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Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable?

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Barry Schwarz <schwarzb@dqel.com> writes:

> On Thu, 01 Dec 2016 10:16:00 -0800, Tim Rentsch
> <txr@alumni.caltech.edu> wrote:
>
>> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>>
>>> Francois Grieu <fgrieu@free.fr> writes:
>>>
>>>> Assume this code, under C99 or later
>>>>
>>>> #include <stdint.h>
>>>> uint32_t foo(uint32_t x) { return (uint32_t)(x*x); }
>>>
>>> The cast is pointless.  The returned value is always converted to the
>>> return type of the function.
>>
>> I agree the cast here is superfluous, but the second sentence is
>> not quite right.  The value of the return expression is converted
>> to the return type of the function /except/ in the case where the
>> type of the return expression is the same as the return type of
>> the function.  For most types this makes no difference, but it
>> can make a difference for floating point types.
>
> Could you please elaborate.  If both the expression and return value
> have type T, isn't any such conversion a no-op?

It is fairly common for floating point operations to be carried
out in higher precision or range than the types of the operands
would imply.  The rule in C is that any higher precision/range
may persist until a result value is converted to a specific type.
Thus if we see, for example,

    double a, b, c;
    ...
    a = b+c;

the result of 'b+c' might have more precision than double does.
When the result is assigned to a (or cast to double) then any
extra precision/range must be discarded.  (Assignment operators
_always_ do a conversion, even if the types are the same.)  The
point of the rule for 'return' is that a statement like

    return  a*b+c;

need not discard any extra range/precision that may have come
about due to the operations performed (provided of course that
the type of the return expression matches the type of the
function).

Personally I think these rules are a bit screwy, but people
better informed about floating point than I am think they
make sense, so take that for what it's worth.

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Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Francois Grieu <fgrieu@free.fr> - 2016-11-24 15:33 +0100
  Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-24 14:59 +0000
    Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Francois Grieu <fgrieu@free.fr> - 2016-11-24 18:34 +0100
      Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-11-24 14:03 -0800
      Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? David Brown <david.brown@hesbynett.no> - 2016-11-25 09:50 +0100
    Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Noob <root@127.0.0.1> - 2016-11-25 09:44 +0100
      Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? David Brown <david.brown@hesbynett.no> - 2016-11-25 11:14 +0100
        Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-25 11:12 +0000
          Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-11-25 10:50 -0800
            Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Keith Thompson <kst-u@mib.org> - 2016-11-25 12:12 -0800
              Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Nick Bowler <nbowler@draconx.ca> - 2016-11-25 21:45 +0000
              Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-11-25 13:58 -0800
                Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Keith Thompson <kst-u@mib.org> - 2016-11-28 23:37 -0800
                Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-29 12:01 +0000
                Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-11-29 07:39 -0800
                Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Keith Thompson <kst-u@mib.org> - 2016-11-29 09:12 -0800
              Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-25 22:54 +0000
          Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Nick Bowler <nbowler@draconx.ca> - 2016-11-25 22:01 +0000
            Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-25 23:22 +0000
              Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Tim Rentsch <txr@alumni.caltech.edu> - 2016-12-10 07:22 -0800
    Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Tim Rentsch <txr@alumni.caltech.edu> - 2016-12-01 10:16 -0800
      Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Barry Schwarz <schwarzb@dqel.com> - 2016-12-01 11:06 -0800
        Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-12-01 12:40 -0800
        Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? David Brown <david.brown@hesbynett.no> - 2016-12-02 09:28 +0100
        Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? Tim Rentsch <txr@alumni.caltech.edu> - 2016-12-10 07:13 -0800
          Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-12-10 10:41 -0800
  Re: Is uint32_t foo(uint32_t x) { return (uint32_t)(x*x); } portable? supercat@casperkitty.com - 2016-11-24 07:40 -0800

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