[llvm-dev] [RFC] Introducing the maxobjsize attribute
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Thu Oct 15 16:00:30 PDT 2020
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.
>
> 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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