[llvm] r251337 - Optimize StringTableBuilder.

[Rui Ueyama via llvm-commits]

On Mon, Oct 26, 2015 at 2:47 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Mon, Oct 26, 2015 at 12:58 PM, Rui Ueyama via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: ruiu
>> Date: Mon Oct 26 14:58:29 2015
>> New Revision: 251337
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=251337&view=rev
>> Log:
>> Optimize StringTableBuilder.
>>
>> This is a patch to improve StringTableBuilder's performance. That class'
>> finalize function is very hot particularly in LLD because the function
>> does tail-merge strings in string tables or SHF_MERGE sections.
>>
>> Generic std::sort-style sorter is not efficient for sorting strings.
>> The function implemented in this patch seems to be more efficient.
>>
>
> array_pod_sort uses a qsort-style comparator. Where is the peformance
> difference you are measuring coming from?
>

The new algorithm does not compare common prefixes once we know that that
are common. https://en.wikipedia.org/wiki/Multi-key_quicksort


>
> Also, did you measure using std::sort? Generally std::sort is generally
> faster due to inlining.
>

I didn't.


>
> E.g.
>
> std::sort(&Strings[0], &Strings[0] + Strings.size(), [](StringPair LHS,
> StringPair RHS) {
>   auto rbegin = [](StringPair SP) { return
> std::reverse_iterator(SP.first.end()); };
>   auto rend = [](StringPair SP) { return
> std::reverse_iterator(SP.first.first()); };
>   return std::lexicographical_compare(rbegin(LHS), rend(LHS), rbegin(RHS),
> rend(RHS));
> });
>
> -- Sean Silva
>
>
>
>>
>> Here's a benchmark of LLD to link Clang with or without this patch.
>> The numbers are medians of 50 runs.
>>
>> -O0
>> real 0m0.455s
>> real 0m0.430s (5.5% faster)
>>
>> -O3
>> real 0m0.487s
>> real 0m0.452s (7.2% faster)
>>
>> Since that is a benchmark of the whole linker, the speedup of
>> StringTableBuilder itself is much more than that.
>>
>> http://reviews.llvm.org/D14053
>>
>>
>> Modified:
>>     llvm/trunk/lib/MC/StringTableBuilder.cpp
>>
>> Modified: llvm/trunk/lib/MC/StringTableBuilder.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/StringTableBuilder.cpp?rev=251337&r1=251336&r2=251337&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/MC/StringTableBuilder.cpp (original)
>> +++ llvm/trunk/lib/MC/StringTableBuilder.cpp Mon Oct 26 14:58:29 2015
>> @@ -18,20 +18,47 @@ using namespace llvm;
>>
>>  StringTableBuilder::StringTableBuilder(Kind K) : K(K) {}
>>
>> -static int compareBySuffix(std::pair<StringRef, size_t> *const *AP,
>> -                           std::pair<StringRef, size_t> *const *BP) {
>> -  StringRef A = (*AP)->first;
>> -  StringRef B = (*BP)->first;
>> -  size_t SizeA = A.size();
>> -  size_t SizeB = B.size();
>> -  size_t Len = std::min(SizeA, SizeB);
>> -  for (size_t I = 0; I < Len; ++I) {
>> -    char CA = A[SizeA - I - 1];
>> -    char CB = B[SizeB - I - 1];
>> -    if (CA != CB)
>> -      return CB - CA;
>> +typedef std::pair<StringRef, size_t> StringPair;
>> +
>> +// Returns the character at Pos from end of a string.
>> +static int charTailAt(StringPair *P, size_t Pos) {
>> +  StringRef S = P->first;
>> +  if (Pos >= S.size())
>> +    return -1;
>> +  return (unsigned char)S[S.size() - Pos - 1];
>> +}
>> +
>> +// Three-way radix quicksort. This is much faster than std::sort with
>> strcmp
>> +// because it does not compare characters that we already know the same.
>> +static void qsort(StringPair **Begin, StringPair **End, int Pos) {
>> +tailcall:
>> +  if (End - Begin <= 1)
>> +    return;
>> +
>> +  // Partition items. Items in [Begin, P) are greater than the pivot,
>> +  // [P, Q) are the same as the pivot, and [Q, End) are less than the
>> pivot.
>> +  int Pivot = charTailAt(*Begin, Pos);
>> +  StringPair **P = Begin;
>> +  StringPair **Q = End;
>> +  for (StringPair **R = Begin + 1; R < Q;) {
>> +    int C = charTailAt(*R, Pos);
>> +    if (C > Pivot)
>> +      std::swap(*P++, *R++);
>> +    else if (C < Pivot)
>> +      std::swap(*--Q, *R);
>> +    else
>> +      R++;
>> +  }
>> +
>> +  qsort(Begin, P, Pos);
>> +  qsort(Q, End, Pos);
>> +  if (Pivot != -1) {
>> +    // qsort(P, Q, Pos + 1), but with tail call optimization.
>> +    Begin = P;
>> +    End = Q;
>> +    ++Pos;
>> +    goto tailcall;
>>    }
>> -  return SizeB - SizeA;
>>  }
>>
>>  void StringTableBuilder::finalize() {
>> @@ -40,7 +67,8 @@ void StringTableBuilder::finalize() {
>>    for (std::pair<StringRef, size_t> &P : StringIndexMap)
>>      Strings.push_back(&P);
>>
>> -  array_pod_sort(Strings.begin(), Strings.end(), compareBySuffix);
>> +  if (!Strings.empty())
>> +    qsort(&Strings[0], &Strings[0] + Strings.size(), 0);
>>
>>    switch (K) {
>>    case RAW:
>>
>>
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>>
>
>
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