[PATCH] Silence misguided GCC warning about alignment issues

[Ralf Friedl]

Johannes Schindelin wrote:
> When compiling xz_dec_stream.c with GCC 7.1.0, it complains thusly:
>
> 	In function 'dec_stream_footer':
> 	error: dereferencing type-punned pointer will break strict-aliasing
> 	      rules [-Werror=strict-aliasing]
> 	   if (xz_crc32(s->temp.buf + 4, 6, 0) != get_le32(s->temp.buf))
> 	       ^~
>
> The thing is, the `buf` field was put just after two fields of type
> size_t for the express purpose of avoiding alignment issues, as per the
> comment above the `temp` struct.
>   
> +#if defined(__GNUC__)
> +	/*
> +	 * The temp.buf field is put just after two fields of type size_t for
> +	 * the express purpose of avoiding alignment issues. But GCC complains
> +	 * about it nevertheless... so: shut GCC up for a few lines.
> +	 */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wstrict-aliasing"
> +#endif
In this discussion, you write repeatedly about the alignment of the 
variable, you even propose a comment to that effect.
Note, this is not about the alignment, it is about aliasing, which means 
two different pointers may access the same memory location.