[llvm] r251337 - Optimize StringTableBuilder.
Rafael Espíndola via llvm-commits
llvm-commits at lists.llvm.org
Tue Oct 27 05:36:35 PDT 2015
We used to use std::sort. I changed to array_pod sort because I could not
measure a difference when trying to optimize string merging.
Cheers,
Rafael
On Oct 26, 2015 3:24 PM, "Sean Silva via llvm-commits" <
llvm-commits at lists.llvm.org> wrote:
>
>
> On Mon, Oct 26, 2015 at 2:55 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> 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
>>
>
> Ah, neat.
>
>
>>
>>
>>>
>>> Also, did you measure using std::sort? Generally std::sort is generally
>>> faster due to inlining.
>>>
>>
>> I didn't.
>>
>
> Probably worth a shot. Writing a custom sorting algorithm is a pretty
> drastic measure to get a performance boost.
>
> -- Sean Silva
>
>
>>
>>
>>>
>>> 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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