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Date: Wed, 6 May 2026 16:37:01 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Will Deacon <will@kernel.org>, Darren Hart <dvhart@infradead.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	=?iso-8859-1?Q?Andr=E9?= Almeida <andrealmeid@igalia.com>,
	linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	Jisheng Zhang <jszhang@kernel.org>
Subject: Re: [PATCH v4 3/8] arm64/runtime-const: Introduce
 runtime_const_mask_32()
Message-ID: <aftgHVEW3dsnblfc@arm.com>
References: <20260430094730.31624-1-kprateek.nayak@amd.com>
 <20260430094730.31624-4-kprateek.nayak@amd.com>
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In-Reply-To: <20260430094730.31624-4-kprateek.nayak@amd.com>

On Thu, Apr 30, 2026 at 09:47:25AM +0000, K Prateek Nayak wrote:
> Futex hash computation requires a mask operation with read-only after
> init data that will be converted to a runtime constant in the subsequent
> commit.
> 
> Introduce runtime_const_mask_32 to further optimize the mask operation
> in the futex hash computation hot path. GCC generates a:
> 
>   movz  w1, #lo16, lsl #0     // w1 = bits [15:0]
>   movk  w1, #hi16, lsl #16    // w1 = full 32-bit value
>   and   w0, w0, w1	      // w0 = w0 & w1
> 
> pattern to tackle arbitrary 32-bit masks and the same was also suggested
> by Claude which is implemented here. The final (__ret & mask) operation
> is intentiaonally placed outside of asm block to allow compilers to
> further optimize it if possible.
> 
> __runtime_fixup_ptr() already patches a "movz, + movk lsl #16" sequence
> which has been reused to patch the same sequence for
> __runtime_fixup_mask().
> 
> Assisted-by: Claude:claude-sonnet-4-5
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>