[llvm-dev] [RFC] Introducing the maxobjsize attribute

Johannes Doerfert via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 15 18:30:39 PDT 2020


On 10/15/20 6:00 PM, Philip Reames via llvm-dev wrote:
>
> On 10/13/20 9:35 AM, Atmn Patel via llvm-dev wrote:
>> Hi All,
>>
>> We've prepared a new attribute `maxobjsize(<n>)` that tracks the 
>> maximum size of the object that a pointer points to. This attribute 
>> will be deduced through the Attributor framework and it is used for 
>> aliasing queries. The `maxobjsize` of an object, and number of 
>> `dereferenceable` bytes can be used as upper and lower bounds on the 
>> object size, and if there is no overlap, we can determine that the 
>> underlying objects cannot alias. Basically, an object that is at most 
>> N bytes long is not aliasing one that is at least N+1 bytes long.
>
> This is commingling two separate concerns.  At minimum, a wording 
> clarification is needed, it's possible the proposed use case does not 
> work.
>
> Deferenceability is the amount of space which can be accessed without 
> a runtime fault.  Of key importance is the dereferenceability is 
> disconnected from object size.  There may be space beyond an object 
> which is dereferenceable, but outside the object.
>
> As a simple example, imagine an allocator which allocates 32 byte 
> blocks of memory, 32 byte aligned.  If the actual object allocated is 
> only 16 bytes, the pointer is still known to be 32 byte aligned and 
> deref for 32 bytes.  The contents past the object are simply unspecified.
>
> Saying that a 32 byte derefenceable pointer doesn't alias one with a 
> maximum object size of 16 bytes would be wrong and lead to miscompiles 
> in practice.

We already perform exactly this deduction in BasicAA right now, except 
that max object size is not made explicit.
You just assumed the allocated object is 16 bytes and therefore it will 
imply `maxobjsize(16)` while the underlying memory region is 
`dereferenceable(32)`.
You cannot have both in our object-driven model.

~ Johannes


>>
>> These changes are in:
>> - D87975 - [IR] Introduce MaxObjSize Attribute
>> - D87978 - [Attributor] Adds deduction for the MaxObjSize Attribute
>> - D88353 - [BasicAA] Integrate MaxobjSize for NoAlias
>>
>> These are the Statistics changes for CTMark *without* the actual 
>> deduction (https://reviews.llvm.org/D88353#2301597):
>> CHANGED: branch-folder                NumHoist              438 ->   
>>      431 (  -1.598%)
>> CHANGED: codegenprepare               NumBlocksElim                   
>> 16093 ->      15885 (  -1.292%)
>> CHANGED: codegenprepare               NumExtsMoved                 
>> 6373 ->       6439 (  +1.036%)
>> CHANGED: gvn  IsValueFullyAvailableInBlockNumSpeculationsMax   6746 
>> ->       6858 (    +1.660%)
>> CHANGED: gvn                          NumGVNInstr                 
>> 78434 ->      79330 (  +1.142%)
>> CHANGED: instcombine                  NumReassoc              22830 
>> ->      23213 (  +1.678%)
>> CHANGED: instsimplify                 NumSimplified                   
>> 21278 ->      21495 (  +1.020%)
>> CHANGED: licm                         NumPromoted                   
>> 407 ->        497 ( +22.113%)
>> CHANGED: loop-rotate  NumNotRotatedDueToHeaderSize             37 -> 
>>         35 (    -5.405%)
>> CHANGED: loop-simplify                NumNested                 126 
>> ->        128 (  +1.587%)
>> CHANGED: machinelicm                  NumPostRAHoisted               
>>        131 ->        134 (  +2.290%)
>> CHANGED: memory-builtins              ObjectVisitorLoad               
>>         96077 ->      97496 (  +1.477%)
>> CHANGED: regalloc                     NumDCEFoldedLoads               
>>            38 ->         37 (  -2.632%)
>> CHANGED: regalloc                     NumLaneConflicts               
>>       4408 ->       4332 (  -1.724%)
>> CHANGED: regalloc                     NumReloadsRemoved               
>>          1062 ->       1050 (  -1.130%)
>> CHANGED: regalloc                     NumSnippets                 
>>  1168 ->       1152 (  -1.370%)
>> CHANGED: regalloc                     NumSpillsRemoved               
>>        672 ->        665 (  -1.042%)
>> CHANGED: stack-slot-coloring          NumDead                14 ->   
>>       18 ( +28.571%)
>> CHANGED: twoaddressinstruction        NumConvertedTo3Addr             
>>             27054 ->      26695 (  -1.327%)
>>
>> These are the Statistic Changes in CTMark w/O3 before/after these 
>> patches (https://reviews.llvm.org/D87978#2307622):
>> CHANGED: codegenprepare               NumExtsMoved                 
>> 3631 ->       3699 (  +1.873%)
>> CHANGED: dse                          NumFastOther                 
>>  192 ->        194 (  +1.042%)
>> CHANGED: gvn  IsValueFullyAvailableInBlockNumSpeculationsMax   4958 
>> ->       5060 (    +2.057%)
>> CHANGED: gvn                          NumGVNInstr                 
>> 46657 ->      47534 (  +1.880%)
>> CHANGED: jump-threading               NumDupes               91 ->   
>>       92 (  +1.099%)
>> CHANGED: licm                         NumMovedLoads                   
>>  6272 ->       6344 (  +1.148%)
>> CHANGED: licm                         NumPromoted                   
>> 381 ->        438 ( +14.961%)
>> CHANGED: loop-rotate  NumNotRotatedDueToHeaderSize             31 -> 
>>         29 (    -6.452%)
>> CHANGED: machinelicm                  NumPostRAHoisted               
>>         88 ->         89 (  +1.136%)
>> CHANGED: memdep                       NumCacheNonLocalPtr             
>>           1005887 ->    1016671 (  +1.072%)
>> CHANGED: memory-builtins              ObjectVisitorLoad               
>>         62048 ->      63473 (  +2.297%)
>> CHANGED: peephole-opt                 NumCmps               532 ->   
>>      526 (  -1.128%)
>> CHANGED: regalloc                     NumDCEFoldedLoads               
>>            27 ->         26 (  -3.704%)
>> CHANGED: regalloc                     NumLocalSplits                 
>>   1891 ->       1870 (  -1.111%)
>>
>> Feedback Welcome.
>>
>> Atmn and Johannes
>>
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