[cfe-commits] [Patch] -Wduplicate-enum which fixes PR6343
Richard Trieu
rtrieu at google.com
Tue Aug 28 15:53:30 PDT 2012
Timing information:
Three clangs were run, clang with no changes (control), duplicate enum with
PointerUnion (most recent patch), duplicate enum with DenseMap without
PointerUnion (next most recent patch). Each run with -fsyntax-only and
-Wduplicate-enum for modified clangs. Runs 1, 2, and 3 are files with only
enums. Run 4 is a preprocessed Clang.
Key:
name: Average (Min-Max)
Run1:
Control: 13.763 (13.66-14.14)
PointerUnion: 14.046 (13.94-14.16)
DenseMap: 14.304 (14.24-14.39)
Run2:
Control: 20.189 (20.1-20.31)
PointerUnion: 20.514 (20.37-20.6)
DenseMap: 20.635 (20.56-20.7)
Run3:
Control: 26.715 (26.66-26.8)
PointerUnion: 26.928 (26.8-27.12)
DenseMap: 27.13 (27.05-27.22)
Run4:
Control: 29.686 (28.98-30.39)
PointerUnion: 29.706 (28.73-30.69)
DenseMap: 29.952 (29.3-30.63)
On Tue, Aug 28, 2012 at 2:12 PM, Ted Kremenek <kremenek at apple.com> wrote:
> How fast? (particularly compared to the first implementation, and
> relative to -fsyntax-only)
>
> On Aug 28, 2012, at 11:48 AM, Richard Trieu <rtrieu at google.com> wrote:
>
> New patch with PointerUnion and DenseMap is slightly faster than the
> previous DenseMap patch.
>
> On Mon, Aug 27, 2012 at 9:51 PM, Ted Kremenek <kremenek at apple.com> wrote:
>
>> Thanks! Quick question before I review it in more details: what is the
>> performance characteristics of this patch compared to the others?
>>
>> On Aug 27, 2012, at 11:35 AM, Richard Trieu <rtrieu at google.com> wrote:
>>
>> Incorporated most of the suggestions into this patch. Still using a
>> double pass over the constants for the reasons outlined below.
>>
>> On Fri, Aug 17, 2012 at 10:00 PM, Ted Kremenek <kremenek at apple.com>wrote:
>>
>>> BTW, I wrote this two days ago. For some reason my mail client didn't
>>> send it out until now. My apologies for the delay.
>>>
>> No worries. Had some issues that cropped up on template diffing that
>> took my time.
>>
>>>
>>> On Aug 15, 2012, at 10:11 PM, Ted Kremenek <kremenek at apple.com> wrote:
>>>
>>> On Aug 15, 2012, at 6:12 PM, Richard Trieu <rtrieu at google.com> wrote:
>>>
>>> On Tue, Aug 14, 2012 at 9:48 PM, Ted Kremenek <kremenek at apple.com>wrote:
>>>
>>>> On Aug 14, 2012, at 2:32 PM, Richard Trieu <rtrieu at google.com> wrote:
>>>>
>>>> At a high level, I honestly find this logic to be more complicated
>>>>> than I would have expected. The sorting seems unnecessary, and will report
>>>>> diagnostics in an unnatural order (first based on enum constant value, then
>>>>> on declaration order). A straight linear pass seems more naturally to me,
>>>>> and DenseMap is very efficient.
>>>>>
>>>> Is there a comparison between the different containers in LLVM and the
>>>> STL containers?
>>>>
>>>>
>>>> This is a reasonable place to start:
>>>>
>>>> http://llvm.org/docs/ProgrammersManual.html#ds_map
>>>>
>>>> The key with DenseMap is that it is probed hashtable. There is one big
>>>> allocation for the entire table, instead of a bunch of buckets. When
>>>> applicable, it can be very fast, and feels like the right data structure to
>>>> use here.
>>>>
>>>
>>> Duplicate enum detection, now with DenseMap. The DenseMap maps a
>>> int64_t to a vector pointer. 0 and 1 were special keys for the DenseMap,
>>> so two separate pointers special cased for them. The vectors pointers are
>>> stored in another vector in declaration order. One pass is made over the
>>> enums to find ones without initializers. These are used to create vectors.
>>> A second pass through the enums populates the vectors. Finally, a pass
>>> over the vector of vectors is used to generate all the warnings and notes.
>>>
>>> Run time is fairly consistent with the sorted vector implementation,
>>> which is max %3 difference against control.
