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

[JF Bastien via llvm-commits]

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!


getelementptr (i32, [5 x i32*] @x, ... )
>
>
>
>
>>
>>
>> If you do care about the (in)equality of address space or vector-ness in
>>> these two GEPs, you'll need to compare those - those would've previously
>>> been caught by just comparing the two GEPs pointer operand types, but won't
>>> be any more with this change.
>>>
>>
>>
>> For reference, the current code is:
>>
>> int FunctionComparator::cmpGEPs(const GEPOperator *GEPL,
>>                                const GEPOperator *GEPR) {
>>
>>   unsigned int ASL = GEPL->getPointerAddressSpace();
>>   unsigned int ASR = GEPR->getPointerAddressSpace();
>>
>>   if (int Res = cmpNumbers(ASL, ASR))
>>
>>
> Ah, thanks - sorry, hadn't looked at the actual code.
>

No worries at all! I'd rather double check that this is correct, versus
maybe getting it work. This pass is finicky in that it needs to know
*everything* about IR, and it'll go out of date when new properties are
added! Jason will propose turning it on by default soon, as far as we know
it's now Bug Free (TM) in that all of Chrome compiles and and the tests
pass.


     return Res;
>
>>
>>   // When we have target data, we can reduce the GEP down to the value in
>> bytes
>>   // added to the address.
>>   const DataLayout &DL = FnL->getParent()->getDataLayout();
>>   unsigned BitWidth = DL.getPointerSizeInBits(ASL);
>>   APInt OffsetL(BitWidth, 0), OffsetR(BitWidth, 0);
>>   if (GEPL->accumulateConstantOffset(DL, OffsetL) &&
>>       GEPR->accumulateConstantOffset(DL, OffsetR))
>>     return cmpAPInts(OffsetL, OffsetR);
>>   if (int Res = cmpTypes(GEPL->getSourceElementType(),
>>                          GEPR->getSourceElementType()))
>>     return Res;
>>
>>   if (int Res = cmpNumbers(GEPL->getNumOperands(),
>> GEPR->getNumOperands()))
>>     return Res;
>>
>>   for (unsigned i = 0, e = GEPL->getNumOperands(); i != e; ++i) {
>>     if (int Res = cmpValues(GEPL->getOperand(i), GEPR->getOperand(i)))
>>       return Res;
>>   }
>>
>>   return 0;
>> }
>>
>>
>>
>
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