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

[Simon Atanasyan]

On Tue, Mar 24, 2015 at 8:57 PM, Simon Atanasyan <simon at atanasyan.com> wrote:
> 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.

I hope the problem has been fixed by r233186. Unfortunately using
getRInfo does not solve the problem because in 32-bit case we extract
too much data and touch symbol index part of r_info field.

So I just factor out the code that gets the "tag" into the separate
function. That allows to keep all bit manipulation code into the
single place and removes the warning.

-- 
Simon Atanasyan