Using RTClib and Time libraries together produces garbage dates due to gcc's const data merging

[hlovdal]

I initially intended to have the title "g++ incorrectly merges PROGMEM and non-PROGMEM data", but opted to focus on the consequences since I think this is a really bad show stopper bug.

The TL;DR version is that because library file RTClib.cpp contains (using PROGMEM)

const uint8_t daysInMonth [] PROGMEM = { 31,28,31,30,31,30,31,31,30,31,30,31 };

and library file Time.cpp contains (not using PROGMEM)

static  const uint8_t monthDays[]={31,28,31,30,31,30,31,31,30,31,30,31}; // API starts months from 1, this array starts from 0

the compiler incorrectly merges them, but due to the micro-controller's modified Harvard architecture they are stored in different memory types and cannot be accessed the same way. Attempts to read one of them will result in garbage data.

---

Initially I debugged the problem by adding some Serial.print to Time.cpp and RTClib.cpp and discovered that in Time.cpp the number of days in a month were garbage values comming from its monthDays array.

I managed to reproduce similar behaviour in a standalone sketch, without requiering modifications of libraries.

#include <Arduino.h>
#include <avr/pgmspace.h>
// #include <Time.h>

// OK to merge
const uint8_t one_to_twelve1[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
const uint8_t one_to_twelve2[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};

// OK to merge
const char abcd1[] PROGMEM = {'a', 'b', 'c', 'd', 'a', 'b', 'c', 'd', 'a', 'b', 'c', 'd'};
const char abcd2[] PROGMEM = {'a', 'b', 'c', 'd', 'a', 'b', 'c', 'd', 'a', 'b', 'c', 'd'};

// NOT OK to merge
const uint8_t month_days1[] PROGMEM = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const uint8_t month_days2[]         = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

// NOT OK to merge
const uint8_t primes1[]         = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37};
const uint8_t primes2[] PROGMEM = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37};

#define PRINT(var) \
    do { \
        Serial.print(#var); \
        Serial.print(" = "); \
        Serial.println(var); \
    } while (0)
static void print_vars() {
    PRINT(one_to_twelve1[0]);
    PRINT(one_to_twelve2[0]);

    PRINT(pgm_read_byte(&abcd1[0]));
    PRINT(pgm_read_byte(&abcd2[0]));

    PRINT(pgm_read_byte(&month_days1[0]));
    PRINT(month_days2[0]);

    PRINT(primes1[0]);
    PRINT(pgm_read_byte(&primes2[0]));
}

const uint8_t monthDays[] = {31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31};

const uint8_t monthDays2[] PROGMEM = {
//  'a', 'b', 'c', 'd'   // OK
//  31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30          // OK
//  31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31      // Fails
//  31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 32  // OK
    31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31 // Fails also when changing Time.cpp to:
//                                                    static  const uint8_t monthDays[]={31,28,31,30,31,99,31,31,30,31,30,31};
};

void dump_monthDays_content() {
    Serial.print("monthDays: ");
    for (int i = 0; i < 12; i++) {
        Serial.print(monthDays[i]);
        Serial.print(", ");
    }
    Serial.println("");
}

void setup() {
    Serial.begin(9600);
    dump_monthDays_content();
    Serial.print("days ");
    uint8_t days = 0;
#if 1
    days = pgm_read_byte(&monthDays2[0]);
#endif
#if 0
    days = (__extension__({
        uint16_t __addr16 = (uint16_t)(&monthDays2[0]);
        uint8_t __result;
        __asm__ __volatile__(
                "lpm"
                "\n\t"
                "mov %0, r0"
                "\n\t"
                : "=r"(__result)
                : "z"(__addr16)
                : "r0");
        __result;
    }));
#endif
    Serial.println(days);
    print_vars();
}

void loop() {
    dump_monthDays_content();
    delay(1000);
}

When compiled it produces warning

 warning: uninitialized variable 'monthDays2' put into program memory area [-Wuninitialized]
 const uint8_t monthDays2[] PROGMEM = {
               ^

but I do not understand why it does not also complain about month_days1/2 and primes1/2, they should have the same issue.

