Original bug ID: 7531
Reporter: jmi
Assigned to: @mshinwell
Status: resolved (set by @mshinwell on 2017-05-08T11:17:10Z)
Resolution: duplicate
Priority: normal
Severity: minor
Version: 4.04.0
Category: middle end (typedtree to clambda)
Related to: #7533

Bug description

The native code compiler erroneously delays the effect of an operator until (if ever) it is fully applied.

The following program outputs a newline when compiled with the bytecode compiler but not with the native code compiler:

(let _i = print_newline ()                                                                                    
 in fun q -> fun i -> "") ()

I believe the issue is described by the following comment on line 828 of asmcomp/closure.ml:

(* We convert [f a] to [let a' = a in fun b c -> f a' b c]
when fun_arity > nargs *)

which instead should use an approach along the lines of:

(* We convert [f a] to [let f’ = f in
let a’ = a in fun b c -> f’ a’ b c]
when fun_arity > nargs *)

The above example illustrates the situation when the effect is delayed indefinitely. For an example, where the effect is delayed until the time of full application, consider:

let k =                                                                                                       
  (let _i = print_int 1                                                                                       
   in fun q -> fun i -> "") ()                                                                                
in k (print_int 0)

which prints 10 when compiled with the bytecode backend and 01 with the native code backend.

The behaviour is the same for version 4.02.3.

Steps to reproduce

$ ocamlc -o func.byte func.ml
$ ./func.byte

$ ocamlopt -o func.native func.ml
$ ./func.native
$