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Date: Thu, 18 Apr 2002 00:06:27 +0200
From: "J.A. Magallon" <jamagallon@able.es>
To: Adam Kropelin <akropel1@rochester.rr.com>
Cc: Rick Stevens <rstevens@vitalstream.com>, linux-kernel@vger.kernel.org
Subject: Re: 2.5.8-dj1 : arch/i386/kernel/smpboot.c error
Message-ID: <20020417220627.GC1728@werewolf.able.es>
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On 2002.04.17 Adam Kropelin wrote:
>Rick Stevens wrote:
>> Adam Kropelin wrote:
>>> Indeed. The optimization step that (presumably) removes the body
>>> of the if() must happen after the body has been fully evaluated.
>>> Makes sense, I guess, now that I think about it...
>> 
>> Right.  If the first condition of a logical AND statement is false,
>> the remainder need not be evaluated at all.  Hence, the entire if
>> statement can (and perhaps should) be eliminated by the compiler,
>> since the condition is false.
>> 
>> I didn't see what the actual message from the compiler was, but it
>> was probably just a warning.
>
>Thanks for the help, but it was an error, not a warning.
>
>It is caused by invalid code inside the body of the if(). The question
>is not what is causing the error but why the compiler is evaluating the
>body at all, given that the conditional is always false.
>
>Apparently the body is evaluated prior to the optimizer getting a 
>chance to nullify the whole construct.
>

AFAIK, you first parse C to build a tree of commands, expresions, etc,
and then you do the rest on the tree (optimizations at source-code level,
like CSE, ie, detection of equal subtrees, or for example
killig and if tree if it is always false).

Hope this helps.

-- 
J.A. Magallon                           #  Let the source be with you...        
mailto:jamagallon@able.es
Mandrake Linux release 8.3 (Cooker) for i586
Linux werewolf 2.4.19-pre7-jam1 #2 SMP Wed Apr 17 21:20:31 CEST 2002 i686
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