Subject:
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H8 bit operations (was Re: GNU C wizards?)
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 4 Jan 1999 22:52:18 GMT
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Original-From:
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John A. Tamplin <jat@traveller.STOPSPAMcom>
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Viewed:
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1309 times
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On Mon, 4 Jan 1999, John A. Tamplin wrote:
> > =>I don't think you need to use asm - the compiler will use those
> > =>instructions. Just do it in C++:
> > => value|=(1<<bit) to set
> > => value&= ~(1<<bit) to clear
> >
> > I saw "bset" and "bclr" generated for these types of statements in
> > some places, but my test case (Port6[3] = true;) resulted in
> >
> > mov.b @0xffbb,r2l ; (this is from memory...)
> > or.b #x08,r2l
> > mov.b r2l,@0xffbb
> >
> > Wastes space, and worse, throws off the timing. (Although the LCD
> > driver doesn't seem to be *that* sensitive to those delay loops
> > getting shortened.)
>
> Hmm, you are correct. It does seem to reliably generate btst instructions
> for things like if(*p & 4) ..., but the set/clear aren't always. That should
> be something that can be fixed in the compiler, possibly creating a peephole
> optimization.
After looking into it further, if the compiler has the address in a
register (or will use it more than once so it will load it in a register),
it does use the bset/bclr instructions. However, there is this in h8300.md:
;; Bitfield operations no longer accept memory operands. We need
;; to add variants which operate on memory back to the MD.
Looking at the instruction patterns, it restricts the type of the second
operand to being non-memory, presumably so it can use the or instruction
if the value isn't suitable for bset. There are constraint types O and P
already, so I think it should be easy to add those instructions as
separate patterns in addition to the patterns already there.
I will play around with the machine description and let you know what I find.
John A. Tamplin Traveller Information Services
jat@Traveller.COM 2104 West Ferry Way
256/705-7007 - FAX 256/705-7100 Huntsville, AL 35801
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message has 1 Reply:
 | | Re: H8 bit operations (was Re: GNU C wizards?)
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| Ok, after more investigation the problem is twofold: 1) The compiler doesn't look at an absolute address (such as generated by #define P6DR (*(volatile unsigned char *)0xFFBB) and then used by P6DR=0). Instead, it looks for an attribute (...) (25 years ago, 5-Jan-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: GNU C wizards?
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| (...) GCC has in general done a very good job removing unnecessary code from templates. You can certainly optimize the generated code further, but you are getting the facilities of a high-level language in exchange for the (usually) small losses in (...) (25 years ago, 4-Jan-99, to lugnet.robotics)
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