[lld] r233088 - [Mips] Suppress "right shift by too large amount" warning

[Simon Atanasyan]

On Tue, Mar 24, 2015 at 8:51 PM, Shankar Easwaran
<shankare at codeaurora.org> wrote:
> On 3/24/2015 12:41 PM, Simon Atanasyan wrote:
>>
>> On Tue, Mar 24, 2015 at 8:35 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>> On Tue, Mar 24, 2015 at 10:28 AM, Simon Atanasyan <simon at atanasyan.com>
>>> wrote:
>>>>
>>>> On Tue, Mar 24, 2015 at 7:16 PM, David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>>
>>>>> On Tue, Mar 24, 2015 at 8:49 AM, Simon Atanasyan <simon at atanasyan.com>
>>>>> wrote:
>>>>>>
>>>>>> Author: atanasyan
>>>>>> Date: Tue Mar 24 10:49:59 2015
>>>>>> New Revision: 233088
>>>>>>
>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=233088&view=rev
>>>>>> Log:
>>>>>> [Mips] Suppress "right shift by too large amount" warning
>>>>>>
>>>>>> Visual C++ shows the "right shift by too large amount" warning if
>>>>>> `MipsELFReference` is instantiated for 32-bit target and
>>>>>> `Elf_Rel_Impl::getType`
>>>>>> method has `unsigned char` return type. We can freely suppress the
>>>>>> warning
>>>>>> in
>>>>>> that case because MIPS 32-bit ABI does not pack multiple relocation
>>>>>> types
>>>>>> into
>>>>>> the single field `r_type` and the `MipsELFReference::_tag` should be
>>>>>> always
>>>>>> zero in that case.
>>>>>
>>>>>
>>>>> Can we just disable the MSVC warning if it's wrong, then?
>>>>
>>>> I thought about an alternative solution - fix Elf_Rel_Impl::getType()
>>>> method so all its variants have the same return type uint32_t. Any
>>>> objections against this idea?
>>>
>>> It still doesn't seem to improve anything - if getType() still only
>>> returns
>>> a number 0-255, the shift still is a zero-ing action. Hiding it from the
>>> compiler doesn't seem to change anything in the semantics of the
>>> operation &
>>> just obscures things further... seems to me, at least.
>>
>> The getType() does not always return 0-255. In case of 32-bit variant
>> of Elf_Rel_Impl it does. In case of 64-bit variant it returns 32-bit
>> integer which can be split up to four parts. In my case I extract
>> lower 8 bits and high 24 bits.
>
> Could you use the r_info (getRInfo) field and extract the needed
> information, seems that is what you need ?

Nice. I think it is the best solution for the problem.

-- 
Simon Atanasyan