a feature to the wishlist

["Rafał Pietrak" <embedded@ztk-rp.eu>]

Hi everybody,

I've been using GCC for a varety of ARM micro controller project. With
embedded systems, one often has to access specific memory locations,
which contain hardware registers, that control device behavior.
"traditionally" or "commonly" that's codded by programmers using
"#define", thus relaying on preprocessor to put specific compiler
constructs, that do the job.

IMHO, this is wrong exactly because of the involvement of preprocessor.
Taking as an example exerpts from libopencm3, like:
	>> USB_SET_EP_RX_STAT(addr, USB_EP_RX_STAT_VALID)
one can understand, that such "code" can actually translate to quite
anything.

Getting to the point, here is code exerpt of by own STM32 usart driver:
------------------------------------------------------------------
#define USART_RE	11
#define USART_RENEIE	8
#define USART_TE	10
#define USART_TXEIE	6
#define USART_UE	0

extern struct usart_s {
	uint sr;
	uint dr;			// offset 0x04
	uint brr;			// offset 0x08
	volatile union { uint cr1;	// offset 0x0C
		struct cr_s { uint sbk:1, rwu:1,
		re:1, te:1, idleie:1, rxneie:1,
		tcie:1, txeie:1, peie:1, ps:1,
		pce:1, wake:1, m:1, ue:1;} cr1_s;
	};
} USART1;

static void EXIT(void *tty) {
	volatile struct usart_s *d = &USART1;

#if VARIANT_TRADITIONAL
	d->cr1 &= ~(1 << USART_RE) & ~(1 << USART_RXNEIE)
		& ~(1 << USART_TE) & ~(1 << USART_TXEIE)
		& ~(1 << USART_UE);
#elif VARIANT_WORKING
	struct cr_s a = (struct cr_s) {
			.re = 1,
			.te = 1,
       			.rxneie = 1,
			.txeie = 1,
			.ue = 1 };
	int *b = (int *) &a;
	d->cr1 &= ~(*b);
#elif VARIANT_BETTER
	(union cr_u) d->cr1 &= ~ {
			.re = 1,
			.te = 1,
       			.rxneie = 1,
			.txeie = 1,
			.ue = 1 };
#elif VARIANT_THE_BEST
	d->cr1_s &= ~ { .re = 1,
			.te = 1,
       			.rxneie = 1,
			.txeie = 1,
			.ue = 1 };
#endif
}
----------------------------------------------------------------

In function EXIT, you can see four variants of "the same code". First
fragment is codded just like todays common practice is. This is BAD
coding, because debugger has no way of presenting "the real thing".

Using gcc-8-branch revision 273027 (arm-none-eabi-gcc linux cross
compiler) I was able to code the very same thing by VARIANT_WORKING,
which is far better to read (IMHO). The disadvantage is, that I had to
use variable "b" to fool gcc syntax parser. No direct multiple "type
overwrite" worked for me.

Then again, it would be even better (and easier to read the code),
should gcc syntax parser recognised VARIANT_BETTER or VARIANT_THE_BEST
as "syntactic sugar" to generate code, that currently gets issued by
VARIANT_WORKING (being the same as issued by VARIANT_TRADITIONAL, which
is very OK).

Pls CC: responses to my address, as I'm not a subscriber to the gcc list.

So my question to the list is: are there chances, the issue can be
recognised as important, and planed for some future gcc?

with best regards,

Rafał Pietrak