Re: RFC: booleans and the kernel

Jeff Garzik
2002-01-24T21:58:13Z

"Richard B. Johnson" wrote:
>
> On Thu, 24 Jan 2002, Oliver Xymoron wrote:
>
> > 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.
>
> Don't you wish!
>
> int foo(int i)
> {
>
> if(i) return 0;
> else
> return 1;
> }

compile with -O2 on a modern compiler :)

.file "x.c"
.text
.align 16
.globl foo
.type foo,@function
foo:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
popl %ebp
testl %eax, %eax
sete %al
andl $255, %eax
ret
.Lfe1:
.size foo,.Lfe1-foo
.ident "GCC: (GNU) 3.0.3 (Mandrake Linux 8.2 3.0.3-1mdk)"

--
Jeff Garzik | "I went through my candy like hot oatmeal
Building 1024 | through an internally-buttered weasel."
MandrakeSoft | - goats.com