[sledge] Fix __llair_alloc

Summary:
`__llair_alloc` is meant to be a drop-in non-failing replacement for
`mallco`. Currently `__llair_alloc(1)` allocates 8 bytes instead of 1 as
`malloc(1)` would. This is because handling of `__llair_alloc` was
merged with handling of `new`. This patch reverts changes to handling of
`new` in D15778817 and adds a new case for `__llair_alloc`.

Reviewed By: jvillard

Differential Revision: D16356865

fbshipit-source-id: 3878d87c3

sledge/src/llair/frontend.ml

@@ -943,10 +943,7 @@ let xlate_instr :
       | ["__llair_throw"] ->
           let exc = xlate_value x (Llvm.operand instr 0) in
           emit_term ~prefix:(pop loc) (Llair.Term.throw ~exc ~loc)
-      | ["_Znwm" (* operator new(size_t num) *)]
+      | ["__llair_alloc" (* void* __llair_alloc(unsigned size) *)] ->
-       |["__llair_alloc" (* void* __llair_alloc(unsigned size) *)]
-       |[ "_ZnwmSt11align_val_t"
-          (* operator new(unsigned long, std::align_val_t) *) ] ->
           let reg = xlate_name instr in
           let num_operand = Llvm.operand instr 0 in
           let num =
@@ -954,6 +951,13 @@ let xlate_instr :
               (xlate_value x num_operand)
               ~src:(xlate_type x (Llvm.type_of num_operand))
           in
+          let len = Exp.integer (Z.of_int 1) Typ.siz in
+          emit_inst (Llair.Inst.alloc ~reg ~num ~len ~loc)
+      | ["_Znwm" (* operator new(size_t num) *)]
+       |[ "_ZnwmSt11align_val_t"
+          (* operator new(unsigned long, std::align_val_t) *) ] ->
+          let reg = xlate_name instr in
+          let num = xlate_value x (Llvm.operand instr 0) in
           let llt = Llvm.type_of instr in
           let len = Exp.integer (Z.of_int (size_of x llt)) Typ.siz in
           emit_inst (Llair.Inst.alloc ~reg ~num ~len ~loc)