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From: Bernd Linsel <bl1-thispartdoesnotbelonghere@gmx.com>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Sat, 14 Sep 2024 22:18:12 +0200
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On 14.09.24 21:57, Kent Dickey wrote:
> In article <vc4mgj$1khmk$1@dont-email.me>,
> Thomas Koenig  <tkoenig@netcologne.de> wrote:
>> Kent Dickey <kegs@provalid.com> schrieb:
>>
>>> When you write code working on signed numbers and do something like:
>>>
>>> 	(a < 0) || (a >= max)
>>>
>>> Then the compiler realizes if you treat 'a' as unsigned, this is just:
>>>
>>> 	(unsigned)a >= max
>>
>> For which definition of a and max exactly?
>>
>> It coertainly does not do so for
>>
>> _Bool foo(int a, int max)
>> {
>>     return (a < 0) || (a >= max);
>> }
> 
> Sorry, I should have made it clear for max >= 0 (but not necessarily an
> unsigned variable), and for my code, a constant, which is how the
> compiler knows it's positive .  I have this in my code all the time to
> validate function inputs--a negative number is bad, and a number beyond
> a certain reasonable value is bad.  And I let the compiler optimize the
> check to (unsigned)a >= (unsigned)max.
> 
> Kent

And that's the information the compiler was missing to optimize foo() in 
the same way:

_Bool foo1(int a, int max)
{
     if (__builtin_expect(max < 0, 0)) __builtin_unreachable();

     return a < 0 || a >= max;
}


_Bool foo2(int a, int max)
{
     return (unsigned)a >= (unsigned)max;
}

compiles to:

foo1:
         cmp     edi, esi
         setnb   al
         ret
foo2:
         cmp     edi, esi
         setnb   al
         ret

(x64-64-gcc 14.2 -Wall -Wextra -Wpedantic -O3 -fexpensive-optimizations)

-- 
Bernd Linsel