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Date: Tue, 22 Dec 2009 08:09:06 -0800
From: Richard Henderson <rth@twiddle.net>
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Subject: Re: [Qemu-devel] [PATCH 1/5] tcg: Generic support for conditional set
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To: Laurent Desnogues <laurent.desnogues@gmail.com>
Cc: qemu-devel@nongnu.org, aurelien@aurel32.net

On 12/22/2009 03:27 AM, Laurent Desnogues wrote:
>> +#if TCG_TARGET_REG_BITS == 64
>> +    tcg_gen_op4i_i64(INDEX_op_setcond_i64, ret, arg1, arg2, cond);
>> +#else
>> +    tcg_gen_op6i_i32(INDEX_op_setcond2_i32, TCGV_LOW(ret),
>> +                     TCGV_LOW(arg1), TCGV_HIGH(arg1),
>> +                     TCGV_LOW(arg2), TCGV_HIGH(arg2), cond);
>> +    tcg_gen_movi_i32(TCGV_HIGH(ret), 0);
>> +#endif
>> +}
>
> I wonder if it wouldn't be better to let the back-ends emit the
> clearing of TCGV_HIGH(ret).  This would reduce the number
> of emitted TCG ops.  Any thoughts?

(1) That would require 6 registers on i386 simultaneously.
(2) You lose the constant propagation that TCG would perform.


r~