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From: "manu at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/42935] Warning "u64 = u32 * u32;" - i.e. not casting one u32 to u64
Date: Wed, 24 Feb 2010 10:23:00 -0000	[thread overview]
Message-ID: <20100224102327.27609.qmail@sourceware.org> (raw)
In-Reply-To: <bug-42935-1034@http.gcc.gnu.org/bugzilla/>



------- Comment #3 from manu at gcc dot gnu dot org  2010-02-24 10:23 -------
(In reply to comment #2)
> It would be nice to have the warning in the second case too, i.e. for
> (unsigned long long)(val1*val2).

Actually, it is a bit weird that the operands are not promoted to unsigned long
long. Normally, I would say that if you do an explicit cast, then we have to
assume that you know what you are doing. However, in this case, the result is
totally unexpected, so I think we could warn.

Also for (double) (int * int) versus ((double) int * int). This is another
subtle case. I think this is an interesting warning.

> Another solution, probably a lot more complex to implement, is to have
> a compilation switch to expand all calculus to 64 bits even when "int"
> is 32 bits, and let the optimiser convert back to simpler assembly code
> when the result used is only 32 bits.

That is independent of this warning. If you want that, open a new PR and see
what others say.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42935


  parent reply	other threads:[~2010-02-24 10:23 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-02-02 10:10 [Bug c/42935] New: " etienne_lorrain at yahoo dot fr
2010-02-24  0:17 ` [Bug c/42935] " manu at gcc dot gnu dot org
2010-02-24  9:45 ` etienne_lorrain at yahoo dot fr
2010-02-24 10:23 ` manu at gcc dot gnu dot org [this message]
2010-06-07 17:52 ` [Bug c/42935] warn if a binary operation is performed on a type but the result is then casted to a larger type manu at gcc dot gnu dot org
2010-06-07 17:53 ` manu at gcc dot gnu dot org
2010-06-07 18:05 ` manu at gcc dot gnu dot org

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