From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cica-lavadora-l1mail3.puc.rediris.es (ci-l1mail3-out05a.puc.rediris.es [IPv6:2001:720:418:ca02::31]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3xHRZ82XwvzDqmV for ; Wed, 26 Jul 2017 17:29:16 +1000 (AEST) Date: Wed, 26 Jul 2017 09:29:00 +0200 From: Gabriel Paubert To: Matt Brown Cc: linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v3 2/5] powerpc/lib/sstep: Add popcnt instruction emulation Message-ID: <20170726072850.vtmqtrgzyhzh6okl@lt-gp.iram.es> References: <20170725033320.17893-1-matthew.brown.dev@gmail.com> <20170725033320.17893-2-matthew.brown.dev@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20170725033320.17893-2-matthew.brown.dev@gmail.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 > --- > 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