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POavr-gcc destructive optimizations
 

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  1. POavr-gcc destructive optimizations
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    <p>I'm programming an Atmel ATtiny13a microcontroller using avr-gcc 4.8.2.</p> <p>This is my c code:</p> <pre><code>#include &lt;avr/io.h&gt; #include &lt;util/delay.h&gt; int main(void) { DDRB = 1; // PB0 is output for (uint8_t i = 0; i &lt; 10; i++) { PORTB = 1; _delay_ms(500); PORTB = 0; _delay_ms(500); } while(1); } void test(void) { DDRB = 1; // PB0 is output for (uint8_t i = 0; i &lt; 10; i++) { PORTB = 1; _delay_ms(100); PORTB = 0; _delay_ms(100); } } </code></pre> <p>The test function (fast blinking of an LED) is never called from the main function, so the controller should only enter the main function (slow blinking).</p> <p>When I compile the code with <code>-O1</code>, everything works fine:</p> <pre><code>avr-gcc -std=gnu99 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -mmcu=attiny13 -DF_CPU=1200000 -Wall -Wstrict-prototypes -Os -c test.c -o test.o avr-gcc test.o -o test.elf avr-objcopy -O ihex -R .eeprom -R .fuse -R .lock -R .signature test.elf test.hex </code></pre> <p>But if I use <code>-Os</code> (optimization for size) or <code>-O2</code>, the microcontroller runs the <code>test</code> function instead of the <code>main</code> function: The LED blinks quickly and never stops.</p> <p>Is the <code>-Os</code> flag simply too dangerous to use, should it be avoided? Or is there something I can change in my code to avoid this kind of bug? The ATtiny13a only has 1K of flash, so size reduction is something important.</p> <hr> <p><em>Edit:</em> As suggested in the comments, here's the assembler diff with <code>-O1</code> and <code>-O2</code>: <a href="http://www.diffchecker.com/3l9cdln6" rel="nofollow">http://www.diffchecker.com/3l9cdln6</a></p> <p>In there you can see that <code>-O2</code> changes the first section from <code>.text</code> to <code>.text.startup</code>.</p> <pre><code>--- test.o1.txt 2013-12-03 19:10:43.874598682 +0100 +++ test.o2.txt 2013-12-03 19:10:50.574674155 +0100 @@ -3,7 +3,7 @@ __SREG__ = 0x3f __tmp_reg__ = 0 __zero_reg__ = 1 - .text + .section .text.startup,"ax",@progbits .global main .type main, @function main: </code></pre> <p>That's probably the main issue here. After some further testing I found that the culprit is the <code>-freorder-functions</code> optimization. Is there a way to prevent this behavior?</p>
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    1. USDanilo Bargen
    1. USDanilo Bargen
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    1. COPerhaps consider compiling with the `-S` option, to compile to assembly only. You could look at or post the assembly for `-Os` and `-O2`, maybe there would be something obvious.
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      1. USMacattack
    2. CO@Macattack: Thanks, you probaby pointed me to the right direction. See edit in original post for diff.
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      1. USDanilo Bargen
    3. COHere are some (nongnu docs)[http://www.nongnu.org/avr-libc/user-manual/mem_sections.html] mentioning `.text` but information is sparse at best. Any idea what goes on in the steps after compilation? I know zero about those build steps.
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