check functions

With indirect branch protection, the `nocf_check` attribute
prevents a function from being called indirectly by omitting the
ENDBR instruction from the beginning of the function body. As
KCFI type prefixes are only needed for indirectly callable
functions, don't emit the unnecessary prefix for `nocf_check`
functions.

Author: samitolvanen

llvm/lib/Target/X86/X86AsmPrinter.cpp

@@ -170,8 +170,9 @@ void X86AsmPrinter::emitKCFITypeId(const MachineFunction &MF) {
     Type = mdconst::extract<ConstantInt>(MD->getOperand(0));
 
   // If we don't have a type to emit, just emit padding if needed to maintain
-  // the same alignment for all functions.
-  if (!Type) {
+  // the same alignment for all functions. Also skip `nocf_check` functions as
+  // they are not indirectly callable due to a missing ENDBR.
+  if (!Type || F.doesNoCfCheck()) {
     EmitKCFITypePadding(MF, /*HasType=*/false);
     return;
   }

llvm/test/CodeGen/X86/kcfi-nocf-check.ll

@@ -0,0 +1,39 @@
+; RUN: llc -mtriple=x86_64-unknown-unknown -x86-indirect-branch-tracking < %s | FileCheck %s
+
+; CHECK-LABEL: __cfi_cf_check_func:
+; CHECK:       movl	$12345678, %eax
+define void @cf_check_func() !kcfi_type !2 {
+; CHECK-LABEL: cf_check_func:
+; CHECK:       endbr64
+; CHECK:       retq
+entry:
+  ret void
+}
+
+; CHECK-NOT:   __cfi_notype_cf_check_func:
+; CHECK-NOT:   movl
+define void @notype_cf_check_func() {
+; CHECK-LABEL: notype_cf_check_func:
+; CHECK:       endbr64
+; CHECK:       retq
+entry:
+  ret void
+}
+
+; CHECK-NOT:   __cfi_nocf_check_func:
+; CHECK-NOT:   movl
+define void @nocf_check_func() #0 !kcfi_type !2 {
+; CHECK-LABEL: nocf_check_func:
+; CHECK-NOT:   endbr64
+; CHECK:       retq
+entry:
+  ret void
+}
+
+attributes #0 = { nocf_check }
+
+!llvm.module.flags = !{!0, !1}
+
+!0 = !{i32 8, !"cf-protection-branch", i32 1}
+!1 = !{i32 4, !"kcfi", i32 1}
+!2 = !{i32 12345678}