[m-users.] Best way to implement constants ?

[Sean Charles (emacstheviking)]

Excellent!

I did a little experiment for myself, first this program, compiled at -O3 without the explicit command line option, using 

Mercury Compiler, version 22.01.6, on aarch64-apple-darwin22.5.0
Copyright (C) 1993-2012 The University of Melbourne
Copyright (C) 2013-2023 The Mercury team
Usage: mmc [<options>] <arguments>
Use `mmc --help' for more information.

The code:

main(!IO) :-

    RESULT = 22.0 / 7.0,
    io.format("Mercury demands focus! %f\n", [f(RESULT)], !IO).

This yields this C code:
void MR_CALL
main_2_p_0(void)
{
  {
    MR_Float RESULT_4;
    MR_Float Var_7 = (MR_Float) 22.000000000000000;
    MR_Float Var_8 = (MR_Float) 7.0000000000000000;
    MR_String Var_15;
    MR_String Var_17;
    MR_Word Var_23;
    MR_Word Var_24;
    MR_String Var_25;

    RESULT_4 = mercury__float__f_slash_2_f_0(Var_7, Var_8);

So I see that yes, it is still performing the evaluation at runtime, but with the --optimize-constant-propogation option:

void MR_CALL
main_2_p_0(void)
{
  {
    MR_Float RESULT_4 = (MR_Float) 3.1428571428571428;

So, thank you Zoltan.



> On 18 Aug 2023, at 10:48, Zoltan Somogyi <zoltan.somogyi at runbox.com> wrote:
> 
> 
> On 2023-08-18 11:37 +02:00 CEST, "Julien Fischer" <jfischer at opturion.com> wrote:
>> At -O3 and above the Mercury compiler will (generally) evaluate a
>> variety of operations (see compiler/const_prop.m for details).
>> Float division is included in those operations
> 
> Actually, constant propagation is not enabled automatically at any
> optimization level; you have to enable manually, with --optimize-constant-propagation.
> What is enabled at -O3 is *constraint* propagation, which I also
> misread as *constant* propagation at first :-(
> 
> I will add constant propagation to -O3.
> 
> Zoltan.
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