[llvm] r247570 - [MergeFuncs] Fix bug in merging GetElementPointers

[JF Bastien via llvm-commits]

On Mon, Sep 14, 2015 at 1:25 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Mon, Sep 14, 2015 at 1:12 PM, JF Bastien <jfb at google.com> wrote:
>
>> On Mon, Sep 14, 2015 at 1:05 PM, David Blaikie <dblaikie at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Mon, Sep 14, 2015 at 12:59 PM, JF Bastien <jfb at google.com> wrote:
>>>
>>>> On Mon, Sep 14, 2015 at 11:25 AM, David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Mon, Sep 14, 2015 at 11:13 AM, JF Bastien <jfb at google.com> wrote:
>>>>>
>>>>>> GetElementPointers must have the first argument's type compared
>>>>>>>> for structural equivalence. Previously the code erroneously
>>>>>>>> compared the
>>>>>>>> pointer's type, but this code was dead because all pointer types
>>>>>>>> (of the
>>>>>>>> same address space) are the same. The pointee must be compared
>>>>>>>> instead
>>>>>>>> (using the type stored in the GEP, not from the pointer type which
>>>>>>>> will
>>>>>>>> be erased anyway).
>>>>>>>>
>>>>>>>
>>>>>>> Thanks so much for being aware of this/using APIs that'll be
>>>>>>> forwards compatible with the eventual removal of typed pointers.
>>>>>>>
>>>>>>> Just in case: do you care about the GEP pointer operands address
>>>>>>> space (I assume not) or vector types (eg: a gep over a vector of pointers)?
>>>>>>> I assume not, but just checking - both those aspects of the pointer operand
>>>>>>> are still in the pointer operand's type, not in the source element type.
>>>>>>>
>>>>>>
>>>>>> We do care about address spaces in general: we can't merge two
>>>>>> functions if they refer to different address spaces. I'm not sure that's
>>>>>> what you're asking for though: in the end we (roughly) only want to merge
>>>>>> functions that would codegen to the same code. I think the same applies to
>>>>>> your vector of pointers question?
>>>>>>
>>>>>
>>>>> My point was - do you care if these two GEPs refer to different
>>>>> pointers from address spaces? What if the GEPs are differently vectored (if
>>>>> vector GEPs can exist in this codepath, etc)?
>>>>>
>>>>
>>>> Yes, we care if the address spaces are different. That's checked at the
>>>> start of the function.
>>>>
>>>> By vectored you mean the indexing? I think this is handled properly,
>>>> but it would be great if you could confirm.
>>>>
>>>
>>> I really don't know enough about this pass or code to know if this is
>>> even reproducible. Can you get vector geps in this codepath?
>>>
>>
>> Yes, that should be possible, but IIUC the code compares the effective
>> byte offsets so everything should "just work", unless I'm missing your
>> concern!
>>
>
> So if I'm understanding correctly, this FunctionComparator is trying to
> test if two functions are equivalent. So I guess the vector-ness of geps
> would then relate to a test case like this (going to be really
> pseudo-code-y because I don't know LLVM IR well enough to write it
> off-the-cuff):
>
> @blob = global /* big global of data, don't care what it is */
> void f1() {
>   store getelementptr (i32, bitcast (@blob to [2 x i32*]), ... ), {1, 2}
> }
> void f2() {
>   store getelementptr (i32, bitcast (@blob to [4 x i32*]), ...), {1, 2, 3,
> 4}
> }
>
> I suppose the store wouldn't match, but I'm not sure - maybe there's a way
> everything else in the IR could be the same, but the vector-ness of the
> geps could be differently sized somehow.
>

I think so? I'm not sure I follow anymore :-)
This pass wants to find functions that have exactly the same effect, so if
the stores end up putting the same bytes at the same place in the same
order then the functions are equivalent and can be merged.
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