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

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Keith Thompson <kst-u@mib.org> writes:

> supercat@casperkitty.com writes:
>> On Friday, November 25, 2016 at 2:11:17 PM UTC-6, Keith Thompson wrote:
>>> supercat writes:
>>> > Implementations where every unsigned value is representable as "int" are
>>> > hardly pathological.
>>> 
>>> I won't argue the meaning of the word "pathological", but such
>>> implementations are certainly rare.
>>
>> I certainly won't dispute that.
>>
>>> >                      That would actually be a common state of affairs in
>>> > sign-magnitude or ones'-complement implementations.
>>> 
>>> I've never used a sign-and-magnitude or ones'-complement implementation,
>>> but I'd expect such an implementation to use all the bits of a word for
>>> both signed and unsigned types.  For example, if int and unsigned int
>>> are, say, 16 bits wide, I'd expect that most likely int would have a
>>> range of -32767..+32767, and unsigned int to have a range of 0..65535.
>>> Certainly there are other possibilities, including ones where the range
>>> of unsigned int matches the non-negative subrange of int, but I'd expect
>>> that to be unusual, perhaps even nonexistent.
>>> 
>>> Do you have a concrete example, or are you just speculating?
>>
>>   http://public.support.unisys.com/2200/docs/cp14.0/pdf/78310422-011.pdf
>
> Thank you for posting that.
>
>> I don't know exactly how such hardware works, but I would expect that since
>> the compiler achieves conformance by looping off a bit, that would imply that
>> having an unsigned type support all 36 bits would be difficult.
>
> (The link is to a download page, not to the actual PDF.)
>
> I think you've misread it.
>
> (Not relevant to the point, but section 4.3 treats "plain integers" and
> "signed integers" as distinct, which is nonsense.)
>
> For example, int and unsigned int are both 36 bits.  The range of int is
> shown as -2**35+1 to +2**35-1, and range of unsigned int as 0 to
> 2**36-2.  (There's a conformance option that makes the upper bound of
> unsigned int 2**36-1.)

There is another Unisys C implementation that is an example of the
problem.  It has 40 bits types and int can store all the values of
unsigned int.  In effect there is a padding bit in the unsigned types
(except for unsigned char).  I suspect this is because it uses floating
point hardware to do all arithmetic, but that's just a guess.

https://public.support.unisys.com/aseries/docs/clearpath-mcp-16.0/pdf/86002268-205.pdf

-- 
Ben.

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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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