[lld] r247770 - ICF: Improve ICF to reduce more sections than before.
Rafael Espíndola via llvm-commits
llvm-commits at lists.llvm.org
Wed Sep 16 11:09:27 PDT 2015
This is impressive, congratulations!
On 15 September 2015 at 23:26, Rui Ueyama via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: ruiu
> Date: Tue Sep 15 22:26:31 2015
> New Revision: 247770
>
> URL: http://llvm.org/viewvc/llvm-project?rev=247770&view=rev
> Log:
> ICF: Improve ICF to reduce more sections than before.
>
> This is a patch to make LLD to be on par with MSVC in terms of ICF
> effectiveness. MSVC produces a 27.14MB executable when linking LLD.
> LLD previously produced a 27.61MB when self-linking. Now the size
> is reduced to 27.11MB. Note that without ICF the size is 29.63MB.
>
> In r247387, I implemented an algorithm that handles section graphs
> as cyclic graphs and merge them using SCC. The algorithm did not
> always work as intended as I demonstrated in r247721. The new
> algortihm implemented in this patch is different from the previous
> one. If you are interested the details, you want to read the file
> comment of ICF.cpp.
>
> Modified:
> lld/trunk/COFF/Chunks.h
> lld/trunk/COFF/ICF.cpp
> lld/trunk/test/COFF/icf-circular.test
> lld/trunk/test/COFF/icf-circular2.test
> lld/trunk/test/COFF/icf-simple.test
>
> Modified: lld/trunk/COFF/Chunks.h
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Chunks.h?rev=247770&r1=247769&r2=247770&view=diff
> ==============================================================================
> --- lld/trunk/COFF/Chunks.h (original)
> +++ lld/trunk/COFF/Chunks.h Tue Sep 15 22:26:31 2015
> @@ -113,16 +113,6 @@ protected:
>
> 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:
> @@ -187,21 +177,21 @@ public:
> // Used for ICF (Identical COMDAT Folding)
> void replaceWith(SectionChunk *Other);
> uint64_t getHash() const;
> - bool equals(const SectionChunk *Other) const;
> + static bool equalsVertex(const SectionChunk *A, const SectionChunk *B);
> + static bool equalsEdge(const SectionChunk *Au, const SectionChunk *B);
>
> // A pointer pointing to a replacement for this chunk.
> // Initially it points to "this" object. If this chunk is merged
> // with other chunk by ICF, it points to another chunk,
> // and this chunk is considrered as dead.
> SectionChunk *Ptr;
> - uint32_t Index = 0;
> - uint32_t LowLink = 0;
> - bool OnStack = false;
> - Component *SCC = nullptr;
> + std::vector<SectionChunk *> Successors;
> + void initSuccessors();
>
> // 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;
> + uint64_t GroupID;
> mutable uint64_t Hash = 0;
>
> private:
>
> Modified: lld/trunk/COFF/ICF.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/ICF.cpp?rev=247770&r1=247769&r2=247770&view=diff
> ==============================================================================
> --- lld/trunk/COFF/ICF.cpp (original)
> +++ lld/trunk/COFF/ICF.cpp Tue Sep 15 22:26:31 2015
> @@ -14,24 +14,32 @@
> // 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
> +// identical 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 same
> // 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.
> +// What we do in this file is this. We split sections into groups. Sections
> +// in the same group are considered identical.
> //
> -// 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.
> +// First, all sections are grouped by their "constant" values. Constant
> +// values are values that never change by ICF, such as section contents,
> +// section name, number of relocations, type and offset of each relocation,
> +// etc. Because we do not care about relocation targets in this spep, two
> +// sections in the same group may be actually not identical, but at least
> +// two sections in different groups can never be identical.
> +//
> +// Then, we try to split each group by relocation targets. Relocations are
> +// considered identical if and only if the relocation targets are in the
> +// same group. Splitting a group may make more groups to be splittable,
> +// because two relocations that were previously considered identical might
> +// now point to different groups. We repeat this step until the convergence
> +// is obtained.
> +//
> +// This algorithm is so-called "optimistic" algorithm described in
> +// http://research.google.com/pubs/pub36912.html.
> //
> //===----------------------------------------------------------------------===//
>
> @@ -51,156 +59,6 @@ using namespace llvm;
>
> namespace lld {
> namespace coff {
> -namespace {
> -
> -struct Hasher {
> - size_t operator()(const SectionChunk *C) const { return C->getHash(); }
> -};
> -
> -struct Equals {
> - bool operator()(const SectionChunk *A, const SectionChunk *B) const {
> - return A->equals(B);
> - }
> -};
> -
> -} // 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 {
> if (Hash == 0) {
> @@ -214,74 +72,143 @@ uint64_t SectionChunk::getHash() const {
> return Hash;
> }
>
> +void SectionChunk::initSuccessors() {
> + Successors = AssocChildren;
> + for (const coff_relocation &R : Relocs) {
> + SymbolBody *B = File->getSymbolBody(R.SymbolTableIndex)->repl();
> + if (auto D = dyn_cast<DefinedRegular>(B))
> + Successors.push_back(D->getChunk());
> + }
> +}
>
> -// Returns true if this and a given chunk are identical COMDAT sections.
