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

From: Blake Leverett
Subject: Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os
Date: Thu, 15 May 2008 12:02:14 -0600
User-agent: KMail/1.9.7

On Thursday 15 May 2008, Thomas D. Dean wrote:
> I changed the code to
>    asm volatile("sbi 0x14, 4"::);
>    atn_rad = atan2(cos_rad,sin_rad);
>    asm volatile("cbi 0x14, 4"::);
> and the compiler still reordered the statements to put both the sbi
> and cbi statements before the atan().
> Changing atan2() to asin() or acos() gives the same incorrect result.
> Changing atan2() to sin() keeps the statements in the correct order.
> tomdean

If you declare atn_rad as a volatile, won't that make the order work out 

As someone already mentioned, the port accesses are already volatile, so you 
should be able to use the C statments for port pin changing, and with a 
volatile atn_rad, the compiler should keep everything in the source order.  
Or else I misunderstand how volatile works.


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