On Thu, Oct 08, 2015 at 12:17:09PM +0100, Russell King - ARM Linux wrote:
> On Thu, Oct 08, 2015 at 12:10:00PM +0100, Catalin Marinas wrote:
> > On Mon, Oct 05, 2015 at 06:02:03PM +0100, Suzuki K. Poulose wrote:
> > > +static bool cpus_have_hwcap(const struct arm64_cpu_capabilities *cap)
> > > +{
> > > + switch(cap->hwcap_type) {
> > > + case CAP_HWCAP:
> > > + return !!(elf_hwcap & cap->hwcap);
> > > +#ifdef CONFIG_COMPAT
> > > + case CAP_COMPAT_HWCAP:
> > > + return !!(compat_elf_hwcap & (u32)cap->hwcap);
> > > + case CAP_COMPAT_HWCAP2:
> > > + return !!(compat_elf_hwcap2 & (u32)cap->hwcap);
> > > +#endif
> > > + default:
> > > + BUG();
> > > + return false;
> > > + }
> > > +}
> >
> > Apart from the multiple returns, you don't really need !! since the
> > return type is bool already.
>
> That's wrong. a & b doesn't return 0 or 1, but the bitwise-and result.
a & b is indeed a bitwise operation and, in this particular case, its
type is an unsigned long. However, because the return type of the
function is a bool, the result of the bitwise operation (unsigned long)
is converted to a bool.
The above may be true only for gcc, I haven't checked other compilers,
nor the standard (AFAIK, it appeared in C99).
On AArch64, the compiler generates something like:
tst x0, x1
cset w0, ne
ret
On AArch32, Thumb-2, I get:
tst r2, r3
ite ne
movne r0, #1
moveq r0, #0
bx lr
So a bool type function always returns 0 or 1 and does the appropriate
conversion.
> http://yarchive.net/comp/linux/bool.html
>
> especially hpa's response.
This seems to be more about a union of int and bool rather than
automatic type conversion. But I can see in the simple test that Linus
did towards the end of the thread that x86 does something similar with
converting a char to a bool:
testb %al, %al
setne %al
ret
I stand by my original comment.
--
Catalin
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