> -bool SectionChunk::equals(const SectionChunk *X) const {
> - // Compare headers
> - if (getPermissions() != X->getPermissions())
> - return false;
> - if (SectionName != X->SectionName)
> - return false;
> - if (Header->SizeOfRawData != X->Header->SizeOfRawData)
> - return false;
> - if (NumRelocs != X->NumRelocs)
> - return false;
> - if (Checksum != X->Checksum)
> - return false;
> -
> - // Compare data
> - if (getContents() != X->getContents())
> +bool SectionChunk::equalsVertex(const SectionChunk *A, const SectionChunk *B) {
> + if (A->getPermissions() != B->getPermissions() ||
> + A->SectionName != B->SectionName ||
> + A->Header->SizeOfRawData != B->Header->SizeOfRawData ||
> + A->NumRelocs != B->NumRelocs ||
> + A->Checksum != B->Checksum ||
> + A->AssocChildren.size() != B->AssocChildren.size() ||
> + A->getContents() != B->getContents()) {
> return false;
> + }
>
> - // Compare associative sections
> - if (AssocChildren.size() != X->AssocChildren.size())
> - return false;
> - for (size_t I = 0, E = AssocChildren.size(); I != E; ++I)
> - if (AssocChildren[I]->Ptr != X->AssocChildren[I]->Ptr)
> + for (size_t I = 0, E = A->AssocChildren.size(); I != E; ++I)
> + if (A->AssocChildren[I]->GroupID != B->AssocChildren[I]->GroupID)
> return false;
>
> // Compare relocations
> auto Eq = [&](const coff_relocation &R1, const coff_relocation &R2) {
> - if (R1.Type != R2.Type)
> + if (R1.Type != R2.Type ||
> + R1.VirtualAddress != R2.VirtualAddress) {
> return false;
> - if (R1.VirtualAddress != R2.VirtualAddress)
> - return false;
> - SymbolBody *B1 = File->getSymbolBody(R1.SymbolTableIndex)->repl();
> - SymbolBody *B2 = X->File->getSymbolBody(R2.SymbolTableIndex)->repl();
> + }
> + SymbolBody *B1 = A->File->getSymbolBody(R1.SymbolTableIndex)->repl();
> + SymbolBody *B2 = B->File->getSymbolBody(R2.SymbolTableIndex)->repl();
> if (B1 == B2)
> return true;
> auto *D1 = dyn_cast<DefinedRegular>(B1);
> auto *D2 = dyn_cast<DefinedRegular>(B2);
> - return (D1 && D2 &&
> - D1->getValue() == D2->getValue() &&
> - D1->getChunk() == D2->getChunk());
> + return D1 && D2 &&
> + D1->getValue() == D2->getValue() &&
> + D1->getChunk()->GroupID == D2->getChunk()->GroupID;
> };
> - return std::equal(Relocs.begin(), Relocs.end(), X->Relocs.begin(), Eq);
> + return std::equal(A->Relocs.begin(), A->Relocs.end(), B->Relocs.begin(), Eq);
> +}
> +
> +bool SectionChunk::equalsEdge(const SectionChunk *A, const SectionChunk *B) {
> + assert(A->Successors.size() == B->Successors.size());
> + return std::equal(A->Successors.begin(), A->Successors.end(),
> + B->Successors.begin(),
> + [](const SectionChunk *X, const SectionChunk *Y) {
> + return X->GroupID == Y->GroupID;
> + });
> +}
> +
> +typedef std::vector<SectionChunk *>::iterator ChunkIterator;
> +typedef bool (*Comparator)(const SectionChunk *, const SectionChunk *);
> +static uint64_t NextID = 0;
> +
> +static bool partition(ChunkIterator Begin, ChunkIterator End, Comparator Eq) {
> + bool R = false;
> + for (auto It = Begin;;) {
> + SectionChunk *Head = *It;
> + auto Bound = std::partition(It + 1, End, [&](SectionChunk *SC) {
> + return Eq(Head, SC);
> + });
> + if (Bound == End)
> + return R;
> + size_t ID = NextID++;
> + std::for_each(It, Bound, [&](SectionChunk *SC) { SC->GroupID = ID; });
> + It = Bound;
> + R = true;
> + }
> +}
> +
> +static bool forEachGroup(std::vector<SectionChunk *> &SChunks,
> + Comparator Eq) {
> + bool R = false;
> + for (auto It = SChunks.begin(), End = SChunks.end(); It != End;) {
> + SectionChunk *Head = *It;
> + auto Bound = std::find_if(It + 1, End, [&](SectionChunk *SC) {
> + return SC->GroupID != Head->GroupID;
> + });
> + if (partition(It, Bound, Eq))
> + R = true;
> + It = Bound;
> + }
> + return R;
> }
>
> // Merge identical COMDAT sections.
