[lld] r247387 - COFF: Teach ICF to merge cyclic graphs.
Rui Ueyama via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 10 22:14:22 PDT 2015
Maybe not. scc_iterator seems to fit here nicely. I'll update the code.
On Thu, Sep 10, 2015 at 10:00 PM, Sean Silva <chisophugis at gmail.com> wrote:
> Is there a reason you're not using scc_iterator?
>
> -- Sean Silva
>
> On Thu, Sep 10, 2015 at 9:29 PM, Rui Ueyama via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: ruiu
>> Date: Thu Sep 10 23:29:03 2015
>> New Revision: 247387
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=247387&view=rev
>> Log:
>> COFF: Teach ICF to merge cyclic graphs.
>>
>> Previously, LLD's ICF couldn't merge cyclic graphs. That was unfortunate
>> because, in COFF, cyclic graphs are not exceptional at all. That is
>> pretty common.
>>
>> In this patch, sections are grouped by Tarjan's strongly connected
>> component algorithm to get acyclic graphs. And then we try to merge
>> SCCs whose outdegree is zero, and remove them from the graph. This
>> makes other SCCs to have outdegree zero, so we can repeat the
>> process until all SCCs are removed. When comparing two SCCs, we handle
>> cycles properly.
>>
>> This algorithm works better than previous one. Previously, self-linking
>> produced a 29.0MB executable. It now produces a 27.7MB. There's still some
>> gap compared to MSVC linker which produces a 27.1MB executable for the
>> same input. So the gap is narrowed, but still LLD is not on par with MSVC.
>> I'll investigate that later.
>>
>> Modified:
>> lld/trunk/COFF/Chunks.h
>> lld/trunk/COFF/ICF.cpp
>> lld/trunk/test/COFF/icf-circular.test
>>
>> Modified: lld/trunk/COFF/Chunks.h
>> URL:
>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Chunks.h?rev=247387&r1=247386&r2=247387&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/COFF/Chunks.h (original)
>> +++ lld/trunk/COFF/Chunks.h Thu Sep 10 23:29:03 2015
>> @@ -111,6 +111,18 @@ protected:
>> uint32_t Align = 1;
>> };
>>
>> +class SectionChunk;
>> +
>> +// A container of SectionChunks. Used by ICF to store computation
>> +// results of strongly connected components. You can ignore this
>> +// unless you are interested in ICF.
>> +struct Component {
>> + Component(std::vector<SectionChunk *> V) : Members(V) {}
>> + std::vector<SectionChunk *> Members;
>> + std::vector<Component *> Predecessors;
>> + int Outdegree = 0;
>> +};
>> +
>> // A chunk corresponding a section of an input file.
>> class SectionChunk : public Chunk {
>> public:
>> @@ -182,12 +194,15 @@ public:
>> // with other chunk by ICF, it points to another chunk,
>> // and this chunk is considrered as dead.
>> SectionChunk *Ptr;
>> - int Outdegree = 0;
>> - std::vector<SectionChunk *> Ins;
>> + uint32_t Index = 0;
>> + uint32_t LowLink = 0;
>> + bool OnStack = false;
>> + Component *SCC = nullptr;
>>
>> // The CRC of the contents as described in the COFF spec 4.5.5.
>> // Auxiliary Format 5: Section Definitions. Used for ICF.
>> uint32_t Checksum = 0;
>> + mutable uint64_t Hash = 0;
>>
>> private:
>> ArrayRef<uint8_t> getContents() const;
>>
>> Modified: lld/trunk/COFF/ICF.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/ICF.cpp?rev=247387&r1=247386&r2=247387&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/COFF/ICF.cpp (original)
>> +++ lld/trunk/COFF/ICF.cpp Thu Sep 10 23:29:03 2015
>> @@ -7,7 +7,31 @@
>> //
>>
>> //===----------------------------------------------------------------------===//
>> //
>> -// Implements ICF (Identical COMDAT Folding)
>> +// Identical COMDAT Folding is a feature to merge COMDAT sections not by
>> +// name (which is regular COMDAT handling) but by contents. If two COMDAT
>> +// sections have the same data, relocations, attributes, etc., then the
>> two
>> +// are considered identical and merged by the linker. This optimization
>> +// makes outputs smaller.
