[llvm] r259419 - SmallSet/SmallPtrSet: Refuse huge Small numbers

[Matthias Braun via llvm-commits]

As this was impossible as compiletime (except that compiletime benefitted in the <1% range for replacing some Small==128 users). To get at 32 I looked at the instruction count reported by valgrind/callgrind which includes the instruction for the linear search and the instructions spend in libc malloc and that was the best cutoff point overall.
I have no problem setting this higher as my measurements may very well be dependent on the malloc implementation and cache effects.

- Matthias

> On Feb 1, 2016, at 3:52 PM, Philip Reames via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Having a cutoff seems entirely reasonable, but why 32?  Linear search performs surprisingly well and avoiding heap allocation can be valuable.  How'd you come up with the threshold chosen?
> 
> Philip
> 
> On 02/01/2016 02:05 PM, Matthias Braun via llvm-commits wrote:
>> Author: matze
>> Date: Mon Feb  1 16:05:16 2016
>> New Revision: 259419
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=259419&view=rev
>> Log:
>> SmallSet/SmallPtrSet: Refuse huge Small numbers
>> 
>> These sets do linear searching in small mode; It is not a good idea to
>> use huge numbers as the small value here, save people from themselves by
>> adding a static_assert.
>> 
>> Differential Revision: http://reviews.llvm.org/D16706
>> 
>> Modified:
>>     llvm/trunk/include/llvm/ADT/SmallPtrSet.h
>>     llvm/trunk/include/llvm/ADT/SmallSet.h
>>     llvm/trunk/lib/Target/CppBackend/CPPBackend.cpp
>> 
>> Modified: llvm/trunk/include/llvm/ADT/SmallPtrSet.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/SmallPtrSet.h?rev=259419&r1=259418&r2=259419&view=diff
>> ==============================================================================
>> --- llvm/trunk/include/llvm/ADT/SmallPtrSet.h (original)
>> +++ llvm/trunk/include/llvm/ADT/SmallPtrSet.h Mon Feb  1 16:05:16 2016
>> @@ -329,6 +329,11 @@ public:
>>  /// SmallPtrSetImplBase for details of the algorithm.
>>  template<class PtrType, unsigned SmallSize>
>>  class SmallPtrSet : public SmallPtrSetImpl<PtrType> {
>> +  // In small mode SmallPtrSet uses linear search for the elements, so it is
>> +  // not a good idea to choose this value too high. You may consider using a
>> +  // DenseSet<> instead if you expect many elements in the set.
>> +  static_assert(SmallSize <= 32, "SmallSize should be small");
>> +
>>    typedef SmallPtrSetImpl<PtrType> BaseT;
>>      // Make sure that SmallSize is a power of two, round up if not.
>> 
>> Modified: llvm/trunk/include/llvm/ADT/SmallSet.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/SmallSet.h?rev=259419&r1=259418&r2=259419&view=diff
>> ==============================================================================
>> --- llvm/trunk/include/llvm/ADT/SmallSet.h (original)
>> +++ llvm/trunk/include/llvm/ADT/SmallSet.h Mon Feb  1 16:05:16 2016
>> @@ -38,6 +38,11 @@ class SmallSet {
>>    typedef typename SmallVector<T, N>::const_iterator VIterator;
>>    typedef typename SmallVector<T, N>::iterator mutable_iterator;
>>  +  // In small mode SmallPtrSet uses linear search for the elements, so it is
>> +  // not a good idea to choose this value too high. You may consider using a
>> +  // DenseSet<> instead if you expect many elements in the set.
>> +  static_assert(N <= 32, "N should be small");
>> +
>>  public:
>>    typedef size_t size_type;
>>    SmallSet() {}
>> 
>> Modified: llvm/trunk/lib/Target/CppBackend/CPPBackend.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/CppBackend/CPPBackend.cpp?rev=259419&r1=259418&r2=259419&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Target/CppBackend/CPPBackend.cpp (original)
>> +++ llvm/trunk/lib/Target/CppBackend/CPPBackend.cpp Mon Feb  1 16:05:16 2016
>> @@ -1651,8 +1651,8 @@ void CppWriter::printFunctionUses(const
>>      // Print type definitions for every type referenced by an instruction and
>>    // make a note of any global values or constants that are referenced
>> -  SmallPtrSet<GlobalValue*,64> gvs;
>> -  SmallPtrSet<Constant*,64> consts;
>> +  SmallPtrSet<GlobalValue*,32> gvs;
>> +  SmallPtrSet<Constant*,32> consts;
>>    for (Function::const_iterator BB = F->begin(), BE = F->end();
>>         BB != BE; ++BB){
>>      for (BasicBlock::const_iterator I = BB->begin(), E = BB->end();
>> 
>> 
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
> 
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits