Strict aliasing-related bug even with optimizations disabled

[Jonathon Reinhart <jonathon dot reinhart at gmail dot com>]

This was originally posted on Stack Overflow:
https://stackoverflow.com/a/49339771/119527

The following program:

    #include <stdio.h>

    static void pshort(short val)
    {
       printf("0x%hx ", val);
    }

    int main(void)
    {
       short A[] = {1, 2, 3, 4, 5, 6};

    #define EXP ((short*)((char*)A + 7))

       short *p = EXP;
       short q = *EXP;

       pshort(*p);
       pshort(q);
       pshort(*EXP);
       printf("\n");

       return 0;
    }

...when compiled on x86-64 with

    gcc -O0 -fno-strict-aliasing -g -Wall -Werror endian.c

...produces the following unexpected output

    0x500 0x500 0x4

...on all available versions of GCC, from 4.9 thru 7.3.1.

It appears that GCC is actually generating different code when the
expression is used directly as an argument, versus when used with
intermediate variables.

The language lawyers declared that UB is UB. But -fno-strict-aliasing
seems to be ineffective here, and this feels like a bug.

Jonathon Reinhart