Re: [avr-gcc-list] Bug with -flto optimization, omits check for variable if weak/zero symbol is present...

[Senthil Kumar Selvaraj]

j
Bill Westfield writes:

> Discovered on Arduino (4.9.2-atmel3.5.4) (
> http://forum.arduino.cc/index.php?topic=556674.0 )
> So I have this pretty trivial avr-gcc program:
>
> #include <avr/io.h>
>
> void serialEvent() __attribute__((weak));
> void loop();
>
> void serialEventRun(void)
> {
> #ifndef OMITBUG
>   if (serialEvent) serialEvent();
> #endif
> }
>
> int main() {
>     for (;;) {
>     loop();
>     if (serialEventRun) serialEventRun();
>     }
> }
>
> int nnn = 1;
>
> void loop()
> {
>   if (nnn <= 3) {
>     char rrr = 0;
>     while ( !rrr ) {
>       if (PORTB)
>         rrr = (PORTB == 'c' );
>     }
>     PORTB |= 1 << 5;
>     nnn++;
>   } else while (1);
> }
>
> If compile with "avr-gcc -g -Os -flto -mmcu=atmega328p bug.c -o bug.elf" it
> will produce code that omits the test for (nnn <= 3)
> Turning off -flto, or adding -DOMITBUG to the compile line (removing the
> call to the non-existent serialEvent() function) results in correct code.

Can repro this on atmel3.6.1 and gcc-5-branch as well with just
-fwhole-program. Bug goes away on gcc-6-branch and trunk [1]. Will try
to isolate fix and see if it can be backported.

[1] 
http://avr-gcc.senthilthecoder.com/#g:!((g:!((g:!((h:codeEditor,i:(j:1,options:(colouriseAsm:'0',compileOnChange:'0'),source:'%23include+%3Cavr/io.h%3E%0A%0Avoid+serialEvent()+__attribute__((weak))%3B%0Avoid+loop()%3B%0A%0Avoid+serialEventRun(void)%0A%7B%0A%23ifndef+OMITBUG%0A++if+(serialEvent)+serialEvent()%3B%0A%23endif%0A%7D%0A%0Aint+main()+%7B%0A++++for+(%3B%3B)+%7B%0A%09loop()%3B%0A%09if+(serialEventRun)+serialEventRun()%3B%0A++++%7D%0A%7D%0A%0Aint+nnn+%3D+1%3B%0A%0Avoid+loop()%0A%7B%0A++if+(nnn+%3C%3D+3)+%7B%0A++++char+rrr+%3D+0%3B%0A++++while+(+!!rrr+)+%7B%0A++++++if+(PORTB)%0A++++++++rrr+%3D+(PORTB+%3D%3D+!'c!'+)%3B%0A++++%7D%0A++++PORTB+%7C%3D+1+%3C%3C+5%3B%0A++++nnn%2B%2B%3B%0A++%7D+else+while+(1)%3B%0A%7D%0A'),l:'5',n:'1',o:'C+source+%231',t:'0')),k:33.333333333333336,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:trunk,filters:(b:'0',commentOnly:'0',intel:'0'),options:'-Os+-mmcu%3Datmega328p+-fwhole-program+-v'),l:'5',n:'0',o:'%231+with+trunk',t:'0')),k:33.333333333333336,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:output,i:(compiler:1,editor:1),l:'5',n:'0',o:'%231+with+trunk',t:'0')),k:33.33333333333333,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4

Regards
Senthil

>
> Discovered on Arduino (4.9.2-atmel3.5.4) (
> http://forum.arduino.cc/index.php?topic=556674.0 )
>
> Also fails with avr-gcc (AVR_8_bit_GNU_Toolchain_3.6.1_496) 5.4.0 and
> avr-gcc (AVR_8_bit_GNU_Toolchain_3.6.1_1750) 5.4.0
> With avr-gcc 8.1, it produce an additional error that may or may not be
> relevant:
>
> C:\Users\billw\Documents\src>\bin\avr-gcc-8.1.0-x86-mingw\bin\avr-gcc -g
> -Os -flto -mmcu=atmega328p bug.c -o bug.elf
> C:\Users\billw\AppData\Local\Temp\ccE52smMdebugobjtem: file not recognized:
> File truncated
> collect2.exe: error: ld returned 1 exit status
> lto-wrapper.exe: fatal error: \bin\avr-gcc-8.1.0-x86-mingw\bin\avr-gcc
> returned 1 exit status
> compilation terminated.
> c:/bin/avr-gcc-8.1.0-x86-mingw/bin/../lib/gcc/avr/8.1.0/../../../../avr/bin/ld.exe:
> error: lto-wrapper failed
> collect2.exe: error: ld returned 1 exit status
>
> (8.1 files in ld even with -DOMITBUG, though slightly differenty.  It works
> without -flto)
>
> Um.  Wild guess:  The   if (serialEvent) serialEvent(); line generates
> slightly odd code, including a call to an absolute address of 0x0.
> Is there something in link that uses a 0x0 address as an "end of list"
> indicator?
>   ba:   20 97           sbiw    r28, 0x00       ; 0
>   bc:   71 f3           breq    .-36            ; 0x9a <main+0x4>
>   be:   0e 94 00 00     call    0       ; 0x0 <__vectors>
>
>
> -save-temp data attached for both working and fail cases.
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