[llvm-dev] Question about Alias Analysis with restrict keyword

[Friedman, Eli via llvm-dev]

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 
> getelementptr
> 1298     // instruction.  If one pointer is a GEP with a non-zero 
> index of the other
> 1299     // pointer, we know they cannot alias.
> 1300
> 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.

> 1305
> 1306     AliasResult R = aliasCheck(UnderlyingV1, 
> MemoryLocation::UnknownSize,
> 1307                                AAMDNodes(), V2, 
> MemoryLocation::UnknownSize,
> 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.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project