[lld] 8389d6f - [lld] Merge equivalent symbols found during ICF (#134342)
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Mon Apr 21 15:17:07 PDT 2025
Author: Pranav Kant
Date: 2025-04-21T15:17:03-07:00
New Revision: 8389d6fad76bd880f02bddce7f0f2612ff0afc40
URL: https://github.com/llvm/llvm-project/commit/8389d6fad76bd880f02bddce7f0f2612ff0afc40
DIFF: https://github.com/llvm/llvm-project/commit/8389d6fad76bd880f02bddce7f0f2612ff0afc40.diff
LOG: [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
https://github.com/llvm/llvm-project/issues/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 https://github.com/llvm/llvm-project/issues/129122
Co-authored by: @jyknight
Added:
lld/test/ELF/aarch64-got-merging-icf.s
Modified:
lld/ELF/ICF.cpp
lld/ELF/SymbolTable.cpp
lld/ELF/SymbolTable.h
lld/test/ELF/icf-preemptible.s
Removed:
################################################################################
diff --git a/lld/ELF/ICF.cpp b/lld/ELF/ICF.cpp
index 1cdcf6be9d8a9..849f6bdd445f9 100644
--- a/lld/ELF/ICF.cpp
+++ b/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().
diff --git a/lld/ELF/SymbolTable.cpp b/lld/ELF/SymbolTable.cpp
index b8a70d4e898fc..91e47e15b01a4 100644
--- a/lld/ELF/SymbolTable.cpp
+++ b/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())];
diff --git a/lld/ELF/SymbolTable.h b/lld/ELF/SymbolTable.h
index d6443742f7baa..e3a39bac85f97 100644
--- a/lld/ELF/SymbolTable.h
+++ b/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);
diff --git a/lld/test/ELF/aarch64-got-merging-icf.s b/lld/test/ELF/aarch64-got-merging-icf.s
new file mode 100644
index 0000000000000..01d7bdfd75185
--- /dev/null
+++ b/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", at 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", at progbits
+f2_\index:
+ldr x0, [x0, :got_lo12:g\index]
+b callee
+
+# Folded
+.ifnb \isglobal
+.globl g\index
+.endif
+.section .rodata.g\index,"a", at progbits
+g_\index:
+.long 111
+.long 122
+
+g\index:
+.byte 123
+
+.section .text._start,"ax", at 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", at progbits
+t1_0:
+adrp x2, :got:g1
+mov x3, #1
+b t2_0
+
+.section .text.t2_0,"ax", at progbits
+t2_0:
+ldr x2, [x2, :got_lo12:g1]
+b callee
+
+.section .text.t1_1,"ax", at progbits
+t1_1:
+adrp x2, :got:g2
+mov x3, #2
+b t2_1
+
+.section .text.t2_1,"ax", at progbits
+t2_1:
+ldr x2, [x2, :got_lo12:g2]
+b callee
+
+.section .text._start,"ax", at progbits
+.globl _start
+_start:
+
+f 0 1
+f 1 1
+f 2 1
+f 3 1
+f 4
diff --git a/lld/test/ELF/icf-preemptible.s b/lld/test/ELF/icf-preemptible.s
index 4bd1eca438b19..9352493600695 100644
--- a/lld/test/ELF/icf-preemptible.s
+++ b/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.
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