Disabling local optimizations with GCC

[John Mills]

Art -

On Wed, 28 Jun 2000, Art Berggreen wrote:
 
> Have you tried declaring your hardware variables as "volatile"?  This
> tends to stop the compiler from many of these optimizations.
> (i.e. try #define IoPortAddr   (volatile (BYTE*)0xa0001))

Thanks for the suggestion. At present, I'm using two types of
construction:

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[...]
#define	EVB_BASE	0xFFFF0000      /* Register-block base address */

/* SCI0 Registers */

/* Serial mode ch 0 */
#define SCI0_SMR   (*(volatile unsigned char *)(EVB_BASE + 0x81a0))
/* Bit rate ch 0 */
#define SCI0_BRR   (*(volatile unsigned char *)(EVB_BASE + 0x81a1))
[...]

then I read or write from appropriately typed or cast variables
{
unsigned char tempch;
...
  tempch = SCI0_SMR | <modes_to_add>;
  SCI0_SMR = tempch;
...
}
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

and in other sources:

[...]
#define readb(addr) (*(volatile unsigned char *) (addr))
#define writeb(b,addr) ((*(volatile unsigned char *) (addr)) = (b)

#define SCI0_SMR    (EVB_BASE + 0x81a0)

used as in:

{
...
unsigned char tempch;

  tempch = readb(SCI0_SMR);
  writeb((tempch | <modes_to_add>), SCI0_SMR);

...
}
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

The second approach is handy in that registers may be grouped as unions
and manipulated jointly or severally without duplicated variant names nor
in-line type-casts (or rather the type casting looks like functions, which
I find handy and readable - YMMV).

I haven't found any construction which seems to work or fail consistently,
but my 'C' coding skills are (to put it gently) 'limited'. I'll try the
formation you suggest and see how it works.

I am also concerned by the inconsistent results I seem to get from code
constructions which (_a_priori_) look acceptable to me.
  
Regards -

   John Mills
   Sr. Software Engineer
   TGA Technologies, Inc.
   100 Pinnacle Way, Suite 140
   Norcross, GA 30071-3633
   e-mail: jmills@tga.com
   Phone: 770-441-2100 ext.124 (voice)
          770-449-7740 (FAX)



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