[avr-gcc-list] Bug with -flto optimization, omits check for variable if

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.

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.

From:	Bill Westfield
Subject:	[avr-gcc-list] Bug with -flto optimization, omits check for variable if weak/zero symbol is present...
Date:	Fri, 6 Jul 2018 18:10:47 -0700