[lld] Merge equivalent symbols found during ICF (#134342)

Fixes a correctness issue for AArch64 when ADRP and LDR instructions are
outlined in separate sections and sections are fed to ICF for
deduplication.

See test case (based on
#129122) for details. All
rodata.* sections are folded into a single section with ICF. This leads
to all f2_* function sections getting folded into one (as their
relocation target symbols g* belong to .rodata.g* sections that have
already been folded into one). Since relocations still refer original g*
symbols, we end up creating duplicate GOT entry for all such symbols.
This PR addresses that by tracking such folded symbols and create one
GOT entry for all such symbols.

Fixes #129122

Co-authored by: @jyknight

Author: pranavk

lld/ELF/ICF.cpp

@@ -80,6 +80,7 @@
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "SyntheticSections.h"
+#include "llvm/ADT/EquivalenceClasses.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Object/ELF.h"
 #include "llvm/Support/Parallel.h"
@@ -333,6 +334,28 @@ bool ICF<ELFT>::equalsConstant(const InputSection *a, const InputSection *b) {
              : constantEq(a, ra.relas, b, rb.relas);
 }
 
+template <class RelTy>
+static SmallVector<Symbol *> getReloc(const InputSection *sec,
+                                      Relocs<RelTy> relocs) {
+  SmallVector<Symbol *> syms;
+  for (auto ri = relocs.begin(), re = relocs.end(); ri != re; ++ri) {
+    Symbol &sym = sec->file->getRelocTargetSym(*ri);
+    syms.push_back(&sym);
+  }
+  return syms;
+}
+
+template <class ELFT>
+static SmallVector<Symbol *> getRelocTargetSyms(const InputSection *sec) {
+  const RelsOrRelas<ELFT> rel = sec->template relsOrRelas<ELFT>();
+  if (rel.areRelocsCrel())
+    return getReloc(sec, rel.crels);
+  if (rel.areRelocsRel())
+    return getReloc(sec, rel.rels);
+
+  return getReloc(sec, rel.relas);
+}
+
 // Compare two lists of relocations. Returns true if all pairs of
 // relocations point to the same section in terms of ICF.
 template <class ELFT>
@@ -535,14 +558,28 @@ template <class ELFT> void ICF<ELFT>::run() {
   auto print = [&ctx = ctx]() -> ELFSyncStream {
     return {ctx, ctx.arg.printIcfSections ? DiagLevel::Msg : DiagLevel::None};
   };
+
+  EquivalenceClasses<Symbol *> symbolEquivalence;
   // Merge sections by the equivalence class.
+  // Merge symbols identified as equivalent during ICF.
   forEachClassRange(0, sections.size(), [&](size_t begin, size_t end) {
     if (end - begin == 1)
       return;
     print() << "selected section " << sections[begin];
+    SmallVector<Symbol *> syms = getRelocTargetSyms<ELFT>(sections[begin]);
     for (size_t i = begin + 1; i < end; ++i) {
       print() << "  removing identical section " << sections[i];
       sections[begin]->replace(sections[i]);
+      SmallVector<Symbol *> replacedSyms =
+          getRelocTargetSyms<ELFT>(sections[i]);
+      assert(syms.size() == replacedSyms.size() &&
+             "Should have same number of syms!");
+      for (size_t i = 0; i < syms.size(); i++) {
+        if (syms[i] == replacedSyms[i] || !syms[i]->isGlobal() ||
+            !replacedSyms[i]->isGlobal())
+          continue;
+        symbolEquivalence.unionSets(syms[i], replacedSyms[i]);
+      }
 
       // At this point we know sections merged are fully identical and hence
       // we want to remove duplicate implicit dependencies such as link order
@@ -561,11 +598,26 @@ template <class ELFT> void ICF<ELFT>::run() {
           d->folded = true;
         }
   };
-  for (Symbol *sym : ctx.symtab->getSymbols())
+  for (Symbol *sym : ctx.symtab->getSymbols()) {
     fold(sym);
+    auto it = symbolEquivalence.findLeader(sym);
+    if (it != symbolEquivalence.member_end() && *it != sym) {
+      print() << "redirecting '" << sym->getName() << "' in symtab to '"
+              << (*it)->getName() << "'";
+      ctx.symtab->redirect(sym, *it);
+    }
+  }
   parallelForEach(ctx.objectFiles, [&](ELFFileBase *file) {
     for (Symbol *sym : file->getLocalSymbols())
       fold(sym);
+    for (Symbol *&sym : file->getMutableGlobalSymbols()) {
+      auto it = symbolEquivalence.findLeader(sym);
+      if (it != symbolEquivalence.member_end() && *it != sym) {
+        print() << "redirecting '" << sym->getName() << "' to '"
+                << (*it)->getName() << "'";
+        sym = *it;
+      }
+    }
   });
 
   // InputSectionDescription::sections is populated by processSectionCommands().

