[Review]: Fix Integer Division Expansion special case of divide by 1

[Aditya Nandakumar]

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Aditya
> On Jul 2, 2015, at 6:05 PM, Aditya Nandakumar <aditya_nandakumar at apple.com> wrote:
> 
> http://reviews.llvm.org/D10904 <https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D10904&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=5-RRC9r0s-Q0lLlak-uzCG7183zyd5M8H9E8g_P5zx8&s=scMfXxEAiLlqHvt3VJ2KrsEBzpaUsGBUGWFyUV6OZLs&e=>
> 
> Added to phabricator.
> 
> Aditya
>> On Jul 2, 2015, at 12:26 PM, Aditya Nandakumar <aditya_nandakumar at apple.com <mailto:aditya_nandakumar at apple.com>> wrote:
>> 
>> Hi All
>> 
>> I think the handling of special case of divide by 1 is incorrect in udivsi3.c (compiler-rt) and it’s equivalent in llvm/lib/Transforms/Utils/IntegerDivision.cpp
>> 
>> I think we want to check if the clz(divisor) == (Num_word_bits - 1) rather than checking
>> (clz(divisor) - clz(dividend)) == (Num_word_bits - 1)
>> 
>> I also think compiler-rt should be updated - 
>> http://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/builtins/udivsi3.c <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_svn_llvm-2Dproject_compiler-2Drt_trunk_lib_builtins_udivsi3.c&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=pW4Q1JdZMBOl8hRJ5f7uP2klbSmXe6EUr3CajMv6lhQ&s=YhRty41MrEPazrXTJ1lTya7lpHm4bykiDBdAK8w2OtU&e=>
>> 
>> sr = __builtin_clz(d) - __builtin_clz(n);
>>     /* 0 <= sr <= n_uword_bits - 1 or sr large */
>>     if (sr > n_uword_bits - 1)  /* d > r */
>>         return 0;
>>     if (sr == n_uword_bits - 1)  /* d == 1 */
>>         return n;
>> The last line should be replaced with 
>> 	
>> 	if ( __buildin_clz(d) == n_uword_bits - 1)
>> 		return n;
>> 
>> I have attached the LLVM patch here for the same.
>> <DiffIntUnsignedDivision.patch>
>> 
>> Thanks
>> Aditya
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