>> +//
>> +// ICF is theoretically a problem of reducing graphs by merging as many
>> +// isomorphic subgraphs as possible, if we consider sections as vertices
>> and
>> +// relocations as edges. This may be a bit more complicated problem than
>> you
>> +// might think. The order of processing sections matters since merging
>> two
>> +// sections can make other sections, whose relocations now point to the
>> +// section, mergeable. Graphs may contain cycles, which is common in
>> COFF.
>> +// We need a sophisticated algorithm to do this properly and efficiently.
>> +//
>> +// What we do in this file is this. We first compute strongly connected
>> +// components of the graphs to get acyclic graphs. Then, we remove SCCs
>> whose
>> +// outdegree is zero from the graphs and try to merge them. This
>> operation
>> +// makes other SCCs to have outdegree zero, so we repeat the process
>> until
>> +// all SCCs are removed.
>> +//
>> +// This algorithm is different from what GNU gold does which is
>> described in
>> +// http://research.google.com/pubs/pub36912.html. I don't know which is
>> +// faster, this or Gold's, in practice. It'd be interesting to implement
>> the
>> +// other algorithm to compare. Note that the gold's algorithm cannot
>> handle
>> +// cycles, so we need to tweak it, though.
>> //
>>
>> //===----------------------------------------------------------------------===//
>>
>> @@ -15,6 +39,10 @@
>> #include "Symbols.h"
>> #include "llvm/ADT/Hashing.h"
>> #include "llvm/ADT/STLExtras.h"
>> +#include "llvm/Support/Debug.h"
>> +#include "llvm/Support/raw_ostream.h"
>> +#include <algorithm>
>> +#include <functional>
>> #include <tuple>
>> #include <unordered_set>
>> #include <vector>
>> @@ -37,15 +65,156 @@ struct Equals {
>>
>> } // anonymous namespace
>>
>> +// Invoke Fn for each live COMDAT successor sections of SC.
>> +static void forEach(SectionChunk *SC, std::function<void(SectionChunk
>> *)> Fn) {
>> + for (SectionChunk *C : SC->children())
>> + Fn(C);
>> + for (SymbolBody *B : SC->symbols()) {
>> + if (auto *D = dyn_cast<DefinedRegular>(B)) {
>> + SectionChunk *C = D->getChunk();
>> + if (C->isCOMDAT() && C->isLive())
>> + Fn(C);
>> + }
>> + }
>> +}
>> +
>> +typedef std::vector<Component *>::iterator ComponentIterator;
>> +
>> +// Try to merge two SCCs, A and B. A and B are likely to be isomorphic
>> +// because all sections have the same hash values.
>> +static void tryMerge(std::vector<SectionChunk *> &A,
>> + std::vector<SectionChunk *> &B) {
>> + // Assume that relocation targets are the same.
>> + size_t End = A.size();
>> + for (size_t I = 0; I != End; ++I) {
>> + assert(B[I] == B[I]->Ptr);
>> + B[I]->Ptr = A[I];
>> + }
>> + for (size_t I = 0; I != End; ++I) {
>> + if (A[I]->equals(B[I]))
>> + continue;
>> + // If we reach here, the assumption was wrong. Reset the pointers
>> + // to the original values and terminate the comparison.
>> + for (size_t I = 0; I != End; ++I)
>> + B[I]->Ptr = B[I];
>> + return;
>> + }
>> + // If we reach here, the assumption was correct. Actually replace them.
>> + for (size_t I = 0; I != End; ++I)
>> + B[I]->replaceWith(A[I]);
>> +}
>> +
>> +// Try to merge components. All components given to this function are
>> +// guaranteed to have the same number of members.
>> +static void doUniquefy(ComponentIterator Begin, ComponentIterator End) {
>> + // Sort component members by hash value.
