linux-kernel - Re: [PATCH 02/10] x86, asm: use bool for bitops and other assembly outputs

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Date:	Wed, 8 Jun 2016 11:39:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 02/10] x86, asm: use bool for bitops and other assembly
 outputs

On Wed, Jun 08, 2016 at 02:31:31AM -0700, H. Peter Anvin wrote:
> On 06/08/16 02:20, Ingo Molnar wrote:
> > 
> > Yeah, absolutely. I hate 'bool' with a vengence but if 'int' generates worse code 
> > with modern compilers then I'm not going to argue for worse code. Would a 'char' 
> > return type be very weird?
> > 
> 
> Yes.  I have to admit I don't share your hatred for "bool" -- it gives
> the compiler a fairly crucial bit of information about what the possible
> values are for a certain piece of data.
> 
> Upcasting to char loses that, and may case gcc to manifest the value as
> an integer instead of retaining it in the flags.  It is, however, less
> likely to cause gcc to then try to widen the value to word size (which
> is an extra instruction on x86), but moving the value out of and back
> into the flags register is the big cost.

So I think using bool as return type or argument is fine, using it in
structures is 'insane'.