[sledge] Protect against misdeclarations of operator new

Summary:
The frontend would implicitly assume there was (at least) one argument
to calls to operator new. If code declares operator new with the wrong
type, this can lead to crashing trying to access a missing arg.

Reviewed By: mbouaziz

Differential Revision: D15098820

fbshipit-source-id: 539281a83

sledge/src/llair/frontend.ml

@@ -1000,15 +1000,15 @@ let xlate_instr :
       let return_dst = label_of_block return_blk in
       let unwind_blk = Llvm.get_unwind_dest instr in
       let unwind_dst = label_of_block unwind_blk in
+      let num_args =
+        if not (Llvm.is_var_arg (Llvm.element_type lltyp)) then
+          Llvm.num_arg_operands instr
+        else (
+          warn "ignoring variable arguments to variadic function: %a"
+            pp_llvalue instr ;
+          Array.length (Llvm.param_types (Llvm.element_type lltyp)) )
+      in
       let args =
-        let num_args =
-          if not (Llvm.is_var_arg (Llvm.element_type lltyp)) then
-            Llvm.num_arg_operands instr
-          else (
-            warn "ignoring variable arguments to variadic function: %a"
-              pp_llvalue instr ;
-            Array.length (Llvm.param_types (Llvm.element_type lltyp)) )
-        in
         List.rev_init num_args ~f:(fun i ->
             xlate_value x (Llvm.operand instr i) )
       in
@@ -1021,7 +1021,7 @@ let xlate_instr :
       | ["__llair_throw"] ->
           let dst = Llair.Jump.mk unwind_dst args in
           emit_term (Llair.Term.goto ~dst ~loc)
-      | ["_Znwm" (* operator new(size_t num) *)] ->
+      | ["_Znwm" (* operator new(size_t num) *)] when num_args = 1 ->
           let reg = xlate_name instr in
           let num = xlate_value x (Llvm.operand instr 0) in
           let llt = Llvm.type_of instr in