Re: [cc65] How to force a read from a memory mapped register w/o volatile

On Mon, Dec 6, 2010 at 10:41 AM, Ullrich von Bassewitz <uz@musoftware.de> wrote:
> On Mon, Dec 06, 2010 at 09:49:08AM -0600, Dave Dribin wrote:
>> I tried using inline assembler, and it still got optimized out:
>
> This is why I didn't write "inline assembler". You may want to use a separate
> assembler module.

Ah... I missed that distinction. ;-)

> At least with the current structure, this is not possible, otherwise I would
> have added this already. The optimizer traces uses of registers, loads and
> stores. And if these information is lost, because parts of a function isn't
> optimized, it is not possible to use optimization for the remainder.

Keep in mind I don't know anything about the optimizer, would it be
easier to optimize out a dummy variable?  i.e. is it difficult for the
optimizer to realize in this code that the pusha/incsp1 is
unnecessary?

000000r 1               ;
000000r 1               ; { uint8_t dummy = IO_Read8(0x2002); }
000000r 1               ;
000000r 1  AD 02 20     	lda     $2002
000003r 1  20 rr rr     	jsr     pusha
000006r 1  20 rr rr     	jsr     incsp1

But.... I think I have found a good work around.  Using inline
assembly + the 'bit' instruction seems to work.  For whatever reason,
the 'bit' instruction does not get optimized out.  For my code, I
don't care if the result goes into A or not, yet it still causes the
hardware register to be read:

#define IO_ForceRead8(_A_) __asm__ ("bit %w", _A_);

000000r 1               .proc	_foo: near
000000r 1
000000r 1               .segment	"CODE"
000000r 1
000000r 1               ;
000000r 1               ; IO_ForceRead8(0x2002);
000000r 1               ;
000000r 1  2C 02 20     	bit     $2002
000003r 1               ;
000003r 1               ; IO_Write8(0x2006, 0x20);
000003r 1               ;
000003r 1  A9 20        	lda     #$20
000005r 1  8D 06 20     	sta     $2006
000008r 1               ;
000008r 1               ; IO_Write8(0x2006, 0x00);
000008r 1               ;
000008r 1  A9 00        	lda     #$00
00000Ar 1  8D 06 20     	sta     $2006
00000Dr 1               ;
00000Dr 1               ; }
00000Dr 1               ;
00000Dr 1  60           	rts
00000Er 1
00000Er 1               .endproc

-Dave
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