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Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os

From: Alex Wenger
Subject: Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os
Date: Fri, 16 May 2008 11:49:57 +0200
User-agent: Thunderbird (Windows/20080421)


Loveny Design schrieb:
> I wouldn't have thought the compiler was allowed to re-order statements
> *around* a volatile access. Perhaps someone can help my understanding, given
> :-
> 1: Volatile Statements<sp>
> 2: Statements<sp>
> 3: Volatile Statements<sp>
> where <sp> is a sequence point.
> Line 3 has a volatile access and therefore has side-effects that the
> compiler doesn't know about, so surely it must complete all preceeding
> statements up to and including the sequence point before the volatile access
> occurs?

the problem here is that atan() is declared as const, because
it is matematicaly only a "table look up". If you make two or
more calls to atan() with the same parameter, it could be
reduced to one.
That is essential in loops:

for(uint8_t i = 0; i < 32; i++)
        PORTB = atan(foobar) + i * 2;

could be reordered to:

tmp = atan(foobar);
for(uint8_t i = 0; i < 32; i++)
        PORTB = tmp + i * 2;


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