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From: Thomas Gleixner <tglx@linutronix.de>
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To: Sean Christopherson <seanjc@google.com>
Cc: Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave
 Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, Paolo Bonzini
 <pbonzini@redhat.com>, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
 Jacob Pan <jacob.jun.pan@linux.intel.com>, Jim Mattson
 <jmattson@google.com>
Subject: Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during
 initial loop (to load PIR vals)
In-Reply-To: <Z9hThFNFrrbXjkjc@google.com>
References: <20250315030630.2371712-1-seanjc@google.com>
 <20250315030630.2371712-3-seanjc@google.com> <87wmcn4x78.ffs@tglx>
 <Z9hThFNFrrbXjkjc@google.com>
Date: Tue, 18 Mar 2025 10:27:23 +0100
Message-ID: <87ldt24rk4.ffs@tglx>
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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.