Re: [boost] aligned_storage in unions

Subject: Re: [boost] aligned_storage in unions
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-09-22 18:34:32

On 09/22/10 17:02, vicente.botet wrote:
> ----- Original Message ----- 
> From: "Larry Evans" <cppljevans_at_[hidden]>
[snip]
>> Note: I did compile with:
>>
>>  -O3 -Wstrict-aliasing
>>
>> as Mathias suggested; however, the compiler didn't issue any
>> warnings.  I'm wondering why?
> 
> Hi,
> 
> I don't know which version are you using, but the options differ from aversion to another.
> 
> 4.1.2
> -Wstrict-aliasing
> This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. 
> 
> -Wstrict-aliasing=2
> This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. This warning catches more cases than -Wstrict-aliasing, but it will also give a warning for some ambiguous cases that are safe. 
> 
> 4.5
> -Wstrict-aliasing
> This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3 
> 
> -Wstrict-aliasing=n
> This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=n, with n=3. 
> Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the frontend only. 
> 
> Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the frontend only. 
> 
> Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the frontend: *(int*)&some_float. If optimization is enabled, it also runs in the backend, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types. 
> 
> HTH,
> 
Thanks Vicente.

I'm using gcc version 4.5.

I changed to to actually dereference the pointer.  I also added the
-fstrict-aliasing flag; however, I'm still not getting any errors.

Code and compilation output attached.

-Larry