[llvm-dev] Question about Alias Analysis with restrict keyword
firstname.lastname@example.org via llvm-dev
llvm-dev at lists.llvm.org
Tue Jun 19 04:08:56 PDT 2018
Ah, the recursive phi and gep caused the may-alias... Eli made my brain
re-work. Thank you very much! To be sure, If we add "-basicaa-recphi"
option, the example does not generate may-alias with restrict keyword.
On 19/06/18 10:33, jingu at codeplay.com via llvm-dev wrote:
> Hi Eli,
> I appreciate your good comments. It is very helpful.
> Thanks again,
> JinGu Kang
> On 18/06/18 23:17, Friedman, Eli wrote:
>> On 6/18/2018 8:16 AM, jingu at codeplay.com wrote:
>>> On BasicAAResult::aliasGEP()
>>> 1297 // Check to see if these two pointers are related by the
>>> 1298 // instruction. If one pointer is a GEP with a non-zero
>>> index of the other
>>> 1299 // pointer, we know they cannot alias.
>>> 1301 // If both accesses are unknown size, we can't do anything
>>> useful here.
>>> 1302 if (V1Size == MemoryLocation::UnknownSize &&
>>> 1303 V2Size == MemoryLocation::UnknownSize)
>>> 1304 return MayAlias;
>> The check for UnknownSize is here because it's impossible to
>> disambiguate the offsets of two pointers which both have unknown
>> size, so the rest of BasicAAResult::aliasGEP can't do anything useful.
>>> 1306 AliasResult R = aliasCheck(UnderlyingV1,
>>> 1307 AAMDNodes(), V2,
>>> 1308 V2AAInfo, nullptr, UnderlyingV2);
>>> On line 1306, we can see the V1Size and V2Size are set up with
>>> "MemoryLocation::UnknownSize" and it causes may-alias. On line 1302
>>> and 1033, there are checking code for
>>> "MemoryLocation::UnknownSize"... I am not sure whether it is bug or
>>> not... Can someone let me know why it passes
>>> "MemoryLocation::UnknownSize" please? If I missed something, please
>>> let me know.
>> UnderlyingV1 is the base of the GEP. So this is asking whether the
>> base of the GEP aliases V2. It uses UnknownSize because the offset
>> is unknown; it can't tell which bytes are accessed, so it's using an
>> approximation instead.
>> We could theoretically try to be a bit more clever here if the size
>> is known and the GEP offset is constant: we could instead pass
>> V1Size+GEP1BaseOffset. Not sure that would have much effect in
>> practice, though.
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