[llvm-dev] Question about Alias Analysis with restrict keyword
email@example.com via llvm-dev
llvm-dev at lists.llvm.org
Tue Jun 19 02:33:57 PDT 2018
I appreciate your good comments. It is very helpful.
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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