>>> <duplicate-enum-densemap.patch>
>>>
>>>
>>> Thanks for working on this. My main concern is this patch now has a lot
>>> of unnecessary malloc() traffic, which will certainly slow it down.
>>> Comments inline:
>>>
>>> +
>>> +static int64_t GetInt64(const llvm::APSInt& Val) {
>>> + return Val.isSigned() ? Val.getSExtValue() : Val.getZExtValue();
>>> +}
>>> +
>>> +struct DenseMapInfoint64_t {
>>> + static int64_t getEmptyKey() { return 0; }
>>> + static int64_t getTombstoneKey() { return 1; }
>>> + static unsigned getHashValue(const int64_t Val) {
>>> + return (unsigned)(Val * 37);
>>> + }
>>> + static bool isEqual(const int64_t& LHS, const int64_t& RHS) {
>>> + return LHS == RHS;
>>> + }
>>> +};
>>>
>>>
>>> This trait class doesn't look like it was actually used. The DenseMap
>>> below just uses the default trait for int64_t.
>>>
>>> I also still think we can so something a bit smarter here. What I think
>>> we need to distinguish between is whether or not a constant has appeared
>>> more than once. We're saving a bit of memory on the keys, but spending
>>> that savings elsewhere when we allocate the vectors unconditionally for
>>> each constant.
>>>
>>> +
>>> +// Emits a warning when an element is implicitly set a value that
>>> +// a previous element has already been set to.
>>> +static void CheckForDuplicateEnumValues(Sema &S, Decl **Elements,
>>> + unsigned NumElements, EnumDecl
>>> *Enum,
>>> + QualType EnumType) {
>>> + if (S.Diags.getDiagnosticLevel(diag::warn_duplicate_enum_values,
>>> + Enum->getLocation()) ==
>>> + DiagnosticsEngine::Ignored)
>>> + return;
>>> + // Avoid anonymous enums
>>> + if (!Enum->getIdentifier())
>>> + return;
>>> +
>>> + // Only check for small enums.
>>> + if (Enum->getNumPositiveBits() > 63 || Enum->getNumNegativeBits() >
>>> 64)
>>> + return;
>>> +
>>> + typedef llvm::SmallVector<EnumConstantDecl*, 4> SameValueVector;
>>> + typedef llvm::DenseMap<int64_t, SameValueVector*> ValueToVectorMap;
>>> + typedef llvm::SmallVector<SameValueVector*, 10> DoubleVector;
>>> + ValueToVectorMap EnumMap;
>>> + DoubleVector EnumVector;
>>> + SameValueVector *ZeroVector = 0, *OneVector = 0;
>>>
>>>
>>> It took me a while to understand what this was doing, so I feel it could
>>> really benefit from a comment. This also appears to result in a ton of
>>> malloc traffic below. Here's my suggestion:
>>>
>>> typedef llvm::SmallVector<EnumConstantDecl*, 3> ECDVector;
>>> typedef llvm::SmallVector<ECDVector *, 3> DuplicatesVector;
>>>
>>> typedef llvm::PointerUnion<EnumConstantDecl*, ECDVector *>
>>> DeclOrVector;
>>> typedef llvm::DenseMap<int64_t, DeclOrVector> ValueToVectorMap;
>>>
>>> DuplicatesVector DupVector;
>>> ValueToVectorMap EnumMap;
>>>
>>> The trick here is that the DenseMap maps from a constant to the first
>>> EnumConstantDecl it encounters. Only if we encounter a second
>>> EnumConstantDecl with the same enum value do we pay the cost of allocating
>>> another vector. This will drastically optimize in the common case, as
>>> calling malloc() is really slow. Right now the code appears to be doing a
>>> malloc() for every enum constant, which is going to really penalize us here.
>>>
>>> +
>>> + for (unsigned i = 0; i < NumElements; ++i) {
>>> + EnumConstantDecl *ECD = cast<EnumConstantDecl>(Elements[i]);
>>> + if (!ECD) {
>>> + for (DoubleVector::iterator I = EnumVector.begin(), E =
>>> EnumVector.end();
>>> + I != E; ++I)
>>> + delete *I;
>>> + return;
>>> + }
>>>
>>>
>>> I don't quite understand this loop through DoubleVector here, but it
>>> looks like logic in case we want to return early and cleanup. Is there a
>>> case where the EnumConstantDecl can be null?
