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[lld][MachO] Tail merge strings (#161262)
Add the flag `--tail-merge-strings` to enable tail merging of cstrings. For example, if we have strings `mystring\0` and `ring\0`, we could place `mystring\0` at address `0x1000` and `ring\0` at address `0x1004` and have them share the same underlying data. It turns out that many ObjC method names can be tail merged. For example, `error:` and `doFoo:error:`. On a large iOS binary, we saw nearly a 15% size improvement in the `__TEXT__objc_methname` section and negligible impact on link time. ``` $ bloaty --domain=vm merged.o.stripped -- base.o.stripped VM SIZE -------------- +95% +5.85Ki [__TEXT] -2.4% -239Ki __TEXT,__cstring -14.5% -710Ki __TEXT,__objc_methname -1.0% -944Ki TOTAL ``` Tail merging for MachO was originally removed in 7c269db. The previous implementation used `StringTableBuilder`, but that was removed in 4308f03 to ensure deduplicated strings are aligned correctly. This implementation ensures that tail merged strings are also aligned correctly. Special thanks to nocchijiang for pointing this out in #158720 (comment). Depends on #161253.
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‎lld/MachO/Config.h‎

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@@ -223,6 +223,7 @@ struct Configuration {
223223
bool warnThinArchiveMissingMembers;
224224
bool disableVerify;
225225
bool separateCstringLiteralSections;
226+
bool tailMergeStrings;
226227

227228
bool callGraphProfileSort = false;
228229
llvm::StringRef printSymbolOrder;

‎lld/MachO/Driver.cpp‎

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Original file line numberDiff line numberDiff line change
@@ -1986,6 +1986,8 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
19861986
config->separateCstringLiteralSections =
19871987
args.hasFlag(OPT_separate_cstring_literal_sections,
19881988
OPT_no_separate_cstring_literal_sections, false);
1989+
config->tailMergeStrings =
1990+
args.hasFlag(OPT_tail_merge_strings, OPT_no_tail_merge_strings, false);
19891991

19901992
auto IncompatWithCGSort = [&](StringRef firstArgStr) {
19911993
// Throw an error only if --call-graph-profile-sort is explicitly specified

‎lld/MachO/Options.td‎

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Original file line numberDiff line numberDiff line change
@@ -1091,6 +1091,10 @@ defm separate_cstring_literal_sections
10911091
"Emit all cstring literals into the __cstring section. As a special "
10921092
"case, the __objc_methname section will still be emitted. (default)">,
10931093
Group<grp_rare>;
1094+
defm tail_merge_strings
1095+
: BB<"tail-merge-strings", "Enable string tail merging",
1096+
"Disable string tail merging to improve link-time performance">,
1097+
Group<grp_rare>;
10941098

10951099
def grp_deprecated : OptionGroup<"deprecated">, HelpText<"DEPRECATED">;
10961100

‎lld/MachO/SyntheticSections.cpp‎

Lines changed: 57 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1746,6 +1746,8 @@ void CStringSection::finalizeContents() {
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void DeduplicatedCStringSection::finalizeContents() {
17471747
// Find the largest alignment required for each string.
17481748
DenseMap<CachedHashStringRef, Align> strToAlignment;
1749+
// Used for tail merging only
1750+
std::vector<CachedHashStringRef> deduplicatedStrs;
17491751
for (const CStringInputSection *isec : inputs) {
17501752
for (const auto &[i, piece] : llvm::enumerate(isec->pieces)) {
17511753
if (!piece.live)
@@ -1754,27 +1756,79 @@ void DeduplicatedCStringSection::finalizeContents() {
17541756
assert(isec->align != 0);
17551757
auto align = getStringPieceAlignment(isec, piece);
17561758
auto [it, wasInserted] = strToAlignment.try_emplace(s, align);
1759+
if (config->tailMergeStrings && wasInserted)
1760+
deduplicatedStrs.push_back(s);
17571761
if (!wasInserted && it->second < align)
17581762
it->second = align;
17591763
}
17601764
}
17611765

