[Bug tree-optimization/43089] Optimizer ignores type in a conversion

[Andrew Pinski]

Sent from my iPhone

On Feb 16, 2010, at 9:37 AM, "0xe2 dot 0x9a dot 0x9b at gmail dot com"  
<gcc-bugzilla@gcc.gnu.org> wrote:

>
>
> ------- Comment #5 from 0xe2 dot 0x9a dot 0x9b at gmail dot com   
> 2010-02-16 17:37 -------
> (In reply to comment #4)
>> There is nothing to fix.  Your program triggers undefined  
>> behavior.  It can do
>> anything, which can include something you'd expect, or something  
>> completely
>> different and it can depend on compiler options, position of stars,  
>> etc.
>
> I understand what you are saying, but I do not agree with that. I my  
> opinion,
> an *optimization* option should never result in any change of a  
> program's
> behavior for this particular kind of undefined behaviors. I mean,  
> there are
> basically two different kinds of undefined behaviors:
>
> 1. Where the compiler has to choose a *particular* implementation.

Huh, this is the opposite effect of undefined behavior. In fact for  
signed interger overflow, gcc sometimes optimizes it as wrapping and  
others as clamping. In this case it is clamping. It is hard sometimes  
to optimize constiently undefined behavior because of inlining and  
other optimizations that can change the ir before the optimization of  
undefined behavior.

>
> 2. Where the compiler does not choose anything or cannot choose  
> anything
> particular. (For example, what happens if accessing deallocated  
> memory.)
>
> The conversion test-case is of the 1st kind. Not of the 2nd kind.  
> GCC -O0
> chooses to generate a particular sequence of instructions to  
> implement the
> undefined behavior. GCC -O2 does not respect the choice made at -O0  
> (or vice
> versa).
>
> So, my question is: If it is possible for the problematic code to be
> implemented in all contexts by the same operation, and in this case  
> it indeed
> is possible, why is GCC using two different operations? How do you  
> justify
> that?
>
>
> -- 
>
> 0xe2 dot 0x9a dot 0x9b at gmail dot com changed:
>
>           What    |Removed                     |Added
> --- 
> --- 
> ----------------------------------------------------------------------
>             Status|RESOLVED                    |UNCONFIRMED
>         Resolution|INVALID                     |
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43089
>