Re: RFC: booleans and the kernel

Richard B. Johnson
2002-01-24T21:55:12Z

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;
}

.file "xxx.c"
.version "01.01"
gcc2_compiled.:
.text
.align 4
.globl foo
.type foo,@function
foo:
pushl %ebp
movl %esp,%ebp
cmpl $0,8(%ebp) <-------------- Compare against zero.
je .L2
xorl %eax,%eax
jmp .L1
jmp .L3
.align 4
.L2:
movl $1,%eax
jmp .L1
.align 4
.L3:
.L1:
movl %ebp,%esp
popl %ebp
ret
.Lfe1:
.size foo,.Lfe1-foo
.ident "GCC: (GNU) egcs-2.91.66 19990314 (egcs-1.1.2 release)"

Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (797.90 BogoMips).

I was going to compile a list of innovations that could be
attributed to Microsoft. Once I realized that Ctrl-Alt-Del
was handled in the BIOS, I found that there aren't any.