> // Two sections are considered the same if their section headers,
> // contents and relocations are all the same.
> void doICF(const std::vector<Chunk *> &Chunks) {
> + if (Config->Verbose)
> + llvm::outs() << "\nICF\n";
> +
> + // Collect only mergeable sections.
> std::vector<SectionChunk *> SChunks;
> - for (Chunk *C : Chunks)
> - if (auto *SC = dyn_cast<SectionChunk>(C))
> - if (SC->isCOMDAT() && SC->isLive())
> + for (Chunk *C : Chunks) {
> + if (auto *SC = dyn_cast<SectionChunk>(C)) {
> + bool Writable = SC->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
> + if (SC->isCOMDAT() && SC->isLive() && !Writable) {
> SChunks.push_back(SC);
> + SC->GroupID = SC->getHash() | (uint64_t(1) << 63);
> + } else {
> + SC->GroupID = NextID++;
> + }
> + }
> + }
>
> - 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;
> + std::sort(SChunks.begin(), SChunks.end(),
> + [](SectionChunk *A, SectionChunk *B) {
> + return A->GroupID < B->GroupID;
> + });
> +
> + // Split groups until we get a convergence.
> + for (SectionChunk *SC : SChunks)
> + SC->initSuccessors();
> + forEachGroup(SChunks, SectionChunk::equalsVertex);
> + while (forEachGroup(SChunks, SectionChunk::equalsEdge));
> +
> + // Merge sections in the same group.
> + for (auto It = SChunks.begin(), End = SChunks.end(); It != End;) {
> + SectionChunk *Head = *It;
> + auto Bound = std::find_if(It + 1, End, [&](SectionChunk *SC) {
> + return Head->GroupID != SC->GroupID;
> + });
> + if (std::distance(It, Bound) == 1) {
> + It = Bound;
> + continue;
> + }
> + if (Config->Verbose)
> + llvm::outs() << "Selected " << Head->getDebugName() << "\n";
> + for (++It; It != Bound; ++It) {
> + SectionChunk *SC = *It;
> + if (Config->Verbose)
> + llvm::outs() << " Removed " << SC->getDebugName() << "\n";
> + SC->replaceWith(Head);
> }
> - 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=247770&r1=247769&r2=247770&view=diff
> ==============================================================================
> --- lld/trunk/test/COFF/icf-circular.test (original)
> +++ lld/trunk/test/COFF/icf-circular.test Tue Sep 15 22:26:31 2015
> @@ -3,7 +3,8 @@
> # RUN: /opt:lldicf /verbose %t.obj > %t.log 2>&1
> # RUN: FileCheck %s < %t.log
>
> -# CHECK: Replaced bar
> +# CHECK: Selected foo
> +# CHECK: Removed bar
>
> ---
> header:
>
> Modified: lld/trunk/test/COFF/icf-circular2.test
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/icf-circular2.test?rev=247770&r1=247769&r2=247770&view=diff
> ==============================================================================
> --- lld/trunk/test/COFF/icf-circular2.test (original)
> +++ lld/trunk/test/COFF/icf-circular2.test Tue Sep 15 22:26:31 2015
> @@ -3,8 +3,8 @@
> # RUN: /opt:lldicf /verbose %t.obj > %t.log 2>&1
> # RUN: FileCheck %s < %t.log
>
> -# TODO: LLD should be able to merge foo and bar.
> -# CHECK-NOT: Replaced bar
> +# CHECK: Selected foo
> +# CHECK: Removed bar
>
> ---
> header:
>
> Modified: lld/trunk/test/COFF/icf-simple.test
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/icf-simple.test?rev=247770&r1=247769&r2=247770&view=diff
> ==============================================================================
> --- lld/trunk/test/COFF/icf-simple.test (original)
> +++ lld/trunk/test/COFF/icf-simple.test Tue Sep 15 22:26:31 2015
> @@ -3,7 +3,8 @@
> # RUN: /opt:lldicf /verbose %t.obj > %t.log 2>&1
> # RUN: FileCheck %s < %t.log
>
> -# CHECK: Replaced bar
> +# CHECK: Selected foo
> +# CHECK: Removed bar
>
> ---
> header:
>
>
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