[PATCH] D32212: Create some helpers for generating bit masks

[Zachary Turner via llvm-commits]

Err, not sure why I'm talking about a static_cast, but in any case can you
test that out?  I wasn't getting a warning with MSVC 2017

On Wed, Apr 19, 2017 at 9:43 AM Zachary Turner <zturner at google.com> wrote:

> We can just use a static_cast to get rid of the warning.  MathExtras.h
> already has such a thing to silence this exact warning:
>
>   // Replace "-Value" by "1+~Value" in the following commented code to
> avoid
>   // MSVC warning C4146
>   //    return (A | B) & -(A | B);
>   return (A | B) & (1 + ~(A | B));
>
>
> On Wed, Apr 19, 2017 at 9:26 AM Adrian McCarthy via Phabricator <
> reviews at reviews.llvm.org> wrote:
>
>> amccarth added inline comments.
>>
>>
>> ================
>> Comment at: llvm/trunk/include/llvm/Support/MathExtras.h:207
>> +  assert(N <= Bits && "Invalid bit index");
>> +  return -T(N != 0) & (T(-1) >> (Bits - N));
>> +}
>> ----------------
>> Be aware that this can lead to compiler warnings, at least in MSVC, if T
>> is an unsigned type:
>>
>>  [[ https://msdn.microsoft.com/en-us/library/4kh09110.aspx |  C4146:
>> unary minus operator applied to unsigned type, result still unsigned ]]
>>
>> Depending on the specific details, this can be a level 2 or level 3
>> warning.  I'm surprised the tests aren't triggering it.  We must disable
>> this warning somewhere.
>>
>>
>>
>>
>> Repository:
>>   rL LLVM
>>
>> https://reviews.llvm.org/D32212
>>
>>
>>
>>
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