Subject: Re: [PATCH v4 2/5] powerpc/lib/sstep: Add popcnt instruction emulation

From: Cyril Bur <[email-masked]>
Date: Mon, 31 Jul 2017 14:15:13 +1000

On Mon, 2017-07-31 at 10:58 +1000, Matt Brown wrote:
> This adds emulations for the popcntb, popcntw, and popcntd instructions.
> Tested for correctness against the popcnt{b,w,d} instructions on ppc64le.
> 
> Signed-off-by: Matt Brown <[email-masked]>

Unlike the rest of this series, it isn't immediately clear that it is
correct, we're definitely on the other side of the optimisation vs
readability line. It looks like it is, perhaps some comments to
clarify.

Otherwise,

Reviewed-by: Cyril Bur <[email-masked]>

> ---
> v4:
> 	- change ifdef macro from __powerpc64__ to CONFIG_PPC64
> 	- slight optimisations 
> 	(now identical to the popcntb implementation in kernel/traps.c)
> v3:
> 	- optimised using the Giles-Miller method of side-ways addition
> v2:
> 	- fixed opcodes
> 	- fixed typecasting
> 	- fixed bitshifting error for both 32 and 64bit arch
> ---
>  arch/powerpc/lib/sstep.c | 42 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index 87d277f..2fd7377 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -612,6 +612,34 @@ static nokprobe_inline void do_cmpb(struct pt_regs *regs, unsigned long v1,
>  	regs->gpr[rd] = out_val;
>  }
>  
> +/*
> + * The size parameter is used to adjust the equivalent popcnt instruction.
> + * popcntb = 8, popcntw = 32, popcntd = 64
> + */
> +static nokprobe_inline void do_popcnt(struct pt_regs *regs, unsigned long v1,
> +				int size, int ra)
> +{
> +	unsigned long long out = v1;
> +
> +	out -= (out >> 1) & 0x5555555555555555;
> +	out = (0x3333333333333333 & out) + (0x3333333333333333 & (out >> 2));
> +	out = (out + (out >> 4)) & 0x0f0f0f0f0f0f0f0f;
> +
> +	if (size == 8) {	/* popcntb */
> +		regs->gpr[ra] = out;
> +		return;
> +	}
> +	out += out >> 8;
> +	out += out >> 16;
> +	if (size == 32) {	/* popcntw */
> +		regs->gpr[ra] = out & 0x0000003f0000003f;
> +		return;
> +	}
> +
> +	out = (out + (out >> 32)) & 0x7f;
> +	regs->gpr[ra] = out;	/* popcntd */
> +}
> +
>  static nokprobe_inline int trap_compare(long v1, long v2)
>  {
>  	int ret = 0;
> @@ -1194,6 +1222,10 @@ int analyse_instr(struct instruction_op *op, struct pt_regs *regs,
>  			regs->gpr[ra] = regs->gpr[rd] & ~regs->gpr[rb];
>  			goto logical_done;
>  
> +		case 122:	/* popcntb */
> +			do_popcnt(regs, regs->gpr[rd], 8, ra);
> +			goto logical_done;
> +
>  		case 124:	/* nor */
>  			regs->gpr[ra] = ~(regs->gpr[rd] | regs->gpr[rb]);
>  			goto logical_done;
> @@ -1206,6 +1238,10 @@ int analyse_instr(struct instruction_op *op, struct pt_regs *regs,
>  			regs->gpr[ra] = regs->gpr[rd] ^ regs->gpr[rb];
>  			goto logical_done;
>  
> +		case 378:	/* popcntw */
> +			do_popcnt(regs, regs->gpr[rd], 32, ra);
> +			goto logical_done;
> +
>  		case 412:	/* orc */
>  			regs->gpr[ra] = regs->gpr[rd] | ~regs->gpr[rb];
>  			goto logical_done;
> @@ -1217,7 +1253,11 @@ int analyse_instr(struct instruction_op *op, struct pt_regs *regs,
>  		case 476:	/* nand */
>  			regs->gpr[ra] = ~(regs->gpr[rd] & regs->gpr[rb]);
>  			goto logical_done;
> -
> +#ifdef CONFIG_PPC64
> +		case 506:	/* popcntd */
> +			do_popcnt(regs, regs->gpr[rd], 64, ra);
> +			goto logical_done;
> +#endif
>  		case 922:	/* extsh */
>  			regs->gpr[ra] = (signed short) regs->gpr[rd];
>  			goto logical_done;