ELF: Add branch-to-branch optimization.

lld/ELF/Arch/AArch64.cpp

@@ -11,6 +11,7 @@
 #include "Symbols.h"
 #include "SyntheticSections.h"
 #include "Target.h"
+#include "TargetImpl.h"
 #include "lld/Common/ErrorHandler.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Support/Endian.h"
@@ -83,6 +84,7 @@ class AArch64 : public TargetInfo {
                 uint64_t val) const override;
   RelExpr adjustTlsExpr(RelType type, RelExpr expr) const override;
   void relocateAlloc(InputSectionBase &sec, uint8_t *buf) const override;
+  void applyBranchToBranchOpt() const override;
 
 private:
   void relaxTlsGdToLe(uint8_t *loc, const Relocation &rel, uint64_t val) const;
@@ -975,6 +977,62 @@ void AArch64::relocateAlloc(InputSectionBase &sec, uint8_t *buf) const {
   }
 }
 
+static std::optional<uint64_t> getControlTransferAddend(InputSection &is,
+                                                        Relocation &r) {
+  // Identify a control transfer relocation for the branch-to-branch
+  // optimization. A "control transfer relocation" means a B or BL
+  // target but it also includes relative vtable relocations for example.
+  //
+  // We require the relocation type to be JUMP26, CALL26 or PLT32. With a
+  // relocation type of PLT32 the value may be assumed to be used for branching
+  // directly to the symbol and the addend is only used to produce the relocated
+  // value (hence the effective addend is always 0). This is because if a PLT is
+  // needed the addend will be added to the address of the PLT, and it doesn't
+  // make sense to branch into the middle of a PLT. For example, relative vtable
+  // relocations use PLT32 and 0 or a positive value as the addend but still are
+  // used to branch to the symbol.
+  //
+  // With JUMP26 or CALL26 the only reasonable interpretation of a non-zero
+  // addend is that we are branching to symbol+addend so that becomes the
+  // effective addend.
+  if (r.type == R_AARCH64_PLT32)
+    return 0;
+  if (r.type == R_AARCH64_JUMP26 || r.type == R_AARCH64_CALL26)
+    return r.addend;
+  return std::nullopt;
+}
+
+static std::pair<Relocation *, uint64_t> getBranchInfo(InputSection &is,
+                                                       uint64_t offset) {
+  auto *i = std::lower_bound(
+      is.relocations.begin(), is.relocations.end(), offset,
+      [](Relocation &r, uint64_t offset) { return r.offset < offset; });
+  if (i != is.relocations.end() && i->offset == offset &&
+      i->type == R_AARCH64_JUMP26) {
+    return {i, i->addend};
+  }
+  return {nullptr, 0};
+}
+
+static void mergeControlTransferRelocations(Relocation &r1,
+                                            const Relocation &r2) {
+  r1.expr = r2.expr;
+  r1.sym = r2.sym;
+  // With PLT32 we must respect the original addend as that affects the value's
+  // interpretation. With the other relocation types the original addend is
+  // irrelevant because it referred to an offset within the original target
+  // section so we overwrite it.
+  if (r1.type == R_AARCH64_PLT32)
+    r1.addend += r2.addend;
+  else
+    r1.addend = r2.addend;
+}
+
+void AArch64::applyBranchToBranchOpt() const {
+  applyBranchToBranchOptImpl(ctx, getBranchInfo, getControlTransferAddend,
+                             mergeControlTransferRelocations);
+}
+
 // AArch64 may use security features in variant PLT sequences. These are:
 // Pointer Authentication (PAC), introduced in armv8.3-a and Branch Target
 // Indicator (BTI) introduced in armv8.5-a. The additional instructions used

lld/ELF/Arch/TargetImpl.h

@@ -0,0 +1,92 @@
+//===- TargetImpl.h ---------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_ELF_ARCH_TARGETIMPL_H
+#define LLD_ELF_ARCH_TARGETIMPL_H
+
+#include "InputFiles.h"
+#include "InputSection.h"
+#include "Relocations.h"
+#include "Symbols.h"
+#include "llvm/BinaryFormat/ELF.h"
+
+namespace lld {
+namespace elf {
+
+// getControlTransferAddend: If this relocation is used for control transfer
+// instructions (e.g. branch, branch-link or call) or code references (e.g.
+// virtual function pointers) and indicates an address-insignificant reference,
+// return the effective addend for the relocation, otherwise return
+// std::nullopt. The effective addend for a relocation is the addend that is
+// used to determine its branch destination.
+//
+// getBranchInfo: If a control transfer relocation referring to is+offset
+// directly transfers control to a relocated branch instruction in the specified
+// section, return the relocation for the branch target as well as its effective
+// addend (see above). Otherwise return {nullptr, 0}.
+//
+// mergeControlTransferRelocations: Given r1, a relocation for which
+// getControlTransferAddend() returned a value, and r2, a relocation returned by
+// getBranchInfo(), modify r1 so that it branches directly to the target of r2.
+template <typename GetBranchInfo, typename GetControlTransferAddend,
+          typename MergeControlTransferRelocations>
+inline void applyBranchToBranchOptImpl(
+    Ctx &ctx, GetBranchInfo getBranchInfo,
+    GetControlTransferAddend getControlTransferAddend,
+    MergeControlTransferRelocations mergeControlTransferRelocations) {
+  // Needs to run serially because it writes to the relocations array as well as
+  // reading relocations of other sections.
+  for (ELFFileBase *f : ctx.objectFiles) {
+    auto getRelocBranchInfo =
+        [&getBranchInfo](Relocation &r,
+                         uint64_t addend) -> std::pair<Relocation *, uint64_t> {
+      auto *target = dyn_cast_or_null<Defined>(r.sym);
+      // We don't allow preemptible symbols or ifuncs (may go somewhere else),
+      // absolute symbols (runtime behavior unknown), non-executable memory
+      // (ditto) or non-regular sections (no section data).
+      if (!target || target->isPreemptible || target->isGnuIFunc() ||
+          !target->section ||
+          !(target->section->flags & llvm::ELF::SHF_EXECINSTR) ||
+          target->section->kind() != SectionBase::Regular)
+        return {nullptr, 0};
+      return getBranchInfo(*cast<InputSection>(target->section),
+                           target->value + addend);
+    };
+    for (InputSectionBase *s : f->getSections()) {
+      if (!s)
+        continue;
+      for (Relocation &r : s->relocations) {
+        if (std::optional<uint64_t> addend =
+                getControlTransferAddend(*cast<InputSection>(s), r)) {
+          std::pair<Relocation *, uint64_t> targetAndAddend =
+              getRelocBranchInfo(r, *addend);
+          if (targetAndAddend.first) {
+            // Avoid getting stuck in an infinite loop if we encounter a branch
+            // that (possibly indirectly) branches to itself. It is unlikely
+            // that more than 5 iterations will ever be needed in practice.
+            size_t iterations = 5;
+            while (iterations--) {
+              std::pair<Relocation *, uint64_t> nextTargetAndAddend =
+                  getRelocBranchInfo(*targetAndAddend.first,
+                                     targetAndAddend.second);
+              if (!nextTargetAndAddend.first)
+                break;
+              targetAndAddend = nextTargetAndAddend;
+            }
+            mergeControlTransferRelocations(r, *targetAndAddend.first);
+          }
+        }
+      }
+    }
+  }
+}
+
+} // namespace elf
+} // namespace lld
+
+#endif

lld/ELF/Arch/X86_64.cpp

@@ -11,6 +11,7 @@
 #include "Symbols.h"
 #include "SyntheticSections.h"
 #include "Target.h"
+#include "TargetImpl.h"
 #include "lld/Common/ErrorHandler.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Support/Endian.h"
@@ -50,6 +51,7 @@ class X86_64 : public TargetInfo {
   bool deleteFallThruJmpInsn(InputSection &is, InputFile *file,
                              InputSection *nextIS) const override;
   bool relaxOnce(int pass) const override;
+  void applyBranchToBranchOpt() const override;
 
 private:
   void relaxTlsGdToLe(uint8_t *loc, const Relocation &rel, uint64_t val) const;
@@ -1162,6 +1164,72 @@ void X86_64::relocateAlloc(InputSectionBase &sec, uint8_t *buf) const {
   }
 }
 
+static std::optional<uint64_t> getControlTransferAddend(InputSection &is,
+                                                        Relocation &r) {
+  // Identify a control transfer relocation for the branch-to-branch
+  // optimization. A "control transfer relocation" usually means a CALL or JMP
+  // target but it also includes relative vtable relocations for example.
+  //
+  // We require the relocation type to be PLT32. With a relocation type of PLT32
+  // the value may be assumed to be used for branching directly to the symbol
+  // and the addend is only used to produce the relocated value (hence the
+  // effective addend is always 0). This is because if a PLT is needed the
+  // addend will be added to the address of the PLT, and it doesn't make sense
+  // to branch into the middle of a PLT. For example, relative vtable
+  // relocations use PLT32 and 0 or a positive value as the addend but still are
+  // used to branch to the symbol.
+  //
+  // STT_SECTION symbols are a special case on x86 because the LLVM assembler
+  // uses them for branches to local symbols which are assembled as referring to
+  // the section symbol with the addend equal to the symbol value - 4.
+  if (r.type == R_X86_64_PLT32) {
+    if (r.sym->isSection())
+      return r.addend + 4;
+    return 0;
+  }
+  return std::nullopt;
+}
+
+static std::pair<Relocation *, uint64_t> getBranchInfo(InputSection &is,
+                                                       uint64_t offset) {
+  auto content = is.contentMaybeDecompress();
+  if (content.size() > offset && content[offset] == 0xe9) { // JMP immediate
+    auto *i = std::lower_bound(
+        is.relocations.begin(), is.relocations.end(), offset + 1,
+        [](Relocation &r, uint64_t offset) { return r.offset < offset; });
+    // Unlike with getControlTransferAddend() it is valid to accept a PC32
+    // relocation here because we know that this is actually a JMP and not some
+    // other reference, so the interpretation is that we add 4 to the addend and
+    // use that as the effective addend.
+    if (i != is.relocations.end() && i->offset == offset + 1 &&
+        (i->type == R_X86_64_PC32 || i->type == R_X86_64_PLT32)) {
+      return {i, i->addend + 4};
+    }
+  }
+  return {nullptr, 0};
+}
+
+static void mergeControlTransferRelocations(Relocation &r1,
+                                            const Relocation &r2) {
+  // The isSection() check handles the STT_SECTION case described above.
+  // In that case the original addend is irrelevant because it referred to an
+  // offset within the original target section so we overwrite it.
+  //
+  // The +4 is here to compensate for r2.addend which will likely be -4,
+  // but may also be addend-4 in case of a PC32 branch to symbol+addend.
+  if (r1.sym->isSection())
+    r1.addend = r2.addend;
+  else
+    r1.addend += r2.addend + 4;
+  r1.expr = r2.expr;
+  r1.sym = r2.sym;
+}
+
+void X86_64::applyBranchToBranchOpt() const {
+  applyBranchToBranchOptImpl(ctx, getBranchInfo, getControlTransferAddend,
+                             mergeControlTransferRelocations);
+}
+
 // If Intel Indirect Branch Tracking is enabled, we have to emit special PLT
 // entries containing endbr64 instructions. A PLT entry will be split into two
 // parts, one in .plt.sec (writePlt), and the other in .plt (writeIBTPlt).

