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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: C wish list
Date: Fri, 22 Mar 2019 13:54:06 -0700
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Bart <bc@freeuk.com> writes:
> On 22/03/2019 19:29, Keith Thompson wrote:
>> Bart <bc@freeuk.com> writes:
[...]
>>> Take my example, and say f() returns 0xFFFFFFFF_00000000 in 64-bit
>>> register R0. The bottom half is stored to x, so x contains 0.
>>>
>>> The next line is:
>>>
>>>        int32_t y = x>>8;
>>>
>>> because R0 could still contain 0xFFFFFFFF_00000000, David Brown is
>>> suggesting that a compiler might make use of that existing value, and
>>> because this processor only has 64-bit ops, it will right shift all 64
>>> bits to end up with 0x00FFFFFF_FF000000.
>>
>> I don't remember what David suggested.
>
> Read his posts today (Mar-22) at 13:26 and 15:10 GMT. He was trying to
> explain why a machine with only 64-bit registers and operations (so
> that intermediate calculations, even for int32, are necessarily
> 64-bits) might give different results.

David was discussing this example:

        int x = 1 << 31;
        int y = x / 5;

which has undefined behavior (again, assuming 32-bit int).  Your example
does not have undefined behavior.  Unless I've missed something, you are
misrepresenting what David wrote.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */