On Tue, Jul 25, 2017 at 01:33:17PM +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 <matthew.brown....@gmail.com>
> ---
> 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 | 40 +++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 39 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index 87d277f..c1f9cdb 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -612,6 +612,32 @@ 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 = (0x5555555555555555 & out) + (0x5555555555555555 & (out >> 1));
This can be simplified in a less obvious way as:
out -= (out >> 1) & 0x5555555555555555;
which maps each pair of bits according to the following:
00 -> 00
01 -> 01
10 -> 01
11 -> 10
This should save one instruction.
> + out = (0x3333333333333333 & out) + (0x3333333333333333 & (out >> 2));
Ok, but now each nibble is between 0 and 4, so the addition of two
nibbles can't overflow or generate carry into the higher one.
> + out = (0x0f0f0f0f0f0f0f0f & out) + (0x0f0f0f0f0f0f0f0f & (out >> 4));
out += out >> 4;
out &= 0x0f0f0f0f0f0f0f0f;
which should also save one instruction
> + if (size == 8) { /* popcntb */
> + regs->gpr[ra] = out;
> + return;
> + }
At this point each count occupies at least one byte and can no more
overflow, so masking is only needed before returning.
> + out = (0x001f001f001f001f & out) + (0x001f001f001f001f & (out >> 8));
out += out >> 8;
> + out = (0x0000003f0000003f & out) + (0x0000003f0000003f & (out >> 16));
out += out >> 16;
> + if (size == 32) { /* popcntw */
> + regs->gpr[ra] = out;
regs->gpr[ra] = out & 0x0000003f0000003f;
> + return;
> + }
> + out = (0x000000000000007f & out) + (0x000000000000007f & (out >> 32));
out = (out + (out >> 32)) & 0x7f;
Gabriel
> + regs->gpr[ra] = out; /* popcntd */
> +}
> +
> static nokprobe_inline int trap_compare(long v1, long v2)
> {
> int ret = 0;
> @@ -1194,6 +1220,10 @@ 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 +1236,10 @@ 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 +1251,11 @@ 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 __powerpc64__
> + 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;
> --
> 2.9.3