>>>
>>> According to ActOnEnumBody, EnumConstantDecl is null if a diagnostic has
>> previously been emitted for the constant. Since the enum
>> is possibly ill-formed, skip checking it.
>>
>>>
>>> +
>>> + if (ECD->getInitExpr())
>>> + continue;
>>> +
>>> + int64_t Val = GetInt64(ECD->getInitVal());
>>> +
>>>
>>>
>>> Looks good.
>>>
>>> + if (Val == 0) {
>>> + if (ZeroVector) continue;
>>> + ZeroVector = new SameValueVector();
>>> + ZeroVector->push_back(ECD);
>>> + EnumVector.push_back(ZeroVector);
>>> + } else if (Val == 1) {
>>> + if (OneVector) continue;
>>> + OneVector = new SameValueVector();
>>> + OneVector->push_back(ECD);
>>> + EnumVector.push_back(OneVector);
>>> + } else {
>>> + if (EnumMap.find(Val) != EnumMap.end())
>>> + continue;
>>> + SameValueVector *ValueVector = new SameValueVector();
>>> + ValueVector->push_back(ECD);
>>> + EnumVector.push_back(ValueVector);
>>> + EnumMap.insert(std::make_pair(Val, ValueVector));
>>>
>>>
>>> The "find()" followed by the "insert()" is wasteful. It results in two
>>> lookups to the hash table when we could have just used one. More on that
>>> later.
>>>
>>> + }
>>> + }
>>>
>>>
>>> IMO, this looks like a lot of complexity just to handle the fact that 0
>>> and 1 are special values for the DenseMap. I don't really see this as the
>>> right tradeoff; the code is more complicated with marginal impact on memory
>>> usage or performance.
>>>
>>> If you humor me for a bit, consider using something else for the key,
>>> e.g.:
>>>
>>> struct DupKey {
>>> int64_t val;
>>> bool isTombstoneOrEmptyKey;
>>> };
>>>
>>> The idea is if 'isTombStoneOrEmptyKey' is true, we can use val = 0 or
>>> val = 1 to represent empty keys or tombstone entries. Otherwise, it's an
>>> int64_t, with the full range of values. We can define a DenseMap trait to
>>> do the right thing. Yes, this costs a tiny bit more in storage, but it
>>> allows the data structure to handle the complete set of values in your
>>> domain, instead of resorting to complicating the core algorithm. What I
>>> see here now is the same code essentially duplicated twice, which makes it
>>> harder to read and more error prone.
>>>
>>> If we use DupKey as our key for the DenseMap, we can instead do
>>> something like this:
>>>
>>> DeclOrVector &entry = EnumMap[Val]; // Use default construction of
>>> 'entry'.
>>> // Is the first time we encountered this constant?
>>> if (entry.isNull()) {
>>> entry = ECD;
>>> continue;
>>> }
>>> // Is this the second time we encountered this constant? If so,
>>> // push the previous decl encountered and the one just encountered
>>> // to a vector of duplicates.
>>> if (EnumConstantDecl *D = entry.dyn_cast<EnumConstantDecl*>()) {
>>> ECDVector *Vec = new ECDVector();
>>> Vec->push_back(D);
>>> Vec->push_back(ECD);
>>>
>>> // Update the entry to refer to the duplicates.
>>> entry = Vec;
>>>
>>> // Store the duplicates in a vector we can consult later for
>>> // quick emission of diagnostics.
>>> DupVector.push_back(Vec);
>>>
>>> // On to the next constant.
>>> continue;
>>> }
>>> // Is this the third (or greater) time we encountered the constant?
>>> If so,
>>> // continue to add it to the existing vector.
>>> ECDVector *Vec = entry.get<ECDVector*>();
>>> Vec->push_back(ECD);
>>>
>>>
>>> With this code, we only allocate memory (beyond the DenseMap) when we
>>> encounter a duplicate that would be worth reporting. In the common case,
>>> this savings in malloc traffic should be noticeable.
>>>
>>> Notice also that I used:
>>>
>>> DeclOrVector &entry = EnumMap[Val]; // Use default construction of
>>> 'entry'.
>>>
>>> This results in a single lookup in the hashtable. Since we plan on
>>> adding a value for a key no matter what, by using this idiom we allow the
>>> DenseMap to default construct an entry if it doesn't exist. This results
>>> in a single hashtable lookup, from which we can modify the value in place.
>>> This is obviously faster than doing a hashtable lookup twice.