lld/ELF/SymbolTable.cpp

@@ -29,6 +29,12 @@ using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
+void SymbolTable::redirect(Symbol *from, Symbol *to) {
+  int &fromIdx = symMap[CachedHashStringRef(from->getName())];
+  const int toIdx = symMap[CachedHashStringRef(to->getName())];
+  fromIdx = toIdx;
+}
+
 void SymbolTable::wrap(Symbol *sym, Symbol *real, Symbol *wrap) {
   // Redirect __real_foo to the original foo and foo to the original __wrap_foo.
   int &idx1 = symMap[CachedHashStringRef(sym->getName())];

lld/ELF/SymbolTable.h

@@ -41,6 +41,7 @@ class SymbolTable {
   SymbolTable(Ctx &ctx) : ctx(ctx) {}
   ArrayRef<Symbol *> getSymbols() const { return symVector; }
 
+  void redirect(Symbol *from, Symbol *to);
   void wrap(Symbol *sym, Symbol *real, Symbol *wrap);
 
   Symbol *insert(StringRef name);

lld/test/ELF/aarch64-got-merging-icf.s

@@ -0,0 +1,95 @@
+## REQUIRES: aarch64
+## Check that symbols that ICF assumes to be the same get a single GOT entry
+
+# RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t
+# RUN: llvm-mc -filetype=obj -crel -triple=aarch64 %s -o %tcrel
+# RUN: ld.lld %t -o %t2 --icf=all
+# RUN: ld.lld %tcrel -o %tcrel2 --icf=all
+
+# RUN: llvm-objdump --section-headers %t2 | FileCheck %s --check-prefix=EXE
+# RUN: llvm-objdump --section-headers %tcrel2 | FileCheck %s --check-prefix=EXE
+
+# RUN: ld.lld -shared %t -o %t3 --icf=all
+# RUN: ld.lld -shared %tcrel -o %tcrel3 --icf=all
+
+# RUN: llvm-objdump --section-headers %t3 | FileCheck %s --check-prefix=DSO
+# RUN: llvm-objdump --section-headers %tcrel3 | FileCheck %s --check-prefix=DSO
+
+## All global g* symbols should merge into a single GOT entry while non-global
+## gets its own GOT entry.
+# EXE: {{.*}}.got 00000010{{.*}}
+
+## When symbols are preemptible in DSO mode, GOT entries wouldn't be merged
+# DSO: {{.*}}.got 00000028{{.*}}
+
+.addrsig
+
+callee:
+ret
+
+.macro f, index, isglobal
+
+# (Kept unique) first instruction of the GOT code sequence
+.section .text.f1_\index,"ax",@progbits
+f1_\index:
+adrp x0, :got:g\index
+mov x1, #\index
+b f2_\index
+
+# Folded, second instruction of the GOT code sequence
+.section .text.f2_\index,"ax",@progbits
+f2_\index:
+ldr x0, [x0, :got_lo12:g\index] 
+b callee
+
+# Folded
+.ifnb \isglobal
+.globl g\index
+.endif
+.section .rodata.g\index,"a",@progbits
+g_\index:
+.long 111
+.long 122
+
+g\index:
+.byte 123
+
+.section .text._start,"ax",@progbits
+bl f1_\index
+
+.endm
+
+## Another set of sections merging: g1 <- g2. Linker should be able to
+## resolve both g1 and g2 to g0 based on ICF on previous sections.
+
+.section .text.t1_0,"ax",@progbits
+t1_0:
+adrp x2, :got:g1
+mov x3, #1
+b t2_0
+
+.section .text.t2_0,"ax",@progbits
+t2_0:
+ldr x2, [x2, :got_lo12:g1]
+b callee
+
+.section .text.t1_1,"ax",@progbits
+t1_1:
+adrp x2, :got:g2
+mov x3, #2
+b t2_1
+
+.section .text.t2_1,"ax",@progbits
+t2_1:
+ldr x2, [x2, :got_lo12:g2]
+b callee
+
+.section .text._start,"ax",@progbits
+.globl _start
+_start:
+
+f 0 1
+f 1 1
+f 2 1
+f 3 1
+f 4

lld/test/ELF/icf-preemptible.s

@@ -17,6 +17,12 @@
 # EXE-NEXT: selected section {{.*}}:(.text.h1)
 # EXE-NEXT:   removing identical section {{.*}}:(.text.h2)
 # EXE-NEXT:   removing identical section {{.*}}:(.text.h3)
+# EXE-NEXT: redirecting 'f2' in symtab to 'f1'
+# EXE-NEXT: redirecting 'g2' in symtab to 'g1'
+# EXE-NEXT: redirecting 'g3' in symtab to 'g1'
+# EXE-NEXT: redirecting 'f2' to 'f1'
+# EXE-NEXT: redirecting 'g2' to 'g1'
+# EXE-NEXT: redirecting 'g3' to 'g1'
 # EXE-NOT:  {{.}}
 
 ## Definitions are preemptible in a DSO. Only leaf functions can be folded.