>> + for (auto It = Begin; It != End; ++It) {
>> + Component *SCC = *It;
>> + auto Comp = [](SectionChunk *A, SectionChunk *B) {
>> + return A->getHash() < B->getHash();
>> + };
>> + std::sort(SCC->Members.begin(), SCC->Members.end(), Comp);
>> + }
>> +
>> + // Merge as much component members as possible.
>> + for (auto It = Begin; It != End;) {
>> + Component *SCC = *It;
>> + auto Bound = std::partition(It + 1, End, [&](Component *C) {
>> + for (size_t I = 0, E = SCC->Members.size(); I != E; ++I)
>> + if (SCC->Members[I]->getHash() != C->Members[I]->getHash())
>> + return false;
>> + return true;
>> + });
>> +
>> + // Components [I, Bound) are likely to have the same members
>> + // because all members have the same hash values. Verify that.
>> + for (auto I = It + 1; I != Bound; ++I)
>> + tryMerge(SCC->Members, (*I)->Members);
>> + It = Bound;
>> + }
>> +}
>> +
>> +static void uniquefy(ComponentIterator Begin, ComponentIterator End) {
>> + for (auto It = Begin; It != End;) {
>> + Component *SCC = *It;
>> + size_t Size = SCC->Members.size();
>> + auto Bound = std::partition(It + 1, End, [&](Component *C) {
>> + return C->Members.size() == Size;
>> + });
>> + doUniquefy(It, Bound);
>> + It = Bound;
>> + }
>> +}
>> +
>> +// Returns strongly connected components of the graph formed by Chunks.
>> +// Chunks (a list of Live COMDAT sections) are considred as vertices,
>> +// and their relocations or association are considered as edges.
>> +static std::vector<Component *>
>> +getSCC(const std::vector<SectionChunk *> &Chunks) {
>> + std::vector<Component *> Ret;
>> + std::vector<SectionChunk *> V;
>> + uint32_t Idx;
>> +
>> + std::function<void(SectionChunk *)> StrongConnect = [&](SectionChunk
>> *SC) {
>> + SC->Index = SC->LowLink = Idx++;
>> + size_t Curr = V.size();
>> + V.push_back(SC);
>> + SC->OnStack = true;
>> +
>> + forEach(SC, [&](SectionChunk *C) {
>> + if (C->Index == 0) {
>> + StrongConnect(C);
>> + SC->LowLink = std::min(SC->LowLink, C->LowLink);
>> + } else if (C->OnStack) {
>> + SC->LowLink = std::min(SC->LowLink, C->Index);
>> + }
>> + });
>> +
>> + if (SC->LowLink != SC->Index)
>> + return;
>> + auto *SCC = new Component(
>> + std::vector<SectionChunk *>(V.begin() + Curr, V.end()));
>> + for (size_t I = Curr, E = V.size(); I != E; ++I) {
>> + V[I]->OnStack = false;
>> + V[I]->SCC = SCC;
>> + }
>> + Ret.push_back(SCC);
>> + V.erase(V.begin() + Curr, V.end());
>> + };
>> +
>> + for (SectionChunk *SC : Chunks) {
>> + if (SC->Index == 0) {
>> + Idx = 1;
>> + StrongConnect(SC);
>> + }
>> + }
>> +
>> + for (Component *SCC : Ret) {
>> + for (SectionChunk *SC : SCC->Members) {
>> + forEach(SC, [&](SectionChunk *C) {
>> + if (SCC == C->SCC)
>> + return;
>> + ++SCC->Outdegree;
>> + C->SCC->Predecessors.push_back(SCC);
>> + });
>> + }
>> + }
>> + return Ret;
>> +}
>> +
>> uint64_t SectionChunk::getHash() const {
>> - return hash_combine(getPermissions(),
>> - hash_value(SectionName),
>> - NumRelocs,
>> - uint32_t(Header->SizeOfRawData),
>> - std::distance(Relocs.end(), Relocs.begin()),
>> - Checksum);
>> + if (Hash == 0) {
>> + Hash = hash_combine(getPermissions(),
>> + hash_value(SectionName),
>> + NumRelocs,
>> + uint32_t(Header->SizeOfRawData),
>> + std::distance(Relocs.end(), Relocs.begin()),
>> + Checksum);
>> + }
>> + return Hash;
>> }
>>
>> +
>> // Returns true if this and a given chunk are identical COMDAT sections.
