[AVR] `-mrelax` does not work for a simple LED blink program

[benshi001]

the following a.cc

// clang --target=avr -gdwarf-4 --sysroot /data/home/bennshi/.arduino15/packages/arduino/tools/avr-gcc/ a.c -mrelax -Os -Wall -mmcu=atmega328
volatile unsigned int bas[200] = {0x20, 0x00, 60000};

void delay(unsigned int a) {
        for (volatile int i = 0; i < a; i++) {}
}

int main() {
        volatile char *p0 = (char *) 0x24;
        volatile char *p1 = (char *) 0x25;
        *p0 = 0x20;
        for (;;)  {
                delay(bas[2]);
                *p1 = bas[0];
                delay(bas[2]);
                *p1 = bas[1];
        }
        return 0;
}

if it is built with -mrelax, we can not see the LED blinking, with -mno-relax it works.

[beakthoven]

I have attached the objdump diff, comparing the given program compiled with -mrelax and with -mno-relax.

$ ./cba-llvm-20.1.1-21032025/bin/clang --target=avr a.cc -mno-relax -Os -Wall -mmcu=atmega328 -o norel.elf
$ ./cba-llvm-20.1.1-21032025/bin/clang --target=avr a.cc -mrelax -Os -Wall -mmcu=atmega328 -o rel.elf
$ ./cba-llvm-20.1.1-21032025/bin/llvm-objdump --mcpu=atmega328 -d norel.elf > norelax.txt
$ ./cba-llvm-20.1.1-21032025/bin/llvm-objdump --mcpu=atmega328 -d rel.elf > relax.dump

diff-relax-norelax-dump.txt
norelax.txt
relax.txt

[beakthoven]

@benshi001 in your commit 366d34b you mentioned that

This is in accordance with avr-gcc, even '-mno-relax' is specified
to avr-gcc, this flag will also be added to the output relocatables.

But in my testing i found that:
$ avr-gcc -Os -Wall -mmcu=atmega328 a.cc -o a.o
Flags: 0x5, EF_AVR_ARCH_AVR5

$ avr-gcc -Os -mno-relax -Wall -mmcu=atmega328 a.cc -o a.o
Flags: 0x5, EF_AVR_ARCH_AVR5

$ avr-gcc -Os -mrelax -Wall -mmcu=atmega328 a.cc -o a.o
Flags: 0x85, EF_AVR_ARCH_AVR5, relaxable

Whereas with clang:
$ clang --target=avr -Os -Wall -mmcu=atmega328 a.cc -o a.o
Flags: 0x85, EF_AVR_ARCH_AVR5, relaxable

$ clang --target=avr -mno-relax -Os -Wall -mmcu=atmega328 a.cc -o a.o
Flags: 0x5, EF_AVR_ARCH_AVR5

$ clang --target=avr -mrelax -Os -Wall -mmcu=atmega328 a.cc -o a.o
Flags: 0x85, EF_AVR_ARCH_AVR5, relaxable

So relaxation support is disabled by default for gcc, can you check if your led blink program works when compiled using gcc with -mrelax

[benshi001]

Yes. avr-gcc -mrelax also works in my side.