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Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling

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  Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-10-08 13:20 +0200
    Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling Catalin Marinas <catalin.marinas@arm.com> - 2015-10-08 15:10 +0200
      Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling Edward Nevill <edward.nevill@linaro.org> - 2015-10-08 17:00 +0200

#1242209 — Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling

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.

http://yarchive.net/comp/linux/bool.html

especially hpa's response.

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#1242329

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.

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#1242464

On Thu, 2015-10-08 at 14:00 +0100, Catalin Marinas wrote:
> 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.

Why not just write what you mean

  return (elf_hwcap & cap->hwcap) != 0;

So much clearer. And every compiler will compile it correctly and
optimally. The !!() syntax is just so ugly it is untrue. Its like people
who write

  if (strcmp(..., ...)) ...

Break their fingers!
Ed.


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