>> bool SectionChunk::equals(const SectionChunk *X) const {
>> // Compare headers
>> @@ -90,28 +259,6 @@ bool SectionChunk::equals(const SectionC
>> return std::equal(Relocs.begin(), Relocs.end(), X->Relocs.begin(), Eq);
>> }
>>
>> -static void link(SectionChunk *From, SectionChunk *To) {
>> - ++From->Outdegree;
>> - To->Ins.push_back(From);
>> -}
>> -
>> -typedef std::vector<SectionChunk *>::iterator ChunkIterator;
>> -
>> -static void uniquefy(ChunkIterator Begin, ChunkIterator End) {
>> - std::unordered_set<SectionChunk *, Hasher, Equals> Set;
>> - for (auto It = Begin; It != End; ++It) {
>> - SectionChunk *SC = *It;
>> - auto P = Set.insert(SC);
>> - bool Inserted = P.second;
>> - if (Inserted)
>> - continue;
>> - SectionChunk *Existing = *P.first;
>> - SC->replaceWith(Existing);
>> - for (SectionChunk *In : SC->Ins)
>> - --In->Outdegree;
>> - }
>> -}
>> -
>> // Merge identical COMDAT sections.
>> // Two sections are considered the same if their section headers,
>> // contents and relocations are all the same.
>> @@ -122,26 +269,19 @@ void doICF(const std::vector<Chunk *> &C
>> if (SC->isCOMDAT() && SC->isLive())
>> SChunks.push_back(SC);
>>
>> - // Initialize SectionChunks' outdegrees and in-chunk lists.
>> - for (SectionChunk *SC : SChunks) {
>> - for (SectionChunk *C : SC->children())
>> - link(SC, C);
>> - for (SymbolBody *B : SC->symbols())
>> - if (auto *D = dyn_cast<DefinedRegular>(B))
>> - link(SC, D->getChunk());
>> - }
>> -
>> - // By merging two sections, more sections can become mergeable
>> - // because two originally different relocations can now point to
>> - // the same section. We process sections whose outdegree is zero
>> - // first to deal with that.
>> - auto Pred = [](SectionChunk *SC) { return SC->Outdegree > 0; };
>> - for (;;) {
>> - auto Bound = std::partition(SChunks.begin(), SChunks.end(), Pred);
>> - if (Bound == SChunks.end())
>> - return;
>> - uniquefy(Bound, SChunks.end());
>> - SChunks.erase(Bound, SChunks.end());
>> + std::vector<Component *> Components = getSCC(SChunks);
>> +
>> + while (Components.size() > 0) {
>> + auto Bound = std::partition(Components.begin(), Components.end(),
>> + [](Component *SCC) { return
>> SCC->Outdegree > 0; });
>> + uniquefy(Bound, Components.end());
>> +
>> + for (auto It = Bound, E = Components.end(); It != E; ++It) {
>> + Component *SCC = *It;
>> + for (Component *Pred : SCC->Predecessors)
>> + --Pred->Outdegree;
>> + }
>> + Components.erase(Bound, Components.end());
>> }
>> }
>>
>>
>> Modified: lld/trunk/test/COFF/icf-circular.test
>> URL:
>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/icf-circular.test?rev=247387&r1=247386&r2=247387&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/test/COFF/icf-circular.test (original)
>> +++ lld/trunk/test/COFF/icf-circular.test Thu Sep 10 23:29:03 2015
>> @@ -3,7 +3,7 @@
>> # RUN: /opt:lldicf /verbose %t.obj > %t.log 2>&1
>> # RUN: FileCheck %s < %t.log
>>
>> -# CHECK-NOT: Replaced bar
>> +# CHECK: Replaced bar
>>
>> ---
>> header:
>>
>>
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