linux-kernel - Re: [PATCH -tip 2/2] x86/hweight: Use POPCNT when available with X86_NATIVE

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Message-ID: <Z-Mme_OxuhYfxgzO@gmail.com>
Date: Tue, 25 Mar 2025 22:56:11 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip 2/2] x86/hweight: Use POPCNT when available with
 X86_NATIVE_CPU option


* Uros Bizjak <ubizjak@...il.com> wrote:

> Emit naked POPCNT instruction when available with X86_NATIVE_CPU
> option. The compiler is not bound by ABI when emitting the instruction
> without the fallback call to __sw_hweight{32,64}() library function
> and has much more freedom to allocate input and output operands,
> including memory input operand.
> 
> The code size of x86_64 defconfig (with X86_NATIVE_CPU option)
> shrinks by 599 bytes:
> 
>   add/remove: 0/0 grow/shrink: 45/197 up/down: 843/-1442 (-599)
>   Total: Before=22710531, After=22709932, chg -0.00%
> 
> The asm changes from e.g.:
> 
> 	   3bf9c:	48 8b 3d 00 00 00 00 	mov    0x0(%rip),%rdi
> 	   3bfa3:	e8 00 00 00 00       	call   3bfa8 <...>
> 	   3bfa8:	90                   	nop
> 	   3bfa9:	90                   	nop
> 
> with:
> 
> 	     34b:	31 c0                	xor    %eax,%eax
> 	     34d:	f3 48 0f b8 c7       	popcnt %rdi,%rax
> 
> in the .altinstr_replacement section
> 
> to:
> 
> 	   3bfdc:	31 c0                	xor    %eax,%eax
> 	   3bfde:	f3 48 0f b8 05 00 00 	popcnt 0x0(%rip),%rax
> 	   3bfe5:	00 00
> 
> where there is no need for an entry in the .altinstr_replacement
> section, shrinking all text sections by 9476 bytes:
> 
> 	    text           data     bss      dec            hex filename
> 	27267068        4643047  814852 32724967        1f357e7 vmlinux-old.o
> 	27257592        4643047  814852 32715491        1f332e3 vmlinux-new.o

> +#ifdef __POPCNT__
> +	asm_inline (ASM_FORCE_CLR "popcntl %[val], %[cnt]"
> +		    : [cnt] "=&r" (res)
> +		    : [val] ASM_INPUT_RM (w));
> +#else
>  	asm_inline (ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE
>  				"call __sw_hweight32",
>  				ASM_CLR "popcntl %[val], %[cnt]",
>  				X86_FEATURE_POPCNT)
>  			 : [cnt] "=a" (res), ASM_CALL_CONSTRAINT
>  			 : [val] REG_IN (w));

So a better optimization I think would be to declare and implement 
__sw_hweight32 with a different, less intrusive function call ABI that 
mirrors that of the instruction in essence, so that we optimize for the 
overwhelmingly common case of having the POPCNT instruction.

Thanks,

	Ingo