[Bug c/43162] New: option to set the "promoted" type of parameters of calculus

[Bug c/43162] New: option to set the "promoted" type of parameters of calculus

[etienne_lorrain at yahoo dot fr]

In C, variables are promoted to unsigned int or int before operations like
adding or multiplying so that we have on ia32 PC:
 unsigned char uc1 = 0x10, uc2 = 0x10;
 printf ("0x%X\n", uc1*uc2); -> display 0x100
 unsigned short us1 = 0x1000, us2 = 0x10;
 printf ("0x%X\n", us1*us2); -> display 0x10000
That doesn't work for unsigned long long, by design we have:
 unsigned u1 = 0x10000000, u2 = 0x10;
 printf ("0x%llX\n", (unsigned long long)(u1*u2)); -> display 0x0
It "would be nice" to be able to set the size to promote operands before
operations to either int/unsigned (as now) or long long/unsigned long long
to have the intended result on lines like:
 unsigned long long ull = u1 * u2; // with previous values of u1 and u2...
Nothing else should be changed by this option, default parameter size
of printf() should still be int/unsigned, so that a line like:
 unsigned u3 = u1 * u2;
would be optimised to the same assembly instructions whatever the value
of the switch.


-- 
           Summary: option to set the "promoted" type of parameters of
                    calculus
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: etienne_lorrain at yahoo dot fr
 GCC build triplet: any
  GCC host triplet: any
GCC target triplet: any


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43162

[Bug c/43162] option to set the "promoted" type of parameters of arithmetic

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2010-02-24 11:33 -------
I think this is the wrong approach really.  Learning C/C++ rules for arithmetic
operations and the promotion of types is not hard.

Also this is not calculus but simple arithmetic operations :).


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|option to set the "promoted"|option to set the "promoted"
                   |type of parameters of       |type of parameters of
                   |calculus                    |arithmetic


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43162

[Bug c/43162] option to set the "promoted" type of parameters of arithmetic

[bangerth at gmail dot com]

------- Comment #2 from bangerth at gmail dot com  2010-02-25 05:12 -------
I don't think we should be doing this. GCC strives to be standards-conforming
and the requested feature would purposefully make us violate the standard.

There is a point for extensions, but I don't think changing the meaning
of a perfectly well-formed program is not what we should be doing. The
same effect can of course be achieved by simply adding the explicit cast
to the code.

W.


-- 

bangerth at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at gmail dot com
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43162