Re: RFC: booleans and the kernel

Oliver Xymoron
2002-01-24T20:40:01Z

On Thu, 24 Jan 2002, Richard B. Johnson wrote:

> On Thu, 24 Jan 2002, Oliver Xymoron wrote:
>
> > On Thu, 24 Jan 2002, Jeff Garzik wrote:
> >
> > > A small issue...
> > >
> > > C99 introduced _Bool as a builtin type. The gcc patch for it went into
> > > cvs around Dec 2000. Any objections to propagating this type and usage
> > > of 'true' and 'false' around the kernel?
> >
> > Ugh, no. C doesn't need booleans, neither do Perl or Python. This is a
> > sickness imported from _recent_ C++ by way of Java by way of Pascal. This
> > just complicates things.
> >
> > > Where variables are truly boolean use of a bool type makes the
> > > intentions of the code more clear. And it also gives the compiler a
> > > slightly better chance to optimize code [I suspect].
> >
> > Unlikely. The compiler can already figure this sort of thing out from
> > context.
>
> IFF the 'C' compiler code-generators start making better code, i.e.,
> ORing a value already in a register, with itself and jumping on
> condition, then bool will be helpful. Right now, I see tests against
> numbers (like 0). This increases the code-size because the 0 is
> in the instruction stream, plus the comparison of an immediate
> value to a register value (on Intel) takes more CPU cycles.

The compiler _will_ turn if(a==0) into a test of a with itself rather than
a comparison against a constant. Since PDP days, no doubt.

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