Re: [PATCH] arch/riscv: Add bitrev.h file to support rev8 and brev8

From: cp0613@linux.alibaba.com
To: ruanjinjie@huawei.com
Cc: akpm@linux-foundation.org, alex@ghiti.fr, aou@eecs.berkeley.edu,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	palmer@dabbelt.com, pjw@kernel.org
Subject: Re: [PATCH] arch/riscv: Add bitrev.h file to support rev8 and brev8
Date: Fri, 10 Apr 2026 19:36:40 +0800	[thread overview]
Message-ID: <20260410113640.106730-1-cp0613@linux.alibaba.com> (raw)
In-Reply-To: <20260311071344.3305640-1-ruanjinjie@huawei.com>

On Wed, 11 Mar 2026 15:13:44 +0800, ruanjinjie@huawei.com wrote:

> +#include <linux/types.h>
> +#include <asm/cpufeature-macros.h>
> +#include <asm/hwcap.h>
> +
> +extern u8 const byte_rev_table[256];
> +static __always_inline __attribute_const__ u32 __arch_bitrev32(u32 x)
> +{
> +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBKB) &&
> +	    riscv_has_extension_likely(RISCV_ISA_EXT_ZBKB)) {
> +		unsigned long result = x;
> +
> +		asm volatile(
> +			".option push\n"
> +			".option arch,+zbkb\n"
> +			"rev8 %0, %0\n"
> +			"brev8 %0, %0\n"
> +			".option pop"
> +			: "+r" (result)
> +		);
> +
> +		if (__riscv_xlen == 64)
> +			return (u32)(result >> 32);
> +
> +		return (u32)result;
> +	}
> +
> +	return (u32)byte_rev_table[x & 0xff] << 24 |
> +	       (u32)byte_rev_table[(x >> 8) & 0xff] << 16 |
> +	       (u32)byte_rev_table[(x >> 16) & 0xff] << 8 |
> +	       (u32)byte_rev_table[x >> 24];
> +}

Hi Jinjie,

Thanks for your patch. I have two suggestions.
1. When ZBKB is not supported, is it simpler to directly use the generic
implementation __bitrev32 in <linux/bitrev.h>.
2. Could you please provide a benchmark test case to illustrate the
performance comparison with and without this extension (refer to
test_bitops.c) and also provide the results by bloat-o-meter.

Thanks,
Pei

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