1766+
// Like lexigraphical sort, except we read strings in reverse and take the
1767+
// longest string first
1768+
// TODO: We could improve performance by implementing our own sort that avoids
1769+
// comparing characters we know to be the same. See
1770+
// StringTableBuilder::multikeySort() for details
1771+
llvm::sort(deduplicatedStrs, [](const auto &left, const auto &right) {
1772+
for (const auto &[leftChar, rightChar] :
1773+
llvm::zip(llvm::reverse(left.val()), llvm::reverse(right.val()))) {
1774+
if (leftChar == rightChar)
1775+
continue;
1776+
return leftChar < rightChar;
1777+
}
1778+
return left.size() > right.size();
1779+
});
1780+
std::optional<CachedHashStringRef> mergeCandidate;
1781+
DenseMap<CachedHashStringRef, std::pair<CachedHashStringRef, uint64_t>>
1782+
tailMergeMap;
1783+
for (auto &s : deduplicatedStrs) {
1784+
if (!mergeCandidate || !mergeCandidate->val().ends_with(s.val())) {
1785+
mergeCandidate = s;
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continue;
1787+
}
1788+
uint64_t tailMergeOffset = mergeCandidate->size() - s.size();
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// TODO: If the tail offset is incompatible with this string's alignment, we
1790+
// might be able to find another superstring with a compatible tail offset.
1791+
// The difficulty is how to do this efficiently
1792+
const auto &align = strToAlignment.at(s);
1793+
if (!isAligned(align, tailMergeOffset))
1794+
continue;
1795+
auto &mergeCandidateAlign = strToAlignment[*mergeCandidate];
1796+
if (align > mergeCandidateAlign)
1797+
mergeCandidateAlign = align;
1798+
tailMergeMap.try_emplace(s, *mergeCandidate, tailMergeOffset);
1799+
}
1800+
17621801
// Sort the strings for performance and compression size win, and then
17631802
// assign an offset for each string and save it to the corresponding
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// StringPieces for easy access.
17651804
for (auto &[isec, i] : priorityBuilder.buildCStringPriorities(inputs)) {
17661805
auto &piece = isec->pieces[i];
17671806
auto s = isec->getCachedHashStringRef(i);
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// Any string can be tail merged with itself with an offset of zero
1808+
uint64_t tailMergeOffset = 0;
1809+
auto mergeIt =
1810+
config->tailMergeStrings ? tailMergeMap.find(s) : tailMergeMap.end();
1811+
if (mergeIt != tailMergeMap.end()) {
1812+
auto &[superString, offset] = mergeIt->second;
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// s can be tail merged with superString. Do not layout s. Instead layout
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// superString if we haven't already
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assert(superString.val().ends_with(s.val()));
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s = superString;
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tailMergeOffset = offset;
1818+
}
17681819
auto [it, wasInserted] = stringOffsetMap.try_emplace(s, /*placeholder*/ 0);
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if (wasInserted) {
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// Avoid computing the offset until we are sure we will need to
17711822
uint64_t offset = alignTo(size, strToAlignment.at(s));
17721823
it->second = offset;
17731824
size = offset + s.size() + 1; // account for null terminator
17741825
}
1775-
// If the string was already in stringOffsetMap, it is a duplicate and we
1776-
// only need to assign the offset.
1777-
piece.outSecOff = it->second;
1826+
piece.outSecOff = it->second + tailMergeOffset;
1827+
if (mergeIt != tailMergeMap.end()) {
1828+
auto &tailMergedString = mergeIt->first;
1829+
stringOffsetMap[tailMergedString] = piece.outSecOff;
1830+
assert(isAligned(strToAlignment.at(tailMergedString), piece.outSecOff));
1831+
}
17781832
}
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for (CStringInputSection *isec : inputs)
17801834
isec->isFinal = true;

‎lld/docs/ReleaseNotes.rst‎

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@@ -46,6 +46,8 @@ MachO Improvements
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* ``--separate-cstring-literal-sections`` emits cstring literal sections into sections defined by their section name.
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(`#158720 <https://github.com/llvm/llvm-project/pull/158720>`_)
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* ``--tail-merge-strings`` enables tail merging of cstring literals.
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(`#161262 <https://github.com/llvm/llvm-project/pull/161262>`_)
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WebAssembly Improvements
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------------------------

