[lld-macho] Fix branch extension logic compatibility with __objc_stubs

[alx32]

Enhance branch extension logic to handle __objc_stubs identically to __stubs

The branch extension algorithm currently has specific handling for the __stubs section:

1. It ensures all __stubs content is directly reachable via branches from the text section.
2. It calculates the latest text section address that might require thunks to reach the end of __stubs.

The __objc_stubs section requires precisely the same handling due to its similar nature, but this was not implemented.

This commit generalizes the existing logic so it applies consistently to both the __stubs and __objc_stubs sections, ensuring correct reachability and thunk placement for both. Without this change it's possible to get relocation errors during linking in scenarios where the __objc_stubs section is large enough.

[llvmbot]

@llvm/pr-subscribers-lld-macho

@llvm/pr-subscribers-lld

Author: None (alx32)

Changes

Extend branch extension logic to handle __objc_stubs identically to __stubs

The branch extension algorithm currently has specific handling for the __stubs section:

1. It ensures all __stubs content is directly reachable via branches from the text section.
2. It calculates the latest text section address that might require thunks to reach the end of __stubs.

The __objc_stubs section requires precisely the same handling due to its similar nature, but this was not implemented.

This commit generalizes the existing logic so it applies consistently to both the __stubs and __objc_stubs sections, ensuring correct reachability and thunk placement for both. Without this change it's possible to get relocation errors during linking in scenarios where the __objc_stubs section is large enough.

---

Full diff: https://github.com/llvm/llvm-project/pull/137913.diff

3 Files Affected:

• (modified) lld/MachO/ConcatOutputSection.cpp (+60-30)
• (modified) lld/MachO/ConcatOutputSection.h (+1-1)
• (modified) lld/test/MachO/arm64-thunks.s (+38-2)

diff --git a/lld/MachO/ConcatOutputSection.cpp b/lld/MachO/ConcatOutputSection.cpp
index d64816ec695ad..48bc06194eef0 100644
--- a/lld/MachO/ConcatOutputSection.cpp
+++ b/lld/MachO/ConcatOutputSection.cpp
@@ -133,8 +133,16 @@ bool TextOutputSection::needsThunks() const {
   // we've already processed in this segment needs thunks, so do the
   // rest.
   bool needsThunks = parent && parent->needsThunks;
+
+  // Calculate the total size of all branch target sections
+  uint64_t branchTargetsSize = in.stubs->getSize();
+
+  // Add the size of __objc_stubs section if it exists
+  if (in.objcStubs && in.objcStubs->isNeeded())
+    branchTargetsSize += in.objcStubs->getSize();
+
   if (!needsThunks &&
-      isecAddr - addr + in.stubs->getSize() <=
+      isecAddr - addr + branchTargetsSize <=
           std::min(target->backwardBranchRange, target->forwardBranchRange))
     return false;
   // Yes, this program is large enough to need thunks.
@@ -148,11 +156,11 @@ bool TextOutputSection::needsThunks() const {
       auto *sym = cast<Symbol *>(r.referent);
       // Pre-populate the thunkMap and memoize call site counts for every
       // InputSection and ThunkInfo. We do this for the benefit of
-      // estimateStubsInRangeVA().
+      // estimateBranchTargetThresholdVA().
       ThunkInfo &thunkInfo = thunkMap[sym];
       // Knowing ThunkInfo call site count will help us know whether or not we
       // might need to create more for this referent at the time we are
-      // estimating distance to __stubs in estimateStubsInRangeVA().
+      // estimating distance to __stubs in estimateBranchTargetThresholdVA().
       ++thunkInfo.callSiteCount;
       // We can avoid work on InputSections that have no BRANCH relocs.
       isec->hasCallSites = true;
@@ -161,10 +169,11 @@ bool TextOutputSection::needsThunks() const {
   return true;
 }
 
-// Since __stubs is placed after __text, we must estimate the address
-// beyond which stubs are within range of a simple forward branch.
-// This is called exactly once, when the last input section has been finalized.
-uint64_t TextOutputSection::estimateStubsInRangeVA(size_t callIdx) const {
+// Estimate the address beyond which branch targets (like __stubs and
+// __objc_stubs) are within range of a simple forward branch. This is called
+// exactly once, when the last input section has been finalized.
+uint64_t
+TextOutputSection::estimateBranchTargetThresholdVA(size_t callIdx) const {
   // Tally the functions which still have call sites remaining to process,
   // which yields the maximum number of thunks we might yet place.
   size_t maxPotentialThunks = 0;
@@ -172,8 +181,8 @@ uint64_t TextOutputSection::estimateStubsInRangeVA(size_t callIdx) const {
     ThunkInfo &ti = tp.second;
     // This overcounts: Only sections that are in forward jump range from the
     // currently-active section get finalized, and all input sections are
-    // finalized when estimateStubsInRangeVA() is called. So only backward
-    // jumps will need thunks, but we count all jumps.
+    // finalized when estimateBranchTargetThresholdVA() is called. So only
+    // backward jumps will need thunks, but we count all jumps.
     if (ti.callSitesUsed < ti.callSiteCount)
       maxPotentialThunks += 1;
   }
@@ -200,7 +209,8 @@ uint64_t TextOutputSection::estimateStubsInRangeVA(size_t callIdx) const {
   assert(isecEnd - isecVA <= forwardBranchRange &&
          "should only finalize sections in jump range");
 