lld/ELF/Config.h

@@ -276,6 +276,7 @@ struct Config {
   bool bpFunctionOrderForCompression = false;
   bool bpDataOrderForCompression = false;
   bool bpVerboseSectionOrderer = false;
+  bool branchToBranch = false;
   bool checkSections;
   bool checkDynamicRelocs;
   std::optional<llvm::DebugCompressionType> compressDebugSections;

lld/ELF/Driver.cpp

@@ -1589,6 +1589,8 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
   ctx.arg.zWxneeded = hasZOption(args, "wxneeded");
   setUnresolvedSymbolPolicy(ctx, args);
   ctx.arg.power10Stubs = args.getLastArgValue(OPT_power10_stubs_eq) != "no";
+  ctx.arg.branchToBranch = args.hasFlag(
+      OPT_branch_to_branch, OPT_no_branch_to_branch, ctx.arg.optimize >= 2);
 
   if (opt::Arg *arg = args.getLastArg(OPT_eb, OPT_el)) {
     if (arg->getOption().matches(OPT_eb))

lld/ELF/Options.td

@@ -59,6 +59,10 @@ def build_id: J<"build-id=">, HelpText<"Generate build ID note">,
   MetaVarName<"[fast,md5,sha1,uuid,0x<hexstring>]">;
 def : F<"build-id">, Alias<build_id>, AliasArgs<["sha1"]>, HelpText<"Alias for --build-id=sha1">;
 
+defm branch_to_branch: BB<"branch-to-branch",
+    "Enable branch-to-branch optimization (default at -O2)",
+    "Disable branch-to-branch optimization (default at -O0 and -O1)">;
+
 defm check_sections: B<"check-sections",
     "Check section addresses for overlaps (default)",
     "Do not check section addresses for overlaps">;

lld/ELF/Relocations.cpp

@@ -1671,9 +1671,10 @@ void RelocationScanner::scan(Relocs<RelTy> rels) {
   }
 
   // Sort relocations by offset for more efficient searching for
-  // R_RISCV_PCREL_HI20 and R_PPC64_ADDR64.
+  // R_RISCV_PCREL_HI20, R_PPC64_ADDR64 and the branch-to-branch optimization.
   if (ctx.arg.emachine == EM_RISCV ||
-      (ctx.arg.emachine == EM_PPC64 && sec->name == ".toc"))
+      (ctx.arg.emachine == EM_PPC64 && sec->name == ".toc") ||
+      ctx.arg.branchToBranch)
     llvm::stable_sort(sec->relocs(),
                       [](const Relocation &lhs, const Relocation &rhs) {
                         return lhs.offset < rhs.offset;
@@ -1964,6 +1965,9 @@ void elf::postScanRelocations(Ctx &ctx) {
   for (ELFFileBase *file : ctx.objectFiles)
     for (Symbol *sym : file->getLocalSymbols())
       fn(*sym);
+
+  if (ctx.arg.branchToBranch)
+    ctx.target->applyBranchToBranchOpt();
 }
 
 static bool mergeCmp(const InputSection *a, const InputSection *b) {

lld/ELF/Target.h

@@ -101,6 +101,7 @@ class TargetInfo {
 
   virtual void applyJumpInstrMod(uint8_t *loc, JumpModType type,
                                  JumpModType val) const {}
+  virtual void applyBranchToBranchOpt() const {}
 
   virtual ~TargetInfo();
 

lld/docs/ld.lld.1

@@ -93,6 +93,10 @@ Bind default visibility defined STB_GLOBAL function symbols locally for
 .Fl shared.
 .It Fl --be8
 Write a Big Endian ELF File using BE8 format(AArch32 only)
+.It Fl -branch-to-branch
+Enable the branch-to-branch optimizations: a branch whose target is
+another branch instruction is rewritten to point to the latter branch
+target (AArch64 and X86_64 only). Enabled by default at -O2.
 .It Fl -build-id Ns = Ns Ar value
 Generate a build ID note.
 .Ar value
@@ -414,7 +418,7 @@ If not specified,
 .Dv a.out
 is used as a default.
 .It Fl O Ns Ar value
-Optimize output file size.
+Optimize output file.
 .Ar value
 may be:
 .Pp
@@ -424,7 +428,7 @@ Disable string merging.
 .It Cm 1
 Enable string merging.
 .It Cm 2
-Enable string tail merging.
+Enable string tail merging and branch-to-branch optimization.
 .El
 .Pp
 .Fl O Ns Cm 1