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Re: [avr-gcc-list] How to (efficeiently !!!) test a bit within a multi-b

From: Vincent Trouilliez
Subject: Re: [avr-gcc-list] How to (efficeiently !!!) test a bit within a multi-byte integer ?
Date: Fri, 04 Nov 2005 13:01:02 +0100

On Fri, 2005-11-04 at 10:17 +0100, Alex Wenger wrote:
> Him
> Vincent Trouilliez schrieb:
> > Thank you very much Ian, your code is highly efficient, and does work
> > perfectly, unlike && or &.  :o) I am in heaven :
> > 
> >             temp = (address >> 16) & 0xFF;
> >     28bc:   ca 01           movw    r24, r20
> >     28be:   aa 27           eor     r26, r26
> >     28c0:   bb 27           eor     r27, r27
> >             if (temp & 0x04)
> >     28c2:   82 ff           sbrs    r24, 2
> >     28c4:   02 c0           rjmp    .+4             ; 0x28ca
> > 
> > can't get any better than that really :o)))
> if you make temp unsigned char, it should be even better.

Yes I see what you mean, the two 'eor' instructions that clear the two
upper bytes.... The problem is that 'temp' IS declared as uint8_t, and I
have no idea why the compiler insists on clearing the upper bytes as if
it were casting it into a 32bit integer (then what about the second byte
in this case, should get cleared as well ?!). It's only testing a bit in
the first (and only) byte, r24, so I don't understand why it care for
upper bytes at all. I tried to reword the statement a bit:

temp = (uint8_t) (address >> 16) 0xFF; 

,as well as change optimisation flags, but it really insists on clearing
these two bytes. God knows why. I guess it's doing something very subtle
that escapes me...


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