[lld] [lld-macho] icf objc stubs (PR #79730)
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Sat Jan 27 22:37:03 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-lld
@llvm/pr-subscribers-lld-macho
Author: Kyungwoo Lee (kyulee-com)
<details>
<summary>Changes</summary>
This supports icf for objc stubs.
---
Full diff: https://github.com/llvm/llvm-project/pull/79730.diff
7 Files Affected:
- (modified) lld/MachO/Driver.cpp (+2-3)
- (modified) lld/MachO/SyntheticSections.cpp (+71-11)
- (modified) lld/MachO/SyntheticSections.h (+9-1)
- (modified) lld/MachO/Writer.cpp (+10-1)
- (added) lld/test/MachO/arm64-objc-stubs-dead.s (+27)
- (modified) lld/test/MachO/objc-selrefs.s (-3)
- (modified) lld/test/MachO/x86-64-objc-stubs.s (+10)
``````````diff
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 411fbcfcf233eb8..519bd483dacb688 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1275,11 +1275,10 @@ static void foldIdenticalLiterals() {
static void addSynthenticMethnames() {
std::string &data = *make<std::string>();
llvm::raw_string_ostream os(data);
- const int prefixLength = ObjCStubsSection::symbolPrefix.size();
for (Symbol *sym : symtab->getSymbols())
if (isa<Undefined>(sym))
- if (sym->getName().starts_with(ObjCStubsSection::symbolPrefix))
- os << sym->getName().drop_front(prefixLength) << '\0';
+ if (ObjCStubsSection::isObjCStubSymbol(sym))
+ os << ObjCStubsSection::getMethName(sym) << '\0';
if (data.empty())
return;
diff --git a/lld/MachO/SyntheticSections.cpp b/lld/MachO/SyntheticSections.cpp
index 53220ad04b842c1..d3674a2b2c5dcc5 100644
--- a/lld/MachO/SyntheticSections.cpp
+++ b/lld/MachO/SyntheticSections.cpp
@@ -814,11 +814,54 @@ ObjCStubsSection::ObjCStubsSection()
: target->objcStubsSmallAlignment;
}
+bool ObjCStubsSection::isObjCStubSymbol(Symbol *sym) {
+ return sym->getName().starts_with(symbolPrefix);
+}
+
+StringRef ObjCStubsSection::getMethName(Symbol *sym) {
+ assert(isObjCStubSymbol(sym) && "not an objc stub");
+ auto name = sym->getName();
+ StringRef methname = name.drop_front(symbolPrefix.size());
+ return methname;
+}
+
+void ObjCStubsSection::initialize() {
+ // Do not fold selrefs without ICF.
+ if (config->icfLevel == ICFLevel::none)
+ return;
+
+ // Search methnames already referenced in __objc_selrefs
+ // Map the name to the corresponding selref entry
+ // which we will reuse when creating objc stubs.
+ for (ConcatInputSection *isec : inputSections) {
+ if (isec->shouldOmitFromOutput())
+ continue;
+ if (isec->getName() != "__objc_selrefs")
+ continue;
+ // We expect a single relocation per selref entry to __objc_methname that
+ // might be aggregated.
+ assert(isec->relocs.size() == 1);
+ auto Reloc = isec->relocs[0];
+ if (const auto *sym = Reloc.referent.dyn_cast<Symbol *>()) {
+ if (const auto *d = dyn_cast<Defined>(sym)) {
+ auto *cisec = cast<CStringInputSection>(d->isec);
+ auto methname = cisec->getStringRefAtOffset(d->value);
+ methnameToselref[methname] = isec;
+ }
+ }
+ }
+}
+
void ObjCStubsSection::addEntry(Symbol *sym) {
- assert(sym->getName().starts_with(symbolPrefix) && "not an objc stub");
- StringRef methname = sym->getName().drop_front(symbolPrefix.size());
- offsets.push_back(
- in.objcMethnameSection->getStringOffset(methname).outSecOff);
+ StringRef methname = getMethName(sym);
+ // We will create a selref entry for each unique methname.
+ if (!methnameToselref.count(methname) &&
+ !methnameToidxOffsetPair.count(methname)) {
+ auto methnameOffset =
+ in.objcMethnameSection->getStringOffset(methname).outSecOff;
+ auto selIndex = methnameToidxOffsetPair.size();
+ methnameToidxOffsetPair[methname] = {selIndex, methnameOffset};
+ }
auto stubSize = config->objcStubsMode == ObjCStubsMode::fast
? target->objcStubsFastSize
@@ -853,10 +896,12 @@ void ObjCStubsSection::setUp() {
in.stubs->addEntry(objcMsgSend);
}
- size_t size = offsets.size() * target->wordSize;
+ size_t size = methnameToidxOffsetPair.size() * target->wordSize;
uint8_t *selrefsData = bAlloc().Allocate<uint8_t>(size);
- for (size_t i = 0, n = offsets.size(); i < n; ++i)
- write64le(&selrefsData[i * target->wordSize], offsets[i]);
+ for (const auto &[methname, idxOffsetPair] : methnameToidxOffsetPair) {
+ const auto &[idx, offset] = idxOffsetPair;
+ write64le(&selrefsData[idx * target->wordSize], offset);
+ }
in.objcSelrefs =
makeSyntheticInputSection(segment_names::data, section_names::objcSelrefs,
@@ -865,12 +910,13 @@ void ObjCStubsSection::setUp() {
/*align=*/target->wordSize);
in.objcSelrefs->live = true;
- for (size_t i = 0, n = offsets.size(); i < n; ++i) {
+ for (const auto &[methname, idxOffsetPair] : methnameToidxOffsetPair) {
+ const auto &[idx, offset] = idxOffsetPair;
in.objcSelrefs->relocs.push_back(
{/*type=*/target->unsignedRelocType,
/*pcrel=*/false, /*length=*/3,
- /*offset=*/static_cast<uint32_t>(i * target->wordSize),
- /*addend=*/offsets[i] * in.objcMethnameSection->align,
+ /*offset=*/static_cast<uint32_t>(idx * target->wordSize),
+ /*addend=*/offset * in.objcMethnameSection->align,
/*referent=*/in.objcMethnameSection->isec});
}
@@ -894,8 +940,22 @@ void ObjCStubsSection::writeTo(uint8_t *buf) const {
uint64_t stubOffset = 0;
for (size_t i = 0, n = symbols.size(); i < n; ++i) {
Defined *sym = symbols[i];
+ uint64_t selBaseAddr;
+ uint64_t selIndex;
+
+ auto methname = getMethName(sym);
+ auto j = methnameToselref.find(methname);
+ if (j != methnameToselref.end()) {
+ selBaseAddr = j->second->getVA(0);
+ selIndex = 0;
+ } else {
+ auto k = methnameToidxOffsetPair.find(methname);
+ assert(k != methnameToIdxOffsetPair.end());
+ selBaseAddr = in.objcSelrefs->getVA();
+ selIndex = k->second.first;
+ }
target->writeObjCMsgSendStub(buf + stubOffset, sym, in.objcStubs->addr,
- stubOffset, in.objcSelrefs->getVA(), i,
+ stubOffset, selBaseAddr, selIndex,
objcMsgSend);
}
}
diff --git a/lld/MachO/SyntheticSections.h b/lld/MachO/SyntheticSections.h
index 5fb7b6e09e8e63e..00944ebfaee5eed 100644
--- a/lld/MachO/SyntheticSections.h
+++ b/lld/MachO/SyntheticSections.h
@@ -324,6 +324,7 @@ class StubHelperSection final : public SyntheticSection {
class ObjCStubsSection final : public SyntheticSection {
public:
ObjCStubsSection();
+ void initialize();
void addEntry(Symbol *sym);
uint64_t getSize() const override;
bool isNeeded() const override { return !symbols.empty(); }
@@ -332,10 +333,17 @@ class ObjCStubsSection final : public SyntheticSection {
void setUp();
static constexpr llvm::StringLiteral symbolPrefix = "_objc_msgSend$";
+ static bool isObjCStubSymbol(Symbol *sym);
+ static StringRef getMethName(Symbol *sym);
private:
std::vector<Defined *> symbols;
- std::vector<uint32_t> offsets;
+ // Existing mapping from methname to selref (0 index is assumed).
+ llvm::StringMap<InputSection *> methnameToselref;
+ // Newly created mapping from methname to the pair of index (selref) and offset
+ // (methname).
+ llvm::MapVector<StringRef, std::pair<uint32_t, uint32_t>>
+ methnameToidxOffsetPair;
Symbol *objcMsgSend = nullptr;
};
diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
index 1b0e64abe843adf..65b598d1d7c422a 100644
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -720,6 +720,7 @@ static void addNonWeakDefinition(const Defined *defined) {
void Writer::scanSymbols() {
TimeTraceScope timeScope("Scan symbols");
+ in.objcStubs->initialize();
for (Symbol *sym : symtab->getSymbols()) {
if (auto *defined = dyn_cast<Defined>(sym)) {
if (!defined->isLive())
@@ -736,8 +737,16 @@ void Writer::scanSymbols() {
dysym->getFile()->refState =
std::max(dysym->getFile()->refState, dysym->getRefState());
} else if (isa<Undefined>(sym)) {
- if (sym->getName().starts_with(ObjCStubsSection::symbolPrefix))
+ if (ObjCStubsSection::isObjCStubSymbol(sym)) {
+ // When -dead_strip is enabled, we don't want to emit any dead stubs.