Initially I thought that this might have something to do with the actual access to the variable through the specific assembly instruction (hence also inlining the macro just to see if it made any difference (which it did not)), but it turns out that with the #ifs disabled the variable is just optimized away. Just referencing the variable like

void *p = &monthDays2[0];
Serial.print((int)p);

will keep it in the translation unit and produce the same issue. Running the above code on an Arduino Uno gives the following, notice the garbage value for days below:

monthDays: 31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31,
days 236
one_to_twelve1[0] = 1
one_to_twelve2[0] = 1
pgm_read_byte(&abcd1[0]) = 97
pgm_read_byte(&abcd2[0]) = 97
pgm_read_byte(&month_days1[0]) = 31
month_days2[0] = 31
primes1[0] = 2
pgm_read_byte(&primes2[0]) = 2
monthDays: 31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31,
monthDays: 31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31,
monthDays: 31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31,
monthDays: 31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31,
monthDays: 31, 28, 31, 30, 31, 99, 31, 31, 30, 31, 30, 31,

I guess this problem is related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80462.

I have tested compiling and flashing both with Arduino studio 1.8.8 & nightly (and with vscode, although it is just reusing the toolchain from the Arduino IDE). I have tested on two different computers. And I have tested with two different arduino boards.

$HOME/.arduino15/packages/arduino/tools/avr-gcc/5.4.0-atmel3.6.1-arduino2/bin/avr-gcc --version | head -2
avr-gcc (GCC) 5.4.0
Copyright (C) 2015 Free Software Foundation, Inc.

The nightly build also uses 5.4.0-atmel3.6.1-arduino2, so no difference.

[hlovdal]

I tried to create a wrapper for all the binaries containing the string merge-constants and rename and symlink the wrapper,

#!/bin/sh

exec $0.real "$@" -fno-merge-constants -fno-merge-all-constants -O0

but it did not help (only triggering a warning from <util/delay.h> that it was not happy with optimizations disabled).

[per1234]

Please give it a try using the beta test version of the upcoming Arduino AVR Boards toolchain, which uses avr-gcc 7.3.0:

1. File > Preferences
2. In the "Additional Boards Manager URLs" field, add: http://downloads.arduino.cc/packages/package_avr_7.3.0_index.json
3. If there are multiple URLs in the field, you should separate them with commas.
4. Click "OK".
5. Tools > Board > Boards Manager
6. Wait for downloads to finish.
7. Click on "Arduino AVR Boards".
8. Click "Update"
9. Wait for the update to finish. You should now have Arduino AVR Boards 1.6.209 installed.
10. Click "Close".

---

Using the sketch you provided with Arduino AVR Boards 1.6.23 (avr-gcc 5.4.0), I also get an incorrect value for days printed to the Serial Monitor. When I use Arduino AVR Boards 1.6.209, I get the expected value of 31.

---

Can we use the following code as a shorter and less convoluted demonstration of the issue?

const uint8_t numbers[]                = {1, 2, 3, 4, 5};
const uint8_t progmemNumbers[] PROGMEM = {1, 2, 3, 4, 5};

void setup() {
  Serial.begin(9600);

  Serial.print("numbers: ");
  for (uint8_t i = 0; i < sizeof(numbers); i++) {
    Serial.print(numbers[i]);
  }
  Serial.println();

  Serial.print("progmemNumbers: ");
  for (uint8_t i = 0; i < sizeof(progmemNumbers); i++) {
    Serial.print(pgm_read_byte(&progmemNumbers[i]));
  }
}

void loop() {}

Result with Arduino AVR Boards 1.6.23:

numbers: 12345
progmemNumbers: 1459143115153

Result with Arduino AVR Boards 1.6.209:

numbers: 12345
progmemNumbers: 12345

[hlovdal]

Thanks, yes updating to AVR boards 1.6.209 makes the problem go away.

[MHotchin]

Problem still exists in gcc 7.3.0. https://github.com/arduino/Arduino/issues/9835 Workaround is adding -fno-ipa-icf-variables.