>>>
>>> +
>>> + for (unsigned i = 0; i < NumElements; ++i) {
>>> + EnumConstantDecl *ECD = cast<EnumConstantDecl>(Elements[i]);
>>> + if (!ValidDuplicateEnum(ECD, Enum))
>>> + continue;
>>> +
>>> + int64_t Val = GetInt64(ECD->getInitVal());
>>> +
>>> + if (Val == 0) {
>>> + if (!ZeroVector || *ZeroVector->begin() == ECD)
>>> + continue;
>>> + ZeroVector->push_back(ECD);
>>> + } else if (Val == 1) {
>>> + if (!OneVector || *OneVector->begin() == ECD)
>>> + continue;
>>> + OneVector->push_back(ECD);
>>> + } else {
>>> + ValueToVectorMap::iterator I = EnumMap.find(Val);
>>> + if (I == EnumMap.end())
>>> + continue;
>>> + SameValueVector *V = I->second;
>>> + if (*V->begin() == ECD)
>>> + continue;
>>> + V->push_back(ECD);
>>> + }
>>> + }
>>>
>>>
>>> This second loop looks unnecessary. I think we can do everything we
>>> need to count duplicates with one loop. Of course the ValidDuplicateEnum()
>>> would need to be hoisted to the first loop.
>>>
>>> Using two traverses allows two things to happen. One, the first element
>> in the ECDVector will not have an initializer and will work with the
>> warning. Otherwise, the vector needs to be searched for a proper enum
>> constant to use. Two, it prevents unneeded creation of ECDVectors. If we
>> have enum A { A1 = 2, A2 = 2, A3 = 1, A4 = 1, A5}; vectors for values 1 and
>> 2 are created using a single pass while only a vector for 2 will be created
>> using a double pass.
>>
>>>
>>> +
>>> + for (DoubleVector::iterator DoubleVectorIter = EnumVector.begin(),
>>> + DoubleVectorEnd = EnumVector.end();
>>> + DoubleVectorIter != DoubleVectorEnd; ++DoubleVectorIter) {
>>> + SameValueVector *V = *DoubleVectorIter;
>>> + if (V->size() == 1)
>>> + continue;
>>> +
>>> + SameValueVector::iterator I = V->begin();
>>> + S.Diag((*I)->getLocation(), diag::warn_duplicate_enum_values)
>>> + << (*I)->getName() << (*I)->getInitVal().toString(10)
>>> + << (*I)->getSourceRange();
>>> + ++I;
>>> + for (SameValueVector::iterator E = V->end(); I != E; ++I)
>>> + S.Diag((*I)->getLocation(), diag::note_duplicate_element)
>>> + << (*I)->getName() << (*I)->getInitVal().toString(10)
>>> + << (*I)->getSourceRange();
>>> + delete V;
>>> + }
>>>
>>>
>>>
>>> This is more or less the same, essentially it becomes:
>>>
>>> for (DuplicateVector::iterator I = DupVector.begin(), E =
>>> DupVector.end(); I != E; ++I) {
>>> ECDVector *Vec = *I;
>>> // do the diagnostic logic ...
>>> delete *I;
>>> }
>>>
>>> Note that with my suggestions the vector has size on order of the number
>>> of duplicate constants, not the number of total constants. If there are no
>>> duplicates, no work is required (including free'ing memory).
>>>
>>> +}
>>> +
>>> void Sema::ActOnEnumBody(SourceLocation EnumLoc, SourceLocation
>>> LBraceLoc,
>>> SourceLocation RBraceLoc, Decl *EnumDeclX,
>>> Decl **Elements, unsigned NumElements,
>>> @@ -10709,6 +10868,7 @@
>>> DeclsInPrototypeScope.push_back(Enum);
>>>
>>> CheckForUniqueEnumValues(*this, Elements, NumElements, Enum,
>>> EnumType);
>>> + CheckForDuplicateEnumValues(*this, Elements, NumElements, Enum,
>>> EnumType);
>>> }
>>>
>>> Decl *Sema::ActOnFileScopeAsmDecl(Expr *expr,
>>>
>>>
>>> I know this may all be nit-picky, but I really think trying to reduce
>>> the malloc() traffic is worth looking at to get a real understanding of the
>>> performance improvement that can be found here.
>>>
>>> Thanks for forging ahead on this.
>>> _______________________________________________
>>> cfe-commits mailing list
>>> cfe-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>>>
>>>
>>>
>> <duplicate-enum-densemap2.patch>
>>
>>
>>
>
>
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