linux-kernel - Re: [PATCH 2/2] bitops: rotate: Add riscv implementation using Zbb extension

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Message-ID: <20250628120816.1679-1-cp0613@linux.alibaba.com>
Date: Sat, 28 Jun 2025 20:08:16 +0800
From: cp0613@...ux.alibaba.com
To: david.laight.linux@...il.com
Cc: alex@...ti.fr,
	aou@...s.berkeley.edu,
	arnd@...db.de,
	cp0613@...ux.alibaba.com,
	linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org,
	linux@...musvillemoes.dk,
	palmer@...belt.com,
	paul.walmsley@...ive.com,
	yury.norov@...il.com
Subject: Re: [PATCH 2/2] bitops: rotate: Add riscv implementation using Zbb extension

On Wed, 25 Jun 2025 17:02:34 +0100, david.laight.linux@...il.com wrote:

> Is it even a gain in the zbb case?
> The "rorw" is only ever going to help full word rotates.
> Here you might as well do ((word << 8 | word) >> shift).
> 
> For "rol8" you'd need ((word << 24 | word) 'rol' shift).
> I still bet the generic code is faster (but see below).
> 
> Same for 16bit rotates.
> 
> Actually the generic version is (probably) horrid for everything except x86.
> See https://www.godbolt.org/z/xTxYj57To

Thanks for your suggestion, this website is very inspiring. According to the
results, the generic version is indeed the most friendly to x86. I think this
is also a reason why other architectures should be optimized. Take the riscv64
ror32 implementation as an example, compare the number of assembly instructions
of the following two functions:
```
u32 zbb_opt_ror32(u32 word, unsigned int shift)
{
	asm volatile(
		".option push\n"
		".option arch,+zbb\n"
		"rorw %0, %1, %2\n"
		".option pop\n"
		: "=r" (word) : "r" (word), "r" (shift) :);

	return word;
}

u16 generic_ror32(u16 word, unsigned int shift)
{
	return (word >> (shift & 31)) | (word << ((-shift) & 31));
}
```
Their disassembly is:
```
zbb_opt_ror32:
<+0>:     addi    sp,sp,-16
<+2>:     sd      s0,0(sp)
<+4>:     sd      ra,8(sp)
<+6>:     addi    s0,sp,16
<+8>:     .insn   4, 0x60b5553b
<+12>:    ld      ra,8(sp)
<+14>:    ld      s0,0(sp)
<+16>:    sext.w  a0,a0
<+18>:    addi    sp,sp,16
<+20>:    ret

generic_ror32:
<+0>:     addi    sp,sp,-16
<+2>:     andi    a1,a1,31
<+4>:     sd      s0,0(sp)
<+6>:     sd      ra,8(sp)
<+8>:     addi    s0,sp,16
<+10>:    negw    a5,a1
<+14>:    sllw    a5,a0,a5
<+18>:    ld      ra,8(sp)
<+20>:    ld      s0,0(sp)
<+22>:    srlw    a0,a0,a1
<+26>:    or      a0,a0,a5
<+28>:    slli    a0,a0,0x30
<+30>:    srli    a0,a0,0x30
<+32>:    addi    sp,sp,16
<+34>:    ret
```
It can be found that the zbb optimized implementation uses fewer instructions,
even for 16-bit and 8-bit data.