[X86] Added support for nocf_check attribute for indirect Branch Tracking

X86 Supports Indirect Branch Tracking (IBT) as part of Control-Flow Enforcement Technology (CET).
IBT instruments ENDBR instructions used to specify valid targets of indirect call / jmp.
The `nocf_check` attribute has two roles in the context of X86 IBT technology:
	1. Appertains to a function - do not add ENDBR instruction at the beginning of the function.
	2. Appertains to a function pointer - do not track the target function of this pointer by adding nocf_check prefix to the indirect-call instruction.

This patch implements `nocf_check` context for Indirect Branch Tracking.
It also auto generates `nocf_check` prefixes before indirect branchs to jump tables that are guarded by range checks.

Differential Revision: https://reviews.llvm.org/D41879

llvm-svn: 327767

2 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
index 30216bcde68..0aea24f22c0 100644
--- a/llvm/lib/IR/Attributes.cpp
+++ b/llvm/lib/IR/Attributes.cpp
@@ -299,6 +299,8 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
     return "noredzone";
   if (hasAttribute(Attribute::NoReturn))
     return "noreturn";
+  if (hasAttribute(Attribute::NoCfCheck))
+    return "nocf_check";
   if (hasAttribute(Attribute::NoRecurse))
     return "norecurse";
   if (hasAttribute(Attribute::NoUnwind))
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index b9df7c91a64..aec5c682296 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1404,6 +1404,7 @@ Verifier::visitModuleFlag(const MDNode *Op,
 static bool isFuncOnlyAttr(Attribute::AttrKind Kind) {
   switch (Kind) {
   case Attribute::NoReturn:
+  case Attribute::NoCfCheck:
   case Attribute::NoUnwind:
   case Attribute::NoInline:
   case Attribute::AlwaysInline: