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-rw-r--r--lld/ELF/Writer.cpp260
1 files changed, 143 insertions, 117 deletions
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index c8f5378aa82..4020fb1bdfe 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -546,32 +546,6 @@ template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
}
}
-// PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections that
-// we would like to make sure appear is a specific order to maximize their
-// coverage by a single signed 16-bit offset from the TOC base pointer.
-// Conversely, the special .tocbss section should be first among all SHT_NOBITS
-// sections. This will put it next to the loaded special PPC64 sections (and,
-// thus, within reach of the TOC base pointer).
-static int getPPC64SectionRank(StringRef SectionName) {
- return StringSwitch<int>(SectionName)
- .Case(".tocbss", 0)
- .Case(".branch_lt", 2)
- .Case(".toc", 3)
- .Case(".toc1", 4)
- .Case(".opd", 5)
- .Default(1);
-}
-
-// All sections with SHF_MIPS_GPREL flag should be grouped together
-// because data in these sections is addressable with a gp relative address.
-static int getMipsSectionRank(const OutputSection *S) {
- if ((S->Flags & SHF_MIPS_GPREL) == 0)
- return 0;
- if (S->Name == ".got")
- return 1;
- return 2;
-}
-
// Today's loaders have a feature to make segments read-only after
// processing dynamic relocations to enhance security. PT_GNU_RELRO
// is defined for that.
@@ -645,99 +619,145 @@ bool elf::isRelroSection(const OutputSection *Sec) {
S == ".eh_frame" || S == ".openbsd.randomdata";
}
-static bool compareSectionsNonScript(const OutputSection *A,
- const OutputSection *B) {
+// We compute a rank for each section. The rank indicates where the
+// section should be placed in the file. Instead of using simple
+// numbers (0,1,2...), we use a series of flags. One for each decision
+// point when placing the section.
+// Using flags has two key properties:
+// * It is easy to check if a give branch was taken.
+// * It is easy two see how similar two ranks are (see getRankProximity).
+enum RankFlags {
+ RF_NOT_ADDR_SET = 1 << 16,
+ RF_NOT_INTERP = 1 << 15,
+ RF_NOT_ALLOC = 1 << 14,
+ RF_WRITE = 1 << 13,
+ RF_EXEC = 1 << 12,
+ RF_NON_TLS_BSS = 1 << 11,
+ RF_NON_TLS_BSS_RO = 1 << 10,
+ RF_NOT_TLS = 1 << 9,
+ RF_BSS = 1 << 8,
+ RF_PPC_NOT_TOCBSS = 1 << 7,
+ RF_PPC_OPD = 1 << 6,
+ RF_PPC_TOCL = 1 << 5,
+ RF_PPC_TOC = 1 << 4,
+ RF_PPC_BRANCH_LT = 1 << 3,
+ RF_MIPS_GPREL = 1 << 2,
+ RF_MIPS_NOT_GOT = 1 << 1
+};
+
+static unsigned getSectionRank(const OutputSection *Sec) {
+ unsigned Rank = 0;
+
+ // We want to put section specified by -T option first, so we
+ // can start assigning VA starting from them later.
+ if (Config->SectionStartMap.count(Sec->Name))
+ return Rank;
+ Rank |= RF_NOT_ADDR_SET;
+
// Put .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
- bool AIsInterp = A->Name == ".interp";
- bool BIsInterp = B->Name == ".interp";
- if (AIsInterp != BIsInterp)
- return AIsInterp;
+ if (Sec->Name == ".interp")
+ return Rank;
+ Rank |= RF_NOT_INTERP;
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
- bool AIsAlloc = A->Flags & SHF_ALLOC;
- bool BIsAlloc = B->Flags & SHF_ALLOC;
- if (AIsAlloc != BIsAlloc)
- return AIsAlloc;
-
- // We don't have any special requirements for the relative order of two non
- // allocatable sections.
- if (!AIsAlloc)
- return false;
-
- // We want to put section specified by -T option first, so we
- // can start assigning VA starting from them later.
- auto AAddrSetI = Config->SectionStartMap.find(A->Name);
- auto BAddrSetI = Config->SectionStartMap.find(B->Name);
- bool AHasAddrSet = AAddrSetI != Config->SectionStartMap.end();
- bool BHasAddrSet = BAddrSetI != Config->SectionStartMap.end();
- if (AHasAddrSet != BHasAddrSet)
- return AHasAddrSet;
- if (AHasAddrSet)
- return AAddrSetI->second < BAddrSetI->second;
+ if (!(Sec->Flags & SHF_ALLOC))
+ return Rank | RF_NOT_ALLOC;
// We want the read only sections first so that they go in the PT_LOAD
// covering the program headers at the start of the file.
- bool AIsWritable = A->Flags & SHF_WRITE;
- bool BIsWritable = B->Flags & SHF_WRITE;
- if (AIsWritable != BIsWritable)
- return BIsWritable;
+ if (Sec->Flags & SHF_WRITE)
+ Rank |= RF_WRITE;
- if (!Config->SingleRoRx) {
+ if (Sec->Flags & SHF_EXECINSTR) {
// For a corresponding reason, put non exec sections first (the program
// header PT_LOAD is not executable).
// We only do that if we are not using linker scripts, since with linker
// scripts ro and rx sections are in the same PT_LOAD, so their relative
// order is not important. The same applies for -no-rosegment.
- bool AIsExec = A->Flags & SHF_EXECINSTR;
- bool BIsExec = B->Flags & SHF_EXECINSTR;
- if (AIsExec != BIsExec)
- return BIsExec;
+ if ((Rank & RF_WRITE) || !Config->SingleRoRx)
+ Rank |= RF_EXEC;
}
// If we got here we know that both A and B are in the same PT_LOAD.
- bool AIsTls = A->Flags & SHF_TLS;
- bool BIsTls = B->Flags & SHF_TLS;
- bool AIsNoBits = A->Type == SHT_NOBITS;
- bool BIsNoBits = B->Type == SHT_NOBITS;
+ bool IsTls = Sec->Flags & SHF_TLS;
+ bool IsNoBits = Sec->Type == SHT_NOBITS;
// The first requirement we have is to put (non-TLS) nobits sections last. The
// reason is that the only thing the dynamic linker will see about them is a
// p_memsz that is larger than p_filesz. Seeing that it zeros the end of the
// PT_LOAD, so that has to correspond to the nobits sections.
- bool AIsNonTlsNoBits = AIsNoBits && !AIsTls;
- bool BIsNonTlsNoBits = BIsNoBits && !BIsTls;
- if (AIsNonTlsNoBits != BIsNonTlsNoBits)
- return BIsNonTlsNoBits;
+ bool IsNonTlsNoBits = IsNoBits && !IsTls;
+ if (IsNonTlsNoBits)
+ Rank |= RF_NON_TLS_BSS;
// We place nobits RelRo sections before plain r/w ones, and non-nobits RelRo
// sections after r/w ones, so that the RelRo sections are contiguous.
- bool AIsRelRo = isRelroSection(A);
- bool BIsRelRo = isRelroSection(B);
- if (AIsRelRo != BIsRelRo)
- return AIsNonTlsNoBits ? AIsRelRo : BIsRelRo;
+ bool IsRelRo = isRelroSection(Sec);
+ if (IsNonTlsNoBits && !IsRelRo)
+ Rank |= RF_NON_TLS_BSS_RO;
+ if (!IsNonTlsNoBits && IsRelRo)
+ Rank |= RF_NON_TLS_BSS_RO;
// The TLS initialization block needs to be a single contiguous block in a R/W
// PT_LOAD, so stick TLS sections directly before the other RelRo R/W
// sections. The TLS NOBITS sections are placed here as they don't take up
// virtual address space in the PT_LOAD.
- if (AIsTls != BIsTls)
- return AIsTls;
+ if (!IsTls)
+ Rank |= RF_NOT_TLS;
// Within the TLS initialization block, the non-nobits sections need to appear
// first.
- if (AIsNoBits != BIsNoBits)
- return BIsNoBits;
+ if (IsNoBits)
+ Rank |= RF_BSS;
+
+ // // Some architectures have additional ordering restrictions for sections
+ // // within the same PT_LOAD.
+ if (Config->EMachine == EM_PPC64) {
+ // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
+ // that we would like to make sure appear is a specific order to maximize
+ // their coverage by a single signed 16-bit offset from the TOC base
+ // pointer. Conversely, the special .tocbss section should be first among
+ // all SHT_NOBITS sections. This will put it next to the loaded special
+ // PPC64 sections (and, thus, within reach of the TOC base pointer).
+ StringRef Name = Sec->Name;
+ if (Name != ".tocbss")
+ Rank |= RF_PPC_NOT_TOCBSS;
+
+ if (Name == ".opd")
+ Rank |= RF_PPC_OPD;
+
+ if (Name == ".toc1")
+ Rank |= RF_PPC_TOCL;
+
+ if (Name == ".toc")
+ Rank |= RF_PPC_TOC;
+
+ if (Name == ".branch_lt")
+ Rank |= RF_PPC_BRANCH_LT;
+ }
+ if (Config->EMachine == EM_MIPS) {
+ // All sections with SHF_MIPS_GPREL flag should be grouped together
+ // because data in these sections is addressable with a gp relative address.
+ if (Sec->Flags & SHF_MIPS_GPREL)
+ Rank |= RF_MIPS_GPREL;
- // Some architectures have additional ordering restrictions for sections
- // within the same PT_LOAD.
- if (Config->EMachine == EM_PPC64)
- return getPPC64SectionRank(A->Name) < getPPC64SectionRank(B->Name);
- if (Config->EMachine == EM_MIPS)
- return getMipsSectionRank(A) < getMipsSectionRank(B);
+ if (Sec->Name != ".got")
+ Rank |= RF_MIPS_NOT_GOT;
+ }
+ return Rank;
+}
+
+static bool compareSectionsNonScript(const OutputSection *A,
+ const OutputSection *B) {
+ if (A->SortRank != B->SortRank)
+ return A->SortRank < B->SortRank;
+ if (!(A->SortRank & RF_NOT_ADDR_SET))
+ return Config->SectionStartMap.lookup(A->Name) <
+ Config->SectionStartMap.lookup(B->Name);
return false;
}
@@ -751,8 +771,7 @@ static bool compareSections(const OutputSection *A, const OutputSection *B) {
if (AIndex != BIndex)
return AIndex < BIndex;
- // The sections are not in the linker script, so don't sort for now.
- return false;
+ return compareSectionsNonScript(A, B);
}
// Program header entry
@@ -962,45 +981,36 @@ template <class ELFT> void Writer<ELFT>::createSections() {
Sec->assignOffsets();
}
-static bool canSharePtLoad(const OutputSection &S1, const OutputSection &S2) {
- if (!(S1.Flags & SHF_ALLOC) || !(S2.Flags & SHF_ALLOC))
- return false;
-
- bool S1IsWrite = S1.Flags & SHF_WRITE;
- bool S2IsWrite = S2.Flags & SHF_WRITE;
- if (S1IsWrite != S2IsWrite)
- return false;
-
- if (!S1IsWrite)
- return true; // RO and RX share a PT_LOAD with linker scripts.
- return (S1.Flags & SHF_EXECINSTR) == (S2.Flags & SHF_EXECINSTR);
+// We want to find how similar two ranks are.
+// The more branches in getSectionRank that match, the more similar they are.
+// Since each branch corresponds to a bit flag, we can just use
+// countLeadingZeros.
+static unsigned getRankProximity(OutputSection *A, OutputSection *B) {
+ return countLeadingZeros(A->SortRank ^ B->SortRank);
}
-// We assume, like createPhdrs that all allocs are at the start.
+// We want to place orphan sections so that they share as much
+// characteristics with their neighbors as possible. For example, if
+// both are rw, or both are tls.
template <typename ELFT>
static std::vector<OutputSection *>::iterator
findOrphanPos(std::vector<OutputSection *>::iterator B,
std::vector<OutputSection *>::iterator E) {
OutputSection *Sec = *E;
- // If it is not allocatable, just leave it at the end.
- if (!(Sec->Flags & SHF_ALLOC))
+ // Find the first element that has as close a rank as possible.
+ auto I = std::max_element(B, E, [=](OutputSection *A, OutputSection *B) {
+ return getRankProximity(Sec, A) < getRankProximity(Sec, B);
+ });
+ if (I == E)
return E;
- // Find the first sharable.
- auto Pos = std::find_if(
- B, E, [=](OutputSection *S) { return canSharePtLoad(*S, *Sec); });
- if (Pos != E) {
- // Ony consider the sharable range.
- B = Pos;
- E = std::find_if(
- B, E, [=](OutputSection *S) { return !canSharePtLoad(*S, *Sec); });
- assert(B != E);
- }
-
- // Find the fist position that Sec compares less to.
- return std::find_if(
- B, E, [=](OutputSection *S) { return compareSectionsNonScript(Sec, S); });
+ // Consider all existing sections with the same proximity.
+ unsigned Proximity = getRankProximity(Sec, *I);
+ while (I != E && getRankProximity(Sec, *I) == Proximity &&
+ Sec->SortRank >= (*I)->SortRank)
+ ++I;
+ return I;
}
template <class ELFT> void Writer<ELFT>::sortSections() {
@@ -1008,12 +1018,18 @@ template <class ELFT> void Writer<ELFT>::sortSections() {
// relative order for SHF_LINK_ORDER sections.
if (Config->Relocatable)
return;
+
+ if (Script->Opt.HasSections)
+ Script->adjustSectionsBeforeSorting();
+
+ for (OutputSection *Sec : OutputSections)
+ Sec->SortRank = getSectionRank(Sec);
+
if (!Script->Opt.HasSections) {
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compareSectionsNonScript);
return;
}
- Script->adjustSectionsBeforeSorting();
// The order of the sections in the script is arbitrary and may not agree with
// compareSectionsNonScript. This means that we cannot easily define a
@@ -1046,8 +1062,18 @@ template <class ELFT> void Writer<ELFT>::sortSections() {
auto NonScriptI =
std::find_if(OutputSections.begin(), E,
[](OutputSection *S) { return S->SectionIndex == INT_MAX; });
- for (; NonScriptI != E; ++NonScriptI)
- std::rotate(findOrphanPos<ELFT>(I, NonScriptI), NonScriptI, NonScriptI + 1);
+ while (NonScriptI != E) {
+ auto Pos = findOrphanPos<ELFT>(I, NonScriptI);
+
+ // As an optimization, find all sections with the same sort rank
+ // and insert them with one rotate.
+ unsigned Rank = (*NonScriptI)->SortRank;
+ auto End = std::find_if(NonScriptI + 1, E, [=](OutputSection *Sec) {
+ return Sec->SortRank != Rank;
+ });
+ std::rotate(Pos, NonScriptI, End);
+ NonScriptI = End;
+ }
Script->adjustSectionsAfterSorting();
}
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