-  // Estimate the maximum size of the code, right before the stubs section.
+  // Estimate the maximum size of the code, right before the branch target
+  // sections.
   uint64_t maxTextSize = 0;
   // Add the size of all the inputs, including the unprocessed ones.
   maxTextSize += isecEnd;
@@ -214,21 +224,35 @@ uint64_t TextOutputSection::estimateStubsInRangeVA(size_t callIdx) const {
   // 'maxPotentialThunks' overcounts, this is an estimate of the upper limit.
   maxTextSize += maxPotentialThunks * target->thunkSize;
 
-  // Estimated maximum VA of last stub.
-  uint64_t maxVAOfLastStub = maxTextSize + in.stubs->getSize();
+  // Calculate the total size of all late branch target sections
+  uint64_t branchTargetsSize = 0;
+
+  // Add the size of __stubs section
+  branchTargetsSize += in.stubs->getSize();
+
+  // Add the size of __objc_stubs section if it exists
+  if (in.objcStubs && in.objcStubs->isNeeded())
+    branchTargetsSize += in.objcStubs->getSize();
+
+  // Estimated maximum VA of the last branch target.
+  uint64_t maxVAOfLastBranchTarget = maxTextSize + branchTargetsSize;
 
-  // Estimate the address after which call sites can safely call stubs
+  // Estimate the address after which call sites can safely call branch targets
   // directly rather than through intermediary thunks.
-  uint64_t stubsInRangeVA = maxVAOfLastStub - forwardBranchRange;
+  uint64_t branchTargetThresholdVA =
+      maxVAOfLastBranchTarget - forwardBranchRange;
 
   log("thunks = " + std::to_string(thunkMap.size()) +
       ", potential = " + std::to_string(maxPotentialThunks) +
-      ", stubs = " + std::to_string(in.stubs->getSize()) + ", isecVA = " +
-      utohexstr(isecVA) + ", threshold = " + utohexstr(stubsInRangeVA) +
-      ", isecEnd = " + utohexstr(isecEnd) +
+      ", stubs = " + std::to_string(in.stubs->getSize()) +
+      (in.objcStubs && in.objcStubs->isNeeded()
+           ? ", objc_stubs = " + std::to_string(in.objcStubs->getSize())
+           : "") +
+      ", isecVA = " + utohexstr(isecVA) + ", threshold = " +
+      utohexstr(branchTargetThresholdVA) + ", isecEnd = " + utohexstr(isecEnd) +
       ", tail = " + utohexstr(isecEnd - isecVA) +
       ", slop = " + utohexstr(forwardBranchRange - (isecEnd - isecVA)));
-  return stubsInRangeVA;
+  return branchTargetThresholdVA;
 }
 
 void ConcatOutputSection::finalizeOne(ConcatInputSection *isec) {
@@ -254,7 +278,7 @@ void TextOutputSection::finalize() {
 
   uint64_t forwardBranchRange = target->forwardBranchRange;
   uint64_t backwardBranchRange = target->backwardBranchRange;
-  uint64_t stubsInRangeVA = TargetInfo::outOfRangeVA;
+  uint64_t branchTargetThresholdVA = TargetInfo::outOfRangeVA;
   size_t thunkSize = target->thunkSize;
   size_t relocCount = 0;
   size_t callSiteCount = 0;
@@ -297,16 +321,18 @@ void TextOutputSection::finalize() {
     if (!isec->hasCallSites)
       continue;
 
