[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.

[tomtor]

A major difference is that clang invokes ld directly and gcc calls collect2

 /home/tom/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../libexec/gcc/avr/7.3.0/c
ollect2 -plugin /home/tom/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../libexec/
gcc/avr/7.3.0/liblto_plugin.so -plugin-opt=/home/tom/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6
.1-arduino7/bin/../libexec/gcc/avr/7.3.0/lto-wrapper -plugin-opt=-fresolution=/tmp/cc0LhxuA.res -plugin-opt=-
pass-through=-lgcc -plugin-opt=-pass-through=-lm -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lat
tiny1614 -mavrxmega3 -Tdata 0x803800 --relax -o gcc.out /home/tom/.arduino15/packages/arduino/tools/avr-gcc/7
.3.0-atmel3.6.1-arduino7/bin/../lib/gcc/avr/7.3.0/../../../../avr/lib/avrxmega3/crtattiny1614.o -L/home/tom/s
rc/rust/avr-gcc-build/output/avr-gcc-14.1.0-x64-linux/lib/gcc/avr/14.1.0/avrxmega3/double64 -L/home/tom/.ardu
ino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../lib/gcc/avr/7.3.0/avrxmega3 -L/home/tom
/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../lib/gcc/avr/7.3.0/../../../../avr
/lib/avrxmega3 -L/home/tom/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../lib/gcc
/avr/7.3.0 -L/home/tom/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../lib/gcc -L/
home/tom/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/../lib/gcc/avr/7.3.0/../../.
./../avr/lib /tmp/cchILy8w.o /tmp/ccBcg6vT.o --start-group -lgcc -lm -lc -lattiny1614 --end-group

[tomtor]

In the clang result a 4 byte call instruction is replaced by a 2 byte rcall, but a backward jump is not adjusted:

00000140 <bug>:
 140:   0f 93           push    r16
 142:   1f 93           push    r17
 144:   80 e0           ldi     r24, 0x00       ; 0
 146:   90 e0           ldi     r25, 0x00       ; 0
 148:   0f 9a           sbi     0x01, 7 ; 1
 14a:   01 96           adiw    r24, 0x01       ; 1
 14c:   8c 01           movw    r16, r24
 14e:   8f 70           andi    r24, 0x0F       ; 15
 150:   90 70           andi    r25, 0x00       ; 0
 152:   01 96           adiw    r24, 0x01       ; 1
 154:   0f d0           rcall   .+30            ; 0x174 <nil>
 156:   c8 01           movw    r24, r16
 158:   f6 cf           rjmp    .-20            ; 0x146 <.Lname46>

The original assembly jump is to the sbi instruction and the rcall was a 4 byte call.

In the gcc result the backward local jump is adjusted.

[tomtor]

This is the root cause:

tom@swan-ubu20:~/src/rust/arduino-hal-test/bug$ avr-objdump -r gcc-bug.o

gcc-bug.o:     file format elf32-avr

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE
00000010 R_AVR_CALL        nil
00000014 R_AVR_13_PCREL    .text+0x00000004


tom@swan-ubu20:~/src/rust/arduino-hal-test/bug$ avr-objdump -r bug.o

bug.o:     file format elf32-avr

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE
00000014 R_AVR_CALL        nil

Gcc emits a relocation record for the backward branch, clang does not

[benshi001]

I need to reconsider @Patryk27's original suggestion, as discussed at #145291 (comment).

[benshi001]

It is strange that the above demo program works even -mrelax specified. How strange it is! Now the LED is blinking with different interval values set !

I just update to the latest llvm main branch, but without #145291 applied.