+ // Although this stub symbol is yet undefined, addSym() was called
+ // during MarkLive.
+ if (config->deadStrip) {
+ if (!sym->isLive())
+ continue;
+ }
in.objcStubs->addEntry(sym);
+ }
}
}
diff --git a/lld/test/MachO/arm64-objc-stubs-dead.s b/lld/test/MachO/arm64-objc-stubs-dead.s
new file mode 100644
index 000000000000000..5dcb171c17eac58
--- /dev/null
+++ b/lld/test/MachO/arm64-objc-stubs-dead.s
@@ -0,0 +1,27 @@
+# REQUIRES: aarch64
+
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
+
+# RUN: %lld -arch arm64 -lSystem -U _objc_msgSend -o %t.out %t.o
+# RUN: llvm-nm %t.out | FileCheck %s
+# RUN: %lld -arch arm64 -lSystem -U _objc_msgSend -dead_strip -o %t.out %t.o
+# RUN: llvm-nm %t.out | FileCheck %s --check-prefix=DEAD
+
+# CHECK: _foo
+# CHECK: _objc_msgSend$length
+
+# DEAD-NOT: _foo
+# DEAD-NOT: _objc_msgSend$length
+
+.section __TEXT,__text
+
+.globl _foo
+_foo:
+ bl _objc_msgSend$length
+ ret
+
+.globl _main
+_main:
+ ret
+
+.subsections_via_symbols
diff --git a/lld/test/MachO/objc-selrefs.s b/lld/test/MachO/objc-selrefs.s
index 9dff440727ac34b..6d144f4938b4542 100644
--- a/lld/test/MachO/objc-selrefs.s
+++ b/lld/test/MachO/objc-selrefs.s
@@ -24,7 +24,6 @@
# SELREFS-NEXT: __TEXT:__objc_methname:length
# SELREFS-EMPTY:
-## We don't yet support dedup'ing implicitly-defined selrefs.
# RUN: %lld -dylib -arch arm64 -lSystem --icf=all -o %t/explicit-and-implicit \
# RUN: %t/explicit-selrefs-1.o %t/explicit-selrefs-2.o %t/implicit-selrefs.o
# RUN: llvm-otool -vs __DATA __objc_selrefs %t/explicit-and-implicit \
@@ -43,8 +42,6 @@
# EXPLICIT-AND-IMPLICIT: Contents of (__DATA,__objc_selrefs) section
# EXPLICIT-AND-IMPLICIT-NEXT: __TEXT:__objc_methname:foo
# EXPLICIT-AND-IMPLICIT-NEXT: __TEXT:__objc_methname:bar
-# NOTE: Ideally this wouldn't exist, but while it does it needs to point to the deduplicated string
-# EXPLICIT-AND-IMPLICIT-NEXT: __TEXT:__objc_methname:foo
# EXPLICIT-AND-IMPLICIT-NEXT: __TEXT:__objc_methname:length
#--- explicit-selrefs-1.s
diff --git a/lld/test/MachO/x86-64-objc-stubs.s b/lld/test/MachO/x86-64-objc-stubs.s
index 2dd8d559377156b..1001d94b8d2cd12 100644
--- a/lld/test/MachO/x86-64-objc-stubs.s
+++ b/lld/test/MachO/x86-64-objc-stubs.s
@@ -10,6 +10,9 @@
# WARNING: warning: -objc_stubs_small is not yet implemented, defaulting to -objc_stubs_fast
+# RUN: %lld -arch x86_64 -lSystem -o %t-icf.out --icf=all %t.o
+# RUN: llvm-otool -vs __DATA __objc_selrefs %t-icf.out | FileCheck %s --check-prefix=ICF
+
# CHECK: Sections:
# CHECK: __got {{[0-9a-f]*}} [[#%x, GOTSTART:]] DATA
# CHECK: __objc_selrefs {{[0-9a-f]*}} [[#%x, SELSTART:]] DATA
@@ -21,6 +24,13 @@
# CHECK-NEXT: [[#%x, FOOSELREF:]] __TEXT:__objc_methname:foo
# CHECK-NEXT: [[#%x, LENGTHSELREF:]] __TEXT:__objc_methname:length
+# ICF: Contents of (__DATA,__objc_selrefs) section
+
+# ICF-NEXT: {{[0-9a-f]*}} __TEXT:__objc_methname:foo
+# ICF-NEXT: {{[0-9a-f]*}} __TEXT:__objc_methname:bar
+# ICF-NEXT: {{[0-9a-f]*}} __TEXT:__objc_methname:length
+# ICF-EMPTY:
+
# CHECK: Contents of (__TEXT,__objc_stubs) section
# CHECK-NEXT: _objc_msgSend$foo:
``````````
</details>
https://github.com/llvm/llvm-project/pull/79730
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