Use _Alignof to query alignments in the struct module

[skirpichev]

Feature or enhancement

Proposal:

The struct module uses pre-C11 hacks to determine alignment for types:

cpython/Modules/_struct.c

Lines 79 to 106 in 5808889

	/* Define various structs to figure out the alignments of types */
	typedef struct { char c; short x; } st_short;
	typedef struct { char c; int x; } st_int;
	typedef struct { char c; long x; } st_long;
	typedef struct { char c; float x; } st_float;
	typedef struct { char c; double x; } st_double;
	#ifdef Py_HAVE_C_COMPLEX
	typedef struct { char c; float complex x; } st_float_complex;
	typedef struct { char c; double complex x; } st_double_complex;
	#endif
	typedef struct { char c; void *x; } st_void_p;
	typedef struct { char c; size_t x; } st_size_t;
	typedef struct { char c; _Bool x; } st_bool;
	#define SHORT_ALIGN (sizeof(st_short) - sizeof(short))
	#define INT_ALIGN (sizeof(st_int) - sizeof(int))
	#define LONG_ALIGN (sizeof(st_long) - sizeof(long))
	#define FLOAT_ALIGN (sizeof(st_float) - sizeof(float))
	#define DOUBLE_ALIGN (sizeof(st_double) - sizeof(double))
	#ifdef Py_HAVE_C_COMPLEX
	# define FLOAT_COMPLEX_ALIGN (sizeof(st_float_complex) - sizeof(float complex))
	# define DOUBLE_COMPLEX_ALIGN (sizeof(st_double_complex) - sizeof(double complex))
	#endif
	#define VOID_P_ALIGN (sizeof(st_void_p) - sizeof(void *))
	#define SIZE_T_ALIGN (sizeof(st_size_t) - sizeof(size_t))
	#define BOOL_ALIGN (sizeof(st_bool) - sizeof(_Bool))

Now we can use _Alignof operator, as C11-compatible compiler is a build requireent.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-132868: use _Alignof() operator in the struct module #132872

[picnixz]

Considering I did use _Alignof in my recent PR (#132428), I support this proposal.

[skirpichev]

PR is ready: #132872