On 31/12/12 06:54, Alex Crawford wrote:
URL:
<http://savannah.nongnu.org/patch/?7909>
Summary: Adding __volatile__ to __asm__ within pgmspace
header
Project: AVR C Runtime Library
Submitted by: crawford
Submitted on: Mon 31 Dec 2012 05:54:17 AM GMT
Category: None
Priority: 5 - Normal
Status: None
Privacy: Public
Assigned to: None
Originator Email:
Open/Closed: Open
Discussion Lock: Any
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Details:
I noticed that gcc was optimizing out calls to pgm_read_word_far() that were
within a while loop. Adding the volatile directive fixes the underlying
problem. I've added the directive to each of the LPM and ELPM calls (20 in
all).
The patch was generated from avr-libc-1.8.0.
-Alex
Could you post an example of the original C code and generated assembly
code demonstrating that there is actually a problem? I would very much
hope that the compiler will optimise out calls to read program memory
within a loop if it does not have to repeat them. Program memory is not
volatile (baring the unusual case of re-programming), and forcing the
compiler to re-read data is a waste of time and space. On the other
hand, if the compiler is making an illegal optimisation here then
perhaps this patch is just hiding a bigger problem.
mvh.,
David
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File Attachments:
-------------------------------------------------------
Date: Mon 31 Dec 2012 05:54:17 AM GMT Name: volatile.patch Size: 13kB By:
crawford
Patch for avr/pgmspace.h
<http://savannah.nongnu.org/patch/download.php?file_id=27168>
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Reply to this item at:
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