[sledge] Add __llair_unreachable intrinsic for use in model code

Reviewed By: jvillard

Differential Revision: D25146156

fbshipit-source-id: cf48508b9

sledge/cli/frontend.ml

@@ -1112,6 +1112,7 @@ let xlate_instr :
           | ["__llair_throw"] ->
               let pre, exc = xlate_value x (Llvm.operand instr 0) in
               emit_term ~prefix:(pop loc @ pre) (Term.throw ~exc ~loc)
+          | ["__llair_unreachable"] -> emit_term Term.unreachable
           (* dropped / handled elsewhere *)
           | ["llvm"; "dbg"; ("declare" | "value")]
            |"llvm" :: ("lifetime" | "invariant") :: ("start" | "end") :: _
@@ -1180,6 +1181,7 @@ let xlate_instr :
                   xlate_jump x instr unwind_blk loc []
                 in
                 emit_term ~prefix (Term.goto ~dst ~loc) ~blocks
+            | ["__llair_unreachable"] -> emit_term Term.unreachable
             (* unimplemented *)
             | "llvm" :: "experimental" :: "gc" :: "statepoint" :: _ ->
                 todo "statepoints:@ %a" pp_llvalue instr ()

sledge/model/llair_intrinsics.h

@@ -13,9 +13,13 @@ extern "C" {
 
 __attribute__((noreturn)) void __llair_throw(void* thrown_exception);
 
-/* This models allocation that cannot fail. */
+/* express assumption that an execution is not possible */
+__attribute__((noreturn)) void __llair_unreachable();
+
+/* allocation that cannot fail. */
 void* __llair_alloc(unsigned size);
 
+/* non-deterministic choice */
 int __llair_choice();
 
 #ifdef __cplusplus