_MMCU_ instructions not working (avr-g++)?

[larsks]

I'm using the [Arduino-Makefile][] project to build a simple program for an attiny85. I wanted to try running this via simavr. I tried adding the following to my code:

const struct avr_mmcu_vcd_trace_t _mytrace[]  _MMCU_ = {
  { AVR_MCU_VCD_SYMBOL("PORTB"), .what = (void*)&PORTB, },
};

When trying to compile the code, I get:

sample.ino:18:1: error: C99 designator 'name' outside aggregate initializer
 };
 ^
sample.ino:18:1: sorry, unimplemented: non-trivial designated initializers not supported

The compile command line looks like this:

avr-g++ -x c++ -include Arduino.h -MMD -c -D__PROG_TYPES_COMPAT__ -I$HOME/src/simavr/ -mmcu=attiny85 -DF_CPU=8000000L -DARDUINO=185 -DARDUINO_ARCH_AVR -I$HOME/lib/arduino/hardware/arduino/avr/cores/arduino -I$HOME/.arduino15/packages/attiny/hardware/avr/1.0.2/variants/tiny8     -I$HOME/Arduino/libraries/DallasTemperature   -I$HOME/Arduino/libraries/OneWire -Wall -ffunction-sections -fdata-sections -Os -fpermissive -fno-exceptions -std=gnu++11 -fno-threadsafe-statics -flto -fno-devirtualize -fdiagnostics-color=always sample.ino -o build-ATtinyX5-attiny85/sample.ino.o

Will the tracing feature work with avg-g++?

[larsks]

$ avr-g++ --version
avr-g++ (GCC) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[hovercraft-github]

@larsks AFAIK, you have to place any MMCU sections into .c file and compile it with avr-gcc.
This is related to c++ structure/class initialization limitations.
Also you can have c++ files in your project too. Preferably, link everything using avr-g++ and remember to add the following linker option: -Wl,--undefined=_mmcu,--section-start=.mmcu=0x910000

[larsks]

Thanks.

So, putting the simavr directives into a .c file (metadata.c), it gets compiled (successfully!) into an object file like this:

avr-gcc -MMD -c -D__PROG_TYPES_COMPAT__ \
  [...a bunch of -I directives...]
  -mmcu=attiny85 -DF_CPU=8000000L -DARDUINO=184 -DARDUINO_ARCH_AVR \
  -Wall -ffunction-sections -fdata-sections \
  -Os -std=gnu11 -flto -fno-fat-lto-objects \
  -fdiagnostics-color=always metadata.c \
  -o build-ATtinyX5-attiny85/metadata.c.o

But the resulting object file has no .mmcu section:

$ objdump -h build-ATtinyX5-attiny85/metadata.c.o
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000000  00000000  00000000  00000034  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .data         00000000  00000000  00000000  00000034  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  00000000  00000000  00000034  2**0
                  ALLOC
  3 .gnu.lto_.profile.45131e2142b701f9 0000000f  00000000  00000000  00000034  2**0
                  CONTENTS, READONLY, EXCLUDE
  4 .gnu.lto_.inline.45131e2142b701f9 0000000e  00000000  00000000  00000043  2**0
                  CONTENTS, READONLY, EXCLUDE
  5 .gnu.lto_.pureconst.45131e2142b701f9 0000000f  00000000  00000000  00000051  2**0
                  CONTENTS, READONLY, EXCLUDE
  6 .gnu.lto_.symbol_nodes.45131e2142b701f9 00000017  00000000  00000000  00000060  2**0
                  CONTENTS, READONLY, EXCLUDE
  7 .gnu.lto_.refs.45131e2142b701f9 0000000f  00000000  00000000  00000077  2**0
                  CONTENTS, READONLY, EXCLUDE
  8 .gnu.lto_.decls.45131e2142b701f9 00000477  00000000  00000000  00000086  2**0
                  CONTENTS, READONLY, EXCLUDE
  9 .gnu.lto_.symtab.45131e2142b701f9 00000018  00000000  00000000  000004fd  2**0
                  CONTENTS, READONLY, EXCLUDE
 10 .gnu.lto_.opts 000000a3  00000000  00000000  00000515  2**0
                  CONTENTS, READONLY, EXCLUDE
 11 .comment      00000012  00000000  00000000  000005b8  2**0
                  CONTENTS, READONLY

And metadata.c is as above:

#include <Arduino.h>
#include <avr_mcu_section.h>

const struct avr_mmcu_vcd_trace_t _mytrace[]  _MMCU_ = {
    { AVR_MCU_VCD_SYMBOL("PORTB"), .what = (void*)&PORTB, },
};

Should I expect to see the .mmcu section in the object file?

[hovercraft-github]

The .mmcu section should present in the object dump indeed.
Just remove those link time optimisation switches - it's totally inapplicable in this case, as .mmcu section doesn't occupy a processor memory. Lto drops it as junk.

[larsks]

That fixed the .o file...

...but it looks as if Arduino-Makefile links the final .elf file with -Wl,--gc-sections, which of course dropped the .mmcu section from the final object. I've modified that as well, and now I'm able to generate traces as expected.

Thanks for your help!

[msquirogac]

Sorry to bring this back, but these are some of my observations.
Certainly, as @hovercraft-github suggested, the flto flag drops all the unused symbols and a possible workaround is to remove said flag.
However, I think a better solution would be to force the compiler/linker to add the symbols by adding the used attribute, like explained here
That way we can be sure that the symbols are always included even if the flto flag is used.
I've tested it and the .mmcu section doesn't get dropped anymore.