Packing booleans?

Packing booleans?

[Sam Lauber]

Would it be possible to have a -fpack-bools option that packs booleans into
the smallest form possible (8 booleans -> 1 8-bit reg, etc.) into a register
(or memory, as the case may be)?

Samuel Lauber
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Re: Packing booleans?

[Nathan Sidwell]

Sam Lauber wrote:
> Would it be possible to have a -fpack-bools option that packs booleans into
> the smallest form possible (8 booleans -> 1 8-bit reg, etc.) into a register
> (or memory, as the case may be)?

why would you want to do this?  Seems to me you might save up to 7 bytes
of data memory at a vastly greater expansion of code size and reduction
in performance.

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

Re: Packing booleans?

[Nix]

On 4 May 2005, Sam Lauber stipulated:
> Would it be possible to have a -fpack-bools option that packs booleans into
> the smallest form possible (8 booleans -> 1 8-bit reg, etc.) into a register
> (or memory, as the case may be)?

How could you do that without breaking the semantics of the program?

Among other things, you couldn't take the address of such a packed
boolean: if the idea is to quietly suppress boolean packing for booleans
whose address is taken, then this will suppress packing for any boolean
shared between translation units, or possibly even between procedures.

-- 
`End users are just test loads for verifying that the system works, kind of
 like resistors in an electrical circuit.' - Kaz Kylheku in c.o.l.d.s

Re: Packing booleans?

[Mattias Karlsson]

On Wed, 4 May 2005, Nathan Sidwell wrote:

> Sam Lauber wrote:
>> Would it be possible to have a -fpack-bools option that packs booleans into
>> the smallest form possible (8 booleans -> 1 8-bit reg, etc.) into a 
>> register
>> (or memory, as the case may be)?
>
> why would you want to do this?  Seems to me you might save up to 7 bytes
> of data memory at a vastly greater expansion of code size and reduction
> in performance.

Unless you are using an architecture with btst, bset, bclr instructions.

Re: Packing booleans?

[Richard Guenther]

On 5/4/05, Sam Lauber <sam124@operamail.com> wrote:
> Would it be possible to have a -fpack-bools option that packs booleans into
> the smallest form possible (8 booleans -> 1 8-bit reg, etc.) into a register
> (or memory, as the case may be)?

Why don't you use bitfields then?

Richard.

Re: Packing booleans?

[Nathan Sidwell]

Mattias Karlsson wrote:
> On Wed, 4 May 2005, Nathan Sidwell wrote:
> 
>> Sam Lauber wrote:
>>
>>> Would it be possible to have a -fpack-bools option that packs 
>>> booleans into
>>> the smallest form possible (8 booleans -> 1 8-bit reg, etc.) into a 
>>> register
>>> (or memory, as the case may be)?
>>
>>
>> why would you want to do this?  Seems to me you might save up to 7 bytes
>> of data memory at a vastly greater expansion of code size and reduction
>> in performance.
> 
> 
> Unless you are using an architecture with btst, bset, bclr instructions.

True, they'd have to have memory operands to be a win though. Otherwise it'd
be a
	load
	bset
	store
or
	load
	btst
sequence, rather than
	store
or
	load

Of course, one might be able to amortize the load and store over several
bit operations, but you'd still end up with at least one extra instruction
over not packing, and that will cost you 4 bytes (or thereabouts).

If this were being done at the level of register allocation, why would
one need a user visible option?  'Just' rearrange the register file to look
like a set of single bit registers overlaid on a more conventional register
file.

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

Re: Packing booleans?

[Sam Lauber]

> If this were being done at the level of register allocation, why would
> one need a user visible option?  'Just' rearrange the register file to look
> like a set of single bit registers overlaid on a more conventional register
> file.
As someone pointed out, this might change semantics (read: screw up ;-)
of the * and & operators: &somebool, though it would be placed into memory,
wouldn't deserve an address, since most[1] architechures aren't bit-
adressable.  

Samuel Lauber

[1] I said ``some'', not ``all'', because the TMS340x0 port is.  
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Re: Packing booleans?

[Sam Lauber]

> Unless you are using an architecture with btst, bset, bclr instructions.
Pretty much any architechure that could be called a binary computer has 
something like that ;-)

Samuel Lauber
-- 
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