linux-kernel - Re: [PATCH] arm: Add condition code check to SWP emulator

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Date:	Sun, 20 Nov 2011 08:41:11 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stepan Moskovchenko <stepanm@...eaurora.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, bryanh@...eaurora.org,
	David Brown <davidb@...eaurora.org>,
	Leif Lindholm <leif.lindholm@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm: Add condition code check to SWP emulator

On Sat, Nov 19, 2011 at 05:04:30PM -0800, Stepan Moskovchenko wrote:
> +static int check_condition(struct pt_regs *regs, unsigned int insn)
> +{
> +	unsigned int base_cond, neg, cond = 0;
> +	unsigned int cpsr_z, cpsr_c, cpsr_n, cpsr_v;
> +
> +	cpsr_n = (regs->ARM_cpsr & PSR_N_BIT) ? 1 : 0;
> +	cpsr_z = (regs->ARM_cpsr & PSR_Z_BIT) ? 1 : 0;
> +	cpsr_c = (regs->ARM_cpsr & PSR_C_BIT) ? 1 : 0;
> +	cpsr_v = (regs->ARM_cpsr & PSR_V_BIT) ? 1 : 0;
> +
> +	/* Upper 3 bits indicate condition, lower bit incicates negation */
> +	base_cond = insn >> 29;
> +	neg = insn & BIT(28) ? 1 : 0;
> +
> +	switch (base_cond) {
> +	case 0x0:	/* equal */
> +		cond = cpsr_z;
> +		break;
> +
> +	case 0x1:	/* carry set */
> +		cond = cpsr_c;
> +		break;
> +
> +	case 0x2:	/* minus / negative */
> +		cond = cpsr_n;
> +		break;
> +
> +	case 0x3:	/* overflow */
> +		cond = cpsr_v;
> +		break;
> +
> +	case 0x4:	/* unsigned higher */
> +		cond = (cpsr_c == 1) && (cpsr_z == 0);
> +		break;
> +
> +	case 0x5:	/* signed greater / equal */
> +		cond = (cpsr_n == cpsr_v);
> +		break;
> +
> +	case 0x6:	/* signed greater */
> +		cond = (cpsr_z == 0) && (cpsr_n == cpsr_v);
> +		break;
> +
> +	case 0x7:	/* always */
> +		cond = 1;
> +		break;
> +	};
> +
> +	return cond && !neg;

There's a much better algorithm to check this.  See the bottom of
arch/arm/nwfpe/fpopcode.c.

It would probably be best for there to be a common function for doing
this kind of check, rather than having several implementations of it
scattered around the kernel.
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