[lld-macho] Enhance safe ICF with thunk-based deduplication

Currently, our `safe` ICF mode only merges non-address-significant code, leaving duplicate address-significant functions in the output. This patch introduces `safe_thunks` ICF mode, which keeps a single master copy of each function and replaces address-significant duplicates with thunks that branch to the master copy.

Author: Alex B

lld/MachO/Arch/ARM64.cpp

@@ -41,6 +41,9 @@ struct ARM64 : ARM64Common {
                             Symbol *objcMsgSend) const override;
   void populateThunk(InputSection *thunk, Symbol *funcSym) override;
   void applyOptimizationHints(uint8_t *, const ObjFile &) const override;
+
+  virtual void initICFSafeThunkBody(InputSection *thunk,
+                                    InputSection *branchTarget) const override;
 };
 
 } // namespace
@@ -175,6 +178,23 @@ void ARM64::populateThunk(InputSection *thunk, Symbol *funcSym) {
                              /*offset=*/0, /*addend=*/0,
                              /*referent=*/funcSym);
 }
+// Just a single direct branch to the target function.
+static constexpr uint32_t icfSafeThunkCode[] = {
+    0x94000000, // 08: b    target
+};
+
+void ARM64::initICFSafeThunkBody(InputSection *thunk,
+                                 InputSection *branchTarget) const {
+  // The base data here will not be itself modified, we'll just be adding a
+  // reloc below. So we can directly use the constexpr above as the data.
+  thunk->data = {reinterpret_cast<const uint8_t *>(icfSafeThunkCode),
+                 sizeof(icfSafeThunkCode)};
+
+  thunk->relocs.emplace_back(/*type=*/ARM64_RELOC_BRANCH26,
+                             /*pcrel=*/true, /*length=*/2,
+                             /*offset=*/0, /*addend=*/0,
+                             /*referent=*/branchTarget);
+}
 
 ARM64::ARM64() : ARM64Common(LP64()) {
   cpuType = CPU_TYPE_ARM64;

lld/MachO/Config.h

@@ -68,6 +68,7 @@ enum class ICFLevel {
   unknown,
   none,
   safe,
+  safe_thunks,
   all,
 };
 

lld/MachO/Driver.cpp

@@ -847,8 +847,15 @@ static ICFLevel getICFLevel(const ArgList &args) {
   auto icfLevel = StringSwitch<ICFLevel>(icfLevelStr)
                       .Cases("none", "", ICFLevel::none)
                       .Case("safe", ICFLevel::safe)
+                      .Case("safe_thunks", ICFLevel::safe_thunks)
                       .Case("all", ICFLevel::all)
                       .Default(ICFLevel::unknown);
+
+  if (icfLevel == ICFLevel::safe_thunks &&
+      !is_contained({AK_x86_64h, AK_arm64}, config->arch())) {
+    error("--icf=safe_thunks is only supported on arm64 targets");
+  }
+
   if (icfLevel == ICFLevel::unknown) {
     warn(Twine("unknown --icf=OPTION `") + icfLevelStr +
          "', defaulting to `none'");
@@ -2104,7 +2111,8 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     // foldIdenticalLiterals before foldIdenticalSections.
     foldIdenticalLiterals();
     if (config->icfLevel != ICFLevel::none) {
-      if (config->icfLevel == ICFLevel::safe)
+      if (config->icfLevel == ICFLevel::safe ||
+          config->icfLevel == ICFLevel::safe_thunks)
         markAddrSigSymbols();
       foldIdenticalSections(/*onlyCfStrings=*/false);
     } else if (config->dedupStrings) {

lld/MachO/ICF.cpp

@@ -45,6 +45,7 @@ class ICF {
                       const ConcatInputSection *ib);
   bool equalsVariable(const ConcatInputSection *ia,
                       const ConcatInputSection *ib);
+  void applySafeThunksToRange(size_t begin, size_t end);
 
   // ICF needs a copy of the inputs vector because its equivalence-class
   // segregation algorithm destroys the proper sequence.
@@ -251,6 +252,38 @@ void ICF::forEachClassRange(size_t begin, size_t end,
   }
 }
 
