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

[Jinjie Ruan]

On 2026/4/10 19:36, cp0613@xxxxxxxxxxxxxxxxx wrote:
> On Wed, 11 Mar 2026 15:13:44 +0800, ruanjinjie@xxxxxxxxxx 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>.

Actually, you can't simply use the default implementation from
linux/bitrev.h. It includes asm/bitrev.h (the architecture-specific
implementation), which would lead to compilation issues. Furthermore,
when ZBKB is not supported, current implementation is identical to the
default one.


> 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.

I don't have access to RISC-V hardware at the moment, so I've only
performed basic functional testing on QEMU, which completed without
issues，could you please help run some benchmarks to verify the performance?

Thanks,
Jinjie

> 
> Thanks,
> Pei