Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #97454

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

Show all headers | View raw

On 25/11/16 09:44, Noob wrote:
> On 24/11/2016 15:59, Ben Bacarisse wrote:
> 
>> Francois Grieu 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.
>>
>>> Is the return value of foo well defined for any uint32_t input?
>>
>> No, for the reasons you go on to explain
>>
>>> In particular: when int is 64-bit, x gets promoted to int and becomes
>>> signed, and x*x can overflow the range of int; e.g. for
>>> x=(uint32_t)(-1);
>>> So, what chapter and verse (if any) saves the day?
>>
>>   uint32_t foo(uint32_t x) { return 1u * x * x; }
> 
> The standard states: "The rank of any unsigned integer type shall
> equal the rank of the corresponding signed integer type, if any."
> 
> So 'int' and 'unsigned int' have the same "rank", whatever their
> actual size?
> 
> Integer promotion is applied to "an object or expression with an
> integer type whose integer conversion rank is less than or equal
> to the rank of int and unsigned int. If an int can represent all
> values of the original type, the value is converted to an int;
> otherwise, it is converted to an unsigned int."
> 
> Assuming a perverse implementation where
> - int is 64 bits
> - unsigned int is 32 bits
> (Is that allowed?)

That is not allowed, so there is no problem :-)

6.2.5p6:

For each of the signed integer types, there is a corresponding (but
different) unsigned integer type (designated with the keyword unsigned)
that uses the same amount of storage (including sign information) and
has the same alignment requirements.

6.2.5p9:

The range of nonnegative values of a signed integer type is a subrange
of the corresponding unsigned integer type, and the representation of
the same value in each type is the same.




> 
> In such an implementation, 1u would be promoted to int for an
> arithmetic operation, right? (Because int could represent all
> values of unsigned int.)
> 
> One would have to write 1ull * x * x;
> 
> Regards.
> 

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

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

csiph-web