[clang][AVR] Alignment discrepancies between clang and avr-gcc for `short`, `long long`, and `double`

[alexrp]

Not sure if this is intentional on the clang side, but I thought I'd at least open an issue to find out. 🙂

❯ avr-gcc --version
avr-gcc (GCC) 5.4.0
❯ clang --version
clang version 18.1.8
❯ avr-gcc -fsyntax-only avr.c
❯ cat avr.c
_Static_assert(sizeof(char) == 1, "sizeof(char) == 1");
_Static_assert(_Alignof(char) == 1, "_Alignof(char) == 1");
_Static_assert(__alignof(char) == 1, "__alignof(char) == 1");

_Static_assert(sizeof(short) == 2, "sizeof(short) == 2");
_Static_assert(_Alignof(short) == 1, "_Alignof(short) == 1");
_Static_assert(__alignof(short) == 1, "__alignof(short) == 1");

_Static_assert(sizeof(unsigned short) == 2, "sizeof(unsigned short) == 2");
_Static_assert(_Alignof(unsigned short) == 1, "_Alignof(unsigned short) == 1");
_Static_assert(__alignof(unsigned short) == 1, "__alignof(unsigned short) == 1");

_Static_assert(sizeof(int) == 2, "sizeof(int) == 2");
_Static_assert(_Alignof(int) == 1, "_Alignof(int) == 1");
_Static_assert(__alignof(int) == 1, "__alignof(int) == 1");

_Static_assert(sizeof(unsigned int) == 2, "sizeof(unsigned int) == 2");
_Static_assert(_Alignof(unsigned int) == 1, "_Alignof(unsigned int) == 1");
_Static_assert(__alignof(unsigned int) == 1, "__alignof(unsigned int) == 1");

_Static_assert(sizeof(long) == 4, "sizeof(long) == 4");
_Static_assert(_Alignof(long) == 1, "_Alignof(long) == 1");
_Static_assert(__alignof(long) == 1, "__alignof(long) == 1");

_Static_assert(sizeof(unsigned long) == 4, "sizeof(unsigned long) == 4");
_Static_assert(_Alignof(unsigned long) == 1, "_Alignof(unsigned long) == 1");
_Static_assert(__alignof(unsigned long) == 1, "__alignof(unsigned long) == 1");

_Static_assert(sizeof(long long) == 8, "sizeof(long long) == 8");
_Static_assert(_Alignof(long long) == 1, "_Alignof(long long) == 1");
_Static_assert(__alignof(long long) == 1, "__alignof(long long) == 1");

_Static_assert(sizeof(unsigned long long) == 8, "sizeof(unsigned long long) == 8");
_Static_assert(_Alignof(unsigned long long) == 1, "_Alignof(unsigned long long) == 1");
_Static_assert(__alignof(unsigned long long) == 1, "__alignof(unsigned long long) == 1");

_Static_assert(sizeof(float) == 4, "sizeof(float) == 4");
_Static_assert(_Alignof(float) == 1, "_Alignof(float) == 1");
_Static_assert(__alignof(float) == 1, "__alignof(float) == 1");

_Static_assert(sizeof(double) == 4, "sizeof(double) == 4");
_Static_assert(_Alignof(double) == 1, "_Alignof(double) == 1");
_Static_assert(__alignof(double) == 1, "__alignof(double) == 1");

_Static_assert(sizeof(long double) == 4, "sizeof(long double) == 4");
_Static_assert(_Alignof(long double) == 1, "_Alignof(long double) == 1");
_Static_assert(__alignof(long double) == 1, "__alignof(long double) == 1");
❯ clang --target=avr -fsyntax-only avr.c
clang: warning: no target microcontroller specified on command line, cannot link standard libraries, please pass -mmcu=<mcu name> [-Wavr-rtlib-linking-quirks]
avr.c:6:16: error: static assertion failed due to requirement '_Alignof(short) == 1': _Alignof(short) == 1
    6 | _Static_assert(_Alignof(short) == 1, "_Alignof(short) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~
avr.c:6:32: note: expression evaluates to '2 == 1'
    6 | _Static_assert(_Alignof(short) == 1, "_Alignof(short) == 1");
      |                ~~~~~~~~~~~~~~~~^~~~
avr.c:7:16: error: static assertion failed due to requirement '__alignof(short) == 1': __alignof(short) == 1
    7 | _Static_assert(__alignof(short) == 1, "__alignof(short) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~~
avr.c:7:33: note: expression evaluates to '2 == 1'
    7 | _Static_assert(__alignof(short) == 1, "__alignof(short) == 1");
      |                ~~~~~~~~~~~~~~~~~^~~~
avr.c:10:16: error: static assertion failed due to requirement '_Alignof(unsigned short) == 1': _Alignof(unsigned short) == 1
   10 | _Static_assert(_Alignof(unsigned short) == 1, "_Alignof(unsigned short) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
avr.c:10:41: note: expression evaluates to '2 == 1'
   10 | _Static_assert(_Alignof(unsigned short) == 1, "_Alignof(unsigned short) == 1");
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
avr.c:11:16: error: static assertion failed due to requirement '__alignof(unsigned short) == 1': __alignof(unsigned short) == 1
   11 | _Static_assert(__alignof(unsigned short) == 1, "__alignof(unsigned short) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
avr.c:11:42: note: expression evaluates to '2 == 1'
   11 | _Static_assert(__alignof(unsigned short) == 1, "__alignof(unsigned short) == 1");
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
avr.c:31:16: error: static assertion failed due to requirement '__alignof(long long) == 1': __alignof(long long) == 1
   31 | _Static_assert(__alignof(long long) == 1, "__alignof(long long) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~
avr.c:31:37: note: expression evaluates to '8 == 1'
   31 | _Static_assert(__alignof(long long) == 1, "__alignof(long long) == 1");
      |                ~~~~~~~~~~~~~~~~~~~~~^~~~
avr.c:35:16: error: static assertion failed due to requirement '__alignof(unsigned long long) == 1': __alignof(unsigned long long) == 1
   35 | _Static_assert(__alignof(unsigned long long) == 1, "__alignof(unsigned long long) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
avr.c:35:46: note: expression evaluates to '8 == 1'
   35 | _Static_assert(__alignof(unsigned long long) == 1, "__alignof(unsigned long long) == 1");
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
avr.c:43:16: error: static assertion failed due to requirement '__alignof(double) == 1': __alignof(double) == 1
   43 | _Static_assert(__alignof(double) == 1, "__alignof(double) == 1");
      |                ^~~~~~~~~~~~~~~~~~~~~~
avr.c:43:34: note: expression evaluates to '4 == 1'
   43 | _Static_assert(__alignof(double) == 1, "__alignof(double) == 1");
      |                ~~~~~~~~~~~~~~~~~~^~~~
7 errors generated.

[alexrp]

cc @aykevl @benshi001 @Patryk27 (based on searching git log)

[aykevl]

I can confirm this issue: https://godbolt.org/z/vEhhs774T
It is indeed a bug because it breaks ABI compatibility with avr-gcc (and it unnecessarily increases alignment requirements while no such requirements exist on AVR).

Interestingly short has an alignment of 2, while int and long (correctly) have an alignment of 1.
Also, long long has an alignment of 1 in Clang (correct), but 8 in the LLVM IR (incorrect). That would definitely lead to miscompilations when calculating struct offsets for example.