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From: Richard Henderson <rth@twiddle.net>
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Date: Thu, 21 Jul 2016 12:11:56 +0530
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Subject: Re: [Qemu-devel] [RFC v1 09/13] target-ppc: add cmpeqb instruction
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To: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>, qemu-ppc@nongnu.org, david@gibson.dropbear.id.au
Cc: Sandipan Das <sandipandas1990@gmail.com>, Swapnil Bokade <bokadeswapnil@gmail.com>, qemu-devel@nongnu.org, aneesh.kumar@linux.vnet.ibm.com

On 07/18/2016 10:35 PM, Nikunj A Dadhania wrote:
> +    tcg_gen_andi_tl(src1, cpu_gpr[rA(ctx->opcode)], 0xFF);
> +    for (i = 0; i < 64; i += 8) {
> +        tcg_gen_shri_tl(t0, arg1, i);
> +        tcg_gen_andi_tl(t0, t0, 0xFF);
> +        tcg_gen_brcond_tl(TCG_COND_EQ, src1, t0, l1);
> +    }
> +    tcg_gen_movi_i32(cpu_crf[crfD(ctx->opcode)], 0);
> +    tcg_gen_br(l2);
> +    gen_set_label(l1);
> +    /* Set match bit, i.e. CRF_GT */
> +    tcg_gen_movi_i32(cpu_crf[crfD(ctx->opcode)], 1 << CRF_GT);

Ew.  This can be done much better as

   http://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord

Which still might be best done in a helper, because of the constants involved 
(tcg is not nearly so good as gcc in building full 64-bit constants).

C.f. target-alpha/int_helper.c, helper_cmpbe0 (which computes different 
information than cmpeqb, but is still helpful as an example).


r~