Tree generation bug or a feature?

Tree generation bug or a feature?

[Yuri Pudgorodsky]

Hi!


With the following two statements, I supposed to get identical result:

x = 2;  y = ++x * ++x;
x = 2;  y = (x+=1) * (x+=1);

However, under gcc  ++x  variant  gives me 16 instead if 12,
as well as x++ * x++ incorrectly (I suppose) gives 4.


Digital C compiler from DU 4.0 show correct values:

++x * ++x = 12
(x+=1) * (x+=1) = 12

as well as gdb builtin C expression parser.


Sounds like a tree generation bug, isn't it?


Regards,
Yuri Pudgorodsky

Re: Tree generation bug or a feature?

[Nathan Sidwell]

Yuri Pudgorodsky wrote:
> With the following two statements, I supposed to get identical result:
> 
> x = 2;  y = ++x * ++x;
> x = 2;  y = (x+=1) * (x+=1);
> 
> Sounds like a tree generation bug, isn't it?
No. you've invoked undefined effects see
http://www.eskimo.com/~scs/C-faq/s3.html

nathan
-- 
Dr Nathan Sidwell   ::   http://www.codesourcery.com   ::   CodeSourcery LLC
         'But that's a lie.' - 'Yes it is. What's your point?'
nathan@codesourcery.com : http://www.cs.bris.ac.uk/~nathan/ : nathan@acm.org

Re: Tree generation bug or a feature?

[Dima Volodin]

On Wed, 24 May 2000 16:48:47 +0400, you wrote:

>Hi!
>
>
>With the following two statements, I supposed to get identical result:
>
>x = 2;  y = ++x * ++x;
>x = 2;  y = (x+=1) * (x+=1);
>
>However, under gcc  ++x  variant  gives me 16 instead if 12,
>as well as x++ * x++ incorrectly (I suppose) gives 4.
>
>
>Digital C compiler from DU 4.0 show correct values:
>
>++x * ++x = 12
>(x+=1) * (x+=1) = 12
>
>as well as gdb builtin C expression parser.
>
>
>Sounds like a tree generation bug, isn't it?

In these code snippets, you are trying to modify x's value more than
once, which makes these snippets undefined as per section 6.3 of the
ANSI/ISO C Standard. That is you cannot expect any particular result at
all.

>Yuri Pudgorodsky

Dima