+// Given a range of identical icfInputs's, replace address significant functions
+// with a thunk that is just a direct branch to the first function in the
+// series. This way we end up we keep only one main body of the function but we
+// still retain address uniqueness of rellevant functions by having them be a
+// direct branch thunk rather than contain a full copy of the actual function
+// body.
+void ICF::applySafeThunksToRange(size_t begin, size_t end) {
+  // If we need to create a unique ICF thunk, use the first section as the
+  // section that all thunks will branch to.
+  ConcatInputSection *masterIsec = icfInputs[begin];
+
+  uint32_t keepUniqueCount = masterIsec->keepUnique ? 1 : 0;
+  for (size_t i = begin + 1; i < end; ++i) {
+    ConcatInputSection *isec = icfInputs[i];
+    if (isec->keepUnique)
+      ++keepUniqueCount;
+
+    // We create thunks for the 2nd, 3rd, ... keepUnique sections. The first
+    // keepUnique section we leave as is - as it will not end up sharing an
+    // address with any other keepUnique section.
+    if (keepUniqueCount >= 2 && isec->keepUnique) {
+      ConcatInputSection *thunk =
+          makeSyntheticInputSection(isec->getSegName(), isec->getName());
+      target->initICFSafeThunkBody(thunk, masterIsec);
+
+      thunk->foldIdentical(isec);
+
+      addInputSection(thunk);
+    }
+  }
+}
+
 // Split icfInputs into shards, then parallelize invocation of FUNC on subranges
 // with matching equivalence class
 void ICF::forEachClass(llvm::function_ref<void(size_t, size_t)> func) {
@@ -335,9 +368,20 @@ void ICF::run() {
   forEachClass([&](size_t begin, size_t end) {
     if (end - begin < 2)
       return;
+    bool useSafeThunks = config->icfLevel == ICFLevel::safe_thunks;
+
+    // For ICF level safe_thunks, replace keepUnique function bodies with
+    // thunks. For all other ICF levles, directly merge the functions.
+    if (useSafeThunks)
+      applySafeThunksToRange(begin, end);
+
     ConcatInputSection *beginIsec = icfInputs[begin];
-    for (size_t i = begin + 1; i < end; ++i)
+    for (size_t i = begin + 1; i < end; ++i) {
+      // When using safe_thunks, keepUnique inputs are already handeled above
+      if (useSafeThunks && icfInputs[i]->keepUnique)
+        continue;
       beginIsec->foldIdentical(icfInputs[i]);
+    }
   });
 }
 
@@ -421,11 +465,22 @@ void macho::foldIdenticalSections(bool onlyCfStrings) {
     // can still fold it.
     bool hasFoldableFlags = (isSelRefsSection(isec) ||
                              sectionType(isec->getFlags()) == MachO::S_REGULAR);
+
+    bool isCodeSec = isCodeSection(isec);
+
+    // When keepUnique is true, the section is not foldable. Unless we are at
+    // icf level safe_thunks, in which case we still want to fold code sections.
+    // When using safe_thunks we'll apply the safe_thunks logic at merge time
+    // based on the 'keepUnique' flag.
+    bool noUniqueRequirement =
+        !isec->keepUnique ||
+        ((config->icfLevel == ICFLevel::safe_thunks) && isCodeSec);
+
     // FIXME: consider non-code __text sections as foldable?
     bool isFoldable = (!onlyCfStrings || isCfStringSection(isec)) &&
-                      (isCodeSection(isec) || isFoldableWithAddendsRemoved ||
+                      (isCodeSec || isFoldableWithAddendsRemoved ||
                        isGccExceptTabSection(isec)) &&
-                      !isec->keepUnique && !isec->hasAltEntry &&
+                      noUniqueRequirement && !isec->hasAltEntry &&
                       !isec->shouldOmitFromOutput() && hasFoldableFlags;
     if (isFoldable) {
       foldable.push_back(isec);

lld/MachO/Target.h

@@ -74,6 +74,12 @@ class TargetInfo {
                                     uint64_t selrefVA,
                                     Symbol *objcMsgSend) const = 0;
 
+  // Init 'thunk' so that it be a direct jump to 'branchTarget'.
+  virtual void initICFSafeThunkBody(InputSection *thunk,
+                                    InputSection *branchTarget) const {
+    llvm_unreachable("target does not support ICF safe thunks");
+  }
+
   // Symbols may be referenced via either the GOT or the stubs section,
   // depending on the relocation type. prepareSymbolRelocation() will set up the
   // GOT/stubs entries, and resolveSymbolVA() will return the addresses of those