Messages in this thread | | | Subject | RE: [PATCH] C undefined behavior fix | Date | Mon, 7 Jan 2002 23:26:01 +0100 | From | "Tim McDaniel" <> |
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-----Original Message----- From: jtv [mailto:jtv@xs4all.nl] Sent: Monday, January 07, 2002 4:16 PM To: Tim Hollebeek Cc: Bernard Dautrevaux; 'dewar@gnat.com'; paulus@samba.org; gcc@gcc.gnu.org; linux-kernel@vger.kernel.org; trini@kernel.crashing.org; velco@fadata.bg Subject: Re: [PATCH] C undefined behavior fix
On Mon, Jan 07, 2002 at 05:28:32PM -0500, Tim Hollebeek wrote: > > You're not allowed to be that smart wrt volatile. If the programmer > says the value might change unpredictably and should not be optimized, > then It Is So and the compiler must respect that even if it determines > It Cannot Possibly Happen.
Naturally I hope you're right. But how does that follow from the Standard? I have to admit I don't have a copy handy. :(
Let's say we have this simplified version of the problem:
int a = 3; { volatile int b = 10; a += b; }
Is there really language in the Standard preventing the compiler from constant-folding this code to "int a = 13;"?
Jeroen
In the above case it is unlikely that folding would present a problem, but volatile was created because hardware, or even seemingly unrelated software, can modify even the most unlikely memory locations. If you want to break device drivers, go ahead and optimize your compiler.
Tim
The only thing dummer than a cow is a man who thinks he's smarter than a cow.
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