[PATCH] D32665: [InstCombine] don't use DeMorgan's Law on integer constants

[Chandler Carruth via llvm-commits]

No issue. You can just use the 'chandlerc' username on Phab. I use my @
gmail.com email for most LLVM-related open source stuff.

On Fri, Apr 28, 2017 at 5:13 PM Sanjay Patel <spatel at rotateright.com> wrote:

> Hi Chandler,
>
> You're listed as an admin of Phab...and yet your email address(es) are
> shown as not verified, so I don't think it sent you mail about this patch!
>
> On Fri, Apr 28, 2017 at 5:55 PM, Sanjay Patel via Phabricator <
> reviews at reviews.llvm.org> wrote:
>
>> spatel created this revision.
>> Herald added subscribers: mcrosier, rengolin, aemerson.
>>
>> This is the fold that causes the infinite loop in BoringSSL (
>> https://github.com/google/boringssl/blob/master/crypto/cipher/e_rc2.c)
>> when we fix instcombine demanded bits to prefer 'not' ops as in
>> https://reviews.llvm.org/D32255.
>>
>> There are 2 or 3 problems with dyn_castNotVal, and I don't think we can
>> reinstate https://reviews.llvm.org/D32255 until dyn_castNotVal is
>> completely eliminated:
>>
>> 1. As shown here, it transforms 'not' into random xor. This transform is
>> harmful to SCEV and codegen because 'not' can often be folded while random
>> xor cannot.
>> 2. It does not transform vector constants. This is actually a good thing,
>> but if you don't believe the above argument, then we shouldn't have
>> excluded vectors.
>> 3. It tries to avoid transforming not(not(X)). That's nice, but it
>> doesn't match the greedy nature of instcombine. If we DeMorganize a pattern
>> that has an extra 'not' in it:
>>
>> ~(~(~X) & Y) --> (~X | ~Y)
>> That's just another case of DeMorgan, so we should trust that we'll get
>> that too:
>> (~X | ~ Y) --> ~(X & Y)
>>
>>
>> https://reviews.llvm.org/D32665
>>
>> Files:
>>   lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
>>   test/Transforms/InstCombine/and-or.ll
>>   test/Transforms/InstCombine/assume2.ll
>>
>>
>
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