linux-kernel - Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)

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Message-ID: <87ldt24rk4.ffs@tglx>
Date: Tue, 18 Mar 2025 10:27:23 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave
 Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, Paolo Bonzini
 <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 Jacob Pan <jacob.jun.pan@...ux.intel.com>, Jim Mattson
 <jmattson@...gle.com>
Subject: Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during
 initial loop (to load PIR vals)

On Mon, Mar 17 2025 at 09:53, Sean Christopherson wrote:
> On Mon, Mar 17, 2025, Thomas Gleixner wrote:
>> > -	for (i = 0; i < 4; i++)
>> > +	for (i = 0; i < 4; i++) {
>> >  		pir_copy[i] = READ_ONCE(pir[i]);
>> > +		if (pir_copy[i])
>> > +			found_irq = true;
>> > +	}
>> 
>> That's four extra conditional branches. You can avoid them completely. See
>> delta patch below.
>
> Huh.  gcc elides the conditional branches when computing found_irq regardless of
> the approach; the JEs in the changelog are from skipping the XCHG.
>
> But clang-14 does not.  I'll slot this in.

Neither does GCC 12. That's why I noticed.