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Date: Fri, 23 Oct 2009 09:04:53 +0200
From: Aurelien Jarno <aurelien@aurel32.net>
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Subject: Re: [Qemu-devel] [PATCH] target-arm: use clz32() instead of a for loop
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	<20091023003417.GA31360@miranda.arrow>
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To: Stuart Brady <sdbrady@ntlworld.com>
Cc: qemu-devel@nongnu.org

Stuart Brady a écrit :
> On Thu, Oct 15, 2009 at 11:14:52PM +0200, Aurelien Jarno wrote:
>> @@ -394,10 +395,7 @@ uint32_t HELPER(uxtb16)(uint32_t x)
>>  
>>  uint32_t HELPER(clz)(uint32_t x)
>>  {
>> -    int count;
>> -    for (count = 32; x; count--)
>> -        x >>= 1;
>> -    return count;
>> +    return clz32(x);
>>  }
>>  
>>  int32_t HELPER(sdiv)(int32_t num, int32_t den)
> 
> Just a quick note that the implementation of clz, ctz and popcnt is
> still listed in the TCG TODO list.  The last time I looked, I noticed
> that quite a few architectures have clz/ctz instructions:
> 
>    http://lkml.indiana.edu/hypermail/linux/kernel/0601.3/1683.html

OTOH, a dump shows that those instruction are not used than often, so I
am not sure it worth implementing it.

> For those that don't, I think a combination the following two hacks at
> http://graphics.stanford.edu/~seander/bithacks.html could be used:

The best is probably to use an helper in that case, calling clz32(x).

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net