[sledge][llvm] Handle nullptr in Llvm.global_initializer

Summary:
LLVMGetInitializer returns nullptr in case there is no
initializer. There is not much that can be done with nullptr in OCaml,
not even test if it is null. Also, there does not seem to be a C or
OCaml API to test if there is an initializer. So this diff changes
Llvm.global_initializer to return an option.

Reviewed By: jvillard

Differential Revision: D27188302

fbshipit-source-id: 3474ec840

sledge/vendor/llvm-dune/llvm-project/llvm/bindings/ocaml/llvm/llvm.ml

@@ -719,7 +719,7 @@ external define_qualified_global : string -> llvalue -> int -> llmodule ->
 external lookup_global : string -> llmodule -> llvalue option
                        = "llvm_lookup_global"
 external delete_global : llvalue -> unit = "llvm_delete_global"
-external global_initializer : llvalue -> llvalue = "LLVMGetInitializer"
+external global_initializer : llvalue -> llvalue option = "llvm_global_initializer"
 external set_initializer : llvalue -> llvalue -> unit = "llvm_set_initializer"
 external remove_initializer : llvalue -> unit = "llvm_remove_initializer"
 external is_thread_local : llvalue -> bool = "llvm_is_thread_local"

sledge/vendor/llvm-dune/llvm-project/llvm/bindings/ocaml/llvm/llvm.mli

@@ -1487,7 +1487,7 @@ val set_global_constant : bool -> llvalue -> unit
 
 (** [global_initializer gv] returns the initializer for the global variable
     [gv]. See the method [llvm::GlobalVariable::getInitializer]. *)
-val global_initializer : llvalue -> llvalue
+val global_initializer : llvalue -> llvalue option
 
 (** [set_initializer c gv] sets the initializer for the global variable
     [gv] to the constant [c].

sledge/vendor/llvm-dune/llvm-project/llvm/bindings/ocaml/llvm/llvm_ocaml.c

@@ -1382,6 +1382,18 @@ CAMLprim value llvm_delete_global(LLVMValueRef GlobalVar) {
   return Val_unit;
 }
 
+/* llvalue -> llvalue option */
+CAMLprim value llvm_global_initializer(LLVMValueRef GlobalVar) {
+  CAMLparam0();
+  LLVMValueRef Init;
+  if ((Init = LLVMGetInitializer(GlobalVar))) {
+    value Option = alloc(1, 0);
+    Field(Option, 0) = (value) Init;
+    CAMLreturn(Option);
+  }
+  CAMLreturn(Val_int(0));
+}
+
 /* llvalue -> llvalue -> unit */
 CAMLprim value llvm_set_initializer(LLVMValueRef ConstantVal,
                                     LLVMValueRef GlobalVar) {