-    if (finalIdx == endIdx && stubsInRangeVA == TargetInfo::outOfRangeVA) {
-      // When we have finalized all input sections, __stubs (destined
-      // to follow __text) comes within range of forward branches and
-      // we can estimate the threshold address after which we can
-      // reach any stub with a forward branch. Note that although it
-      // sits in the middle of a loop, this code executes only once.
+    if (finalIdx == endIdx &&
+        branchTargetThresholdVA == TargetInfo::outOfRangeVA) {
+      // When we have finalized all input sections, branch target sections (like
+      // __stubs and __objc_stubs) (destined to follow __text) come within range
+      // of forward branches and we can estimate the threshold address after
+      // which we can reach any branch target with a forward branch. Note that
+      // although it sits in the middle of a loop, this code executes only once.
       // It is in the loop because we need to call it at the proper
       // time: the earliest call site from which the end of __text
-      // (and start of __stubs) comes within range of a forward branch.
-      stubsInRangeVA = estimateStubsInRangeVA(callIdx);
+      // (and start of branch target sections) comes within range of a forward
+      // branch.
+      branchTargetThresholdVA = estimateBranchTargetThresholdVA(callIdx);
     }
     // Process relocs by ascending address, i.e., ascending offset within isec
     std::vector<Reloc> &relocs = isec->relocs;
@@ -328,10 +354,14 @@ void TextOutputSection::finalize() {
       auto *funcSym = cast<Symbol *>(r.referent);
       ThunkInfo &thunkInfo = thunkMap[funcSym];
       // The referent is not reachable, so we need to use a thunk ...
-      if (funcSym->isInStubs() && callVA >= stubsInRangeVA) {
+      if ((funcSym->isInStubs() ||
+           (in.objcStubs && in.objcStubs->isNeeded() &&
+            ObjCStubsSection::isObjCStubSymbol(funcSym))) &&
+          callVA >= branchTargetThresholdVA) {
         assert(callVA != TargetInfo::outOfRangeVA);
-        // ... Oh, wait! We are close enough to the end that __stubs
-        // are now within range of a simple forward branch.
+        // ... Oh, wait! We are close enough to the end that branch target
+        // sections (__stubs, __objc_stubs) are now within range of a simple
+        // forward branch.
         continue;
       }
       uint64_t funcVA = funcSym->resolveBranchVA();
diff --git a/lld/MachO/ConcatOutputSection.h b/lld/MachO/ConcatOutputSection.h
index 8131c48d31113..1c68018f97552 100644
--- a/lld/MachO/ConcatOutputSection.h
+++ b/lld/MachO/ConcatOutputSection.h
@@ -80,7 +80,7 @@ class TextOutputSection : public ConcatOutputSection {
   }
 
 private:
-  uint64_t estimateStubsInRangeVA(size_t callIdx) const;
+  uint64_t estimateBranchTargetThresholdVA(size_t callIdx) const;
 
   std::vector<ConcatInputSection *> thunks;
 };
diff --git a/lld/test/MachO/arm64-thunks.s b/lld/test/MachO/arm64-thunks.s
index b48abec71f63a..cd3842895c47e 100644
--- a/lld/test/MachO/arm64-thunks.s
+++ b/lld/test/MachO/arm64-thunks.s
@@ -18,9 +18,28 @@
 ## with safe_thunks ICF.
 # RUN: %lld -arch arm64 -dead_strip -lSystem -U _extern_sym -map %t/thunk.map -o %t/thunk %t/input.o --icf=safe_thunks
 # RUN: llvm-objdump --no-print-imm-hex -d --no-show-raw-insn %t/thunk | FileCheck %s
+# RUN: llvm-objdump --macho --section-headers %t/thunk > %t/headers.txt
+# RUN: llvm-otool -vs __DATA __objc_selrefs %t/thunk >> %t/headers.txt
+# RUN: llvm-otool -vs __TEXT __objc_stubs %t/thunk >> %t/headers.txt
+# RUN: FileCheck %s --check-prefix=OBJC < %t/headers.txt
 
 # RUN: FileCheck %s --input-file %t/thunk.map --check-prefix=MAP
- 
+
+# OBJC: Sections:
+# OBJC: __text
+# OBJC-NEXT: __lcxx_override
+# OBJC-NEXT: __stubs
+# OBJC-NEXT: __stub_helper
+# OBJC-NEXT: __objc_stubs
+
+# OBJC: Contents of (__DATA,__objc_selrefs) section
+# OBJC-NEXT: {{[0-9a-f]*}}  __TEXT:__objc_methname:foo
+# OBJC-NEXT: {{[0-9a-f]*}}  __TEXT:__objc_methname:bar
+
+# OBJC: Contents of (__TEXT,__objc_stubs) section
+# OBJC: _objc_msgSend$bar:
+# OBJC: _objc_msgSend$foo:
+
 # MAP:      0x{{[[:xdigit:]]+}} {{.*}} _fold_func_low_addr
 # MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _a
 # MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _b
@@ -45,7 +64,6 @@
 # MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _e.thunk.1
 # MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _f.thunk.1
 # MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_low_addr.thunk.0
-# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} ltmp0.thunk.0
 # MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _z
 
 
@@ -206,6 +224,22 @@
 
 # CHECK: [[#%x, NAN_PAGE + NAN_OFFSET]] <__stubs>:
 
+.section  __TEXT,__objc_methname,cstring_literals
+lselref1:
+  .asciz  "foo"
+lselref2:
+  .asciz  "bar"
+
+.section  __DATA,__objc_selrefs,literal_pointers,no_dead_strip
+.p2align  3
+.quad lselref1
+.quad lselref2
+
+.text
+.globl _objc_msgSend
+_objc_msgSend:
+  ret
+
 .subsections_via_symbols
 
 .addrsig
@@ -352,6 +386,8 @@ _main:
   bl _fold_func_low_addr
   bl _fold_func_high_addr
   bl ___nan
+  bl _objc_msgSend$foo
+  bl _objc_msgSend$bar
   ret
 
 .globl _fold_func_high_addr