Subject:
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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 18:18:06 GMT
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Original-From:
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dave madden <dhm@paradigm.webvision^saynotospam^.com>
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Viewed:
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1489 times
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On Sun, 3 Jan 1999, I wrote:
=>>
=>> Are there any GNU C/C++ wizards out there? I'm trying to write an
=>> asm() statement to describe the H8's bclr/bset instructions. I have a
=>> wicked sweet tooth for C++ syntactic sugar, and I'd like to be able to
=>> say (for example):
=>>
=>> Port6.Bit3 = true;
=>>
=>> and have it turn into:
=>>
=>> bset #3,@0xbb:8
=>>
=>> [Actually, I'd *really* like to say: "Port6[3] = true;" but after
=>> beating my head against the compiler for an hour or two, I don't think
=>> it's going to happen.]
Then "John A. Tamplin" <jat@Traveller.COM> replied:
=>
=>Actually, you can do that -- look at the STL implementation for
=>BitVector<bool>.
=>[...]
=>This is similar to the STL implementation using a helper class for the
=>assignments. All you need to do is pass a reference to the port object
=>into the bit when you create it (in the subscript opererator), and then
=>the helper object calls members on the Port object when you access the
=>value (via cast operators) or assign the value (via the assignment
=>operator).
I'd tried doing this, but the generated code (-O6) only infrequently
reduces to "bset #n,...", and never to "bset #n,@xx:8". (It also
seemed to allocate/construct stack space for the Bit sometimes...) If
you need to pass Bit references around, of course, then you have to do
something like BitVector<T>, but for simple linear code, keeping the
port backreference and and mask around just make the compiler work
harder.
Anyway, I kept at it a little longer last night and got the templates
working in the "Port6.Bit3 = true; if (Port6.Bit5) do_something( );"
mode. The DDR access is clumsier because, as you note, the register
is write-only, so you have to keep a shadow copy around. I'll take
another shot at expressing things as arrays and post the best version
I can come up with.
=>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.)
BTW, won't the "-r" option to gnu-ld work for your load-on-the-fly
problem? The info page says:
`-r'
`--relocateable'
Generate relocatable output--i.e., generate an output file that
can in turn serve as input to `ld'. This is often called "partial
linking". As a side effect, in environments that support standard
Unix magic numbers, this option also sets the output file's magic
number to `OMAGIC'. If this option is not specified, an absolute
file is produced. When linking C++ programs, this option *will
not* resolve references to constructors; to do that, use `-Ur'.
d.
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message has 1 Reply:
 | | 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 (...) (26 years ago, 4-Jan-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: GNU C wizards?
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| (...) Actually, you can do that -- look at the STL implementation for BitVector<bool>. (...) This is similar to the STL implementation using a helper class for the assignments. All you need to do is pass a reference to the port object into the bit (...) (26 years ago, 4-Jan-99, to lugnet.robotics)
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