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

From: Paulo Marques
Subject: Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os
Date: Thu, 15 May 2008 18:03:41 +0100
User-agent: Thunderbird (X11/20071210)

Thomas D. Dean wrote:
From 'info gcc',

   You can prevent an `asm' instruction from being deleted, moved
significantly, or combined, by writing the keyword `volatile' after the

This only means that they are not reordered with respect to other "volatile asm" statements. Note the "moved significantly" part: if the other instruction is not "significant", then it might be moved over it.

Yes, I do agree that the outcome might be somewhat surprising, though.

Changing the code fragment to
  line3[20] = ' ';
  line4[20] = ' ';
asm volatile("sbi 0x1b,4" : :); atan_rad = atan2(cos_rad,sin_rad); asm volatile("cbi 0x1b,4" : :); dtostrf(atan_rad,6,3,&line4[26]);

does not alter the result.

The PORTx accessors are already volatile, so this doesn't change much.

The main problem here is probably that the atan2 function is declared to be __attribute__((const)), which gives teh compiler more leeway to move it around...

Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

"Power corrupts. Absolute power is kind of neat." John Lehman

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