netdev - Re: [PATCH net] bpf: do not use reciprocal divide

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Message-Id: <20140114.232846.647632973151148430.davem@davemloft.net>
Date:	Tue, 14 Jan 2014 23:28:46 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	hannes@...essinduktion.org, netdev@...r.kernel.org,
	dborkman@...hat.com, darkjames-ws@...kjames.pl, mgherzan@...il.com,
	rmk+kernel@....linux.org.uk, matt@...abs.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH net] bpf: do not use reciprocal divide

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 14 Jan 2014 23:02:41 -0800

> diff --git a/arch/sparc/net/bpf_jit_comp.c b/arch/sparc/net/bpf_jit_comp.c
> index 218b6b23c378..125045063b91 100644
> --- a/arch/sparc/net/bpf_jit_comp.c
> +++ b/arch/sparc/net/bpf_jit_comp.c
> @@ -498,8 +498,7 @@ void bpf_jit_compile(struct sk_filter *fp)
>  				emit_alu_K(MUL, K);
>  				break;
>  			case BPF_S_ALU_DIV_K:	/* A /= K */
> -				emit_alu_K(MUL, K);
> -				emit_read_y(r_A);
> +				emit_alu_K(DIV, K);
>  				break;
>  			case BPF_S_ALU_DIV_X:	/* A /= X; */
>  				emit_cmpi(r_X, 0);

You have to clear the Y register before a divide, as it provides
the top 32-bits of the 64-bit numerator.

You can just cut and paste the sequence used for BPF_S_ALU_DIV_X:

				emit_write_y(G0);
#ifdef CONFIG_SPARC32
				/* The Sparc v8 architecture requires
				 * three instructions between a %y
				 * register write and the first use.
				 */
				emit_nop();
				emit_nop();
				emit_nop();
#endif
				emit_alu_X(DIV);
--
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