[Patch]New InstCombine pattern for Icmp

[Yi Jiang]

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On Jul 25, 2014, at 4:11 PM, Yi Jiang <yjiang at apple.com> wrote:

> Hi Ben,
> 
> Thank you for your comments! Here is a new version.  In this version, I still kept some “if” thinking that some operand has been already extracted so I would like to use it directly.  Please let me know if any other comments. 
> <combinecmpV2.patch>
> -Yi
> On Jul 25, 2014, at 2:12 AM, Benjamin Kramer <benny.kra at gmail.com> wrote:
> 
>> On Fri, Jul 25, 2014 at 1:24 AM, Yi Jiang <yjiang at apple.com> wrote:
>>> Hi,
>>> 
>>> his patch is trying to fold (icmp ult/ule (A + C1), C3) | (icmp ult/ule (A +
>>> C2), C3)  to (icmp ult/ule ((A & ~(C1 ^ C2)) + max(C1, C2)), C3)  .
>>> This transformation is legal if C1 ^ C2 is one-bit mask, in other word, C1
>>> is only one bit different from C2. In this case, we can “mask” that bit and
>>> do just one comparison.
>>> A typical example is:
>>> isALPHA(c)    ((c) >= 'A' && (c) <= 'Z') || ((c) >= 'a' && (c) <= 'z’)
>>> 
>>> Now llvm will optimize it to ((c + 191) <=25) || ((c + 159) <=25)
>>> 
>>> With this patch, we can optimize it further to:
>>> (c & 223) + 191 <= 25
>>> 
>>> The binary format of the constants are:
>>> 191   10111111
>>> 159   10011111
>>> 223   11011111
>>> 
>>> Here is some experiment result on arm64:
>>> The patch shows no regression and improve spec2000 perlbmk 3.8% in
>>> test-suite under -O3.
>>> We also test the spec2006 400.perlbench with ref size input, it will improve
>>> 1% under -O3 and 1.2% under -O3+lto+pgo.
>> 
>> Wow, very nice.
>> 
>>> Any comments are appreciated.
>> 
>> The patch could be significantly simplified with the PatternMatch
>> tool. Something like match(LHS, m_Add(m_OneUse(m_Value(A),
>> m_ConstantInt(LAddCst)... could replace your if chain.
>> 
>> - Ben
> 

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