SafeStack: Add builtins to read unsafe stack top/bottom

Summary:
Introduce built-ins to read the unsafe stack top and bottom. The unsafe
stack top is required to implement garbage collection scanning for
Oilpan. Currently there is already a built-in 'get_unsafe_stack_start'
to read the bottom of the unsafe stack, but I chose to duplicate this
API because 'start' is ambiguous (e.g. Oilpan uses WTF::GetStackStart to
read the safe stack top.)

Reviewers: pcc

Reviewed By: pcc

Subscribers: llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D49152

llvm-svn: 337037

3 files changed, 26 insertions, 3 deletions

diff --git a/clang/docs/SafeStack.rst b/clang/docs/SafeStack.rst
index f01b75f5cb3..866a8060aa4 100644
--- a/clang/docs/SafeStack.rst
+++ b/clang/docs/SafeStack.rst
@@ -165,11 +165,23 @@ never be stored on the heap, as it would leak the location of the SafeStack.
 This builtin function returns current unsafe stack pointer of the current
 thread.
 
+``__builtin___get_unsafe_stack_bottom()``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This builtin function returns a pointer to the bottom of the unsafe stack of the
+current thread.
+
+``__builtin___get_unsafe_stack_top()``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This builtin function returns a pointer to the top of the unsafe stack of the
+current thread.
+
 ``__builtin___get_unsafe_stack_start()``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-This builtin function returns a pointer to the start of the unsafe stack of the
-current thread.
+Deprecated: This builtin function is an alias for
+``__builtin___get_unsafe_stack_bottom()``.
 
 Design
 ======
diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
index 39105a9b372..edd823754a3 100644
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -1412,6 +1412,8 @@ BUILTIN(__builtin_dump_struct, "ivC*v*", "tn")
 
 // Safestack builtins
 BUILTIN(__builtin___get_unsafe_stack_start, "v*", "Fn")
+BUILTIN(__builtin___get_unsafe_stack_bottom, "v*", "Fn")
+BUILTIN(__builtin___get_unsafe_stack_top, "v*", "Fn")
 BUILTIN(__builtin___get_unsafe_stack_ptr, "v*", "Fn")
 
 // Nontemporal loads/stores builtins
diff --git a/compiler-rt/lib/safestack/safestack.cc b/compiler-rt/lib/safestack/safestack.cc
index 8b1fdb788fe..cc6d81b2eef 100644
--- a/compiler-rt/lib/safestack/safestack.cc
+++ b/compiler-rt/lib/safestack/safestack.cc
@@ -257,11 +257,20 @@ __attribute__((section(".preinit_array"),
 #endif
 
 extern "C"
-    __attribute__((visibility("default"))) void *__get_unsafe_stack_start() {
+    __attribute__((visibility("default"))) void *__get_unsafe_stack_bottom() {
   return unsafe_stack_start;
 }
 
 extern "C"
+    __attribute__((visibility("default"))) void *__get_unsafe_stack_top() {
+  return (char*)unsafe_stack_start + unsafe_stack_size;
+}
+
+extern "C"
+    __attribute__((visibility("default"), alias("__get_unsafe_stack_bottom")))
+    void *__get_unsafe_stack_start();
+
+extern "C"
     __attribute__((visibility("default"))) void *__get_unsafe_stack_ptr() {
   return __safestack_unsafe_stack_ptr;
 }