‎lld/test/MachO/cstring-tailmerge-objc.s‎

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@@ -0,0 +1,144 @@
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; REQUIRES: aarch64
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; RUN: rm -rf %t && split-file %s %t
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; Test that ObjC method names are tail merged and
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; ObjCSelRefsHelper::makeSelRef() still works correctly
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; RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/a.s -o %t/a.o
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; RUN: %lld -dylib -arch arm64 --tail-merge-strings %t/a.o -o %t/a
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; RUN: llvm-objdump --macho --section="__TEXT,__objc_methname" %t/a | FileCheck %s --implicit-check-not=error
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; RUN: %lld -dylib -arch arm64 --no-tail-merge-strings %t/a.o -o %t/nomerge
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; RUN: llvm-objdump --macho --section="__TEXT,__objc_methname" %t/nomerge | FileCheck %s --check-prefixes=CHECK,NOMERGE --implicit-check-not=error
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; CHECK: withBar:error:
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; NOMERGE: error:
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;--- a.mm
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__attribute__((objc_root_class))
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@interface Foo
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- (void)withBar:(int)bar error:(int)error;
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- (void)error:(int)error;
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@end
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@implementation Foo
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- (void)withBar:(int)bar error:(int)error {}
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- (void)error:(int)error {}
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@end
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void *_objc_empty_cache;
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void *_objc_empty_vtable;
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;--- gen
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clang -Oz -target arm64-apple-darwin a.mm -S -o -
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;--- a.s
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.build_version macos, 11, 0
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.section __TEXT,__text,regular,pure_instructions
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.p2align 2 ; -- Begin function -[Foo withBar:error:]
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"-[Foo withBar:error:]": ; @"\01-[Foo withBar:error:]"
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.cfi_startproc
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; %bb.0:
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ret
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.cfi_endproc
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; -- End function
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.p2align 2 ; -- Begin function -[Foo error:]
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"-[Foo error:]": ; @"\01-[Foo error:]"
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.cfi_startproc
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; %bb.0:
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ret
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.cfi_endproc
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; -- End function
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.globl __objc_empty_vtable ; @_objc_empty_vtable
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.zerofill __DATA,__common,__objc_empty_vtable,8,3
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.section __DATA,__objc_data
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.globl _OBJC_CLASS_$_Foo ; @"OBJC_CLASS_$_Foo"
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.p2align 3, 0x0
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_OBJC_CLASS_$_Foo:
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.quad _OBJC_METACLASS_$_Foo
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.quad 0
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.quad __objc_empty_cache
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.quad __objc_empty_vtable
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.quad __OBJC_CLASS_RO_$_Foo
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.globl _OBJC_METACLASS_$_Foo ; @"OBJC_METACLASS_$_Foo"
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.p2align 3, 0x0
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_OBJC_METACLASS_$_Foo:
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.quad _OBJC_METACLASS_$_Foo
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.quad _OBJC_CLASS_$_Foo
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.quad __objc_empty_cache
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.quad __objc_empty_vtable
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.quad __OBJC_METACLASS_RO_$_Foo
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.section __TEXT,__objc_classname,cstring_literals
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l_OBJC_CLASS_NAME_: ; @OBJC_CLASS_NAME_
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.asciz "Foo"
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.section __DATA,__objc_const
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.p2align 3, 0x0 ; @"_OBJC_METACLASS_RO_$_Foo"
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__OBJC_METACLASS_RO_$_Foo:
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.long 3 ; 0x3
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.long 40 ; 0x28
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.long 40 ; 0x28
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.space 4
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.quad 0
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.quad l_OBJC_CLASS_NAME_
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.quad 0
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.quad 0
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.quad 0
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.quad 0
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.quad 0
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.section __TEXT,__objc_methname,cstring_literals
91+
l_OBJC_METH_VAR_NAME_: ; @OBJC_METH_VAR_NAME_
92+
.asciz "withBar:error:"
93+
94+
.section __TEXT,__objc_methtype,cstring_literals
95+
l_OBJC_METH_VAR_TYPE_: ; @OBJC_METH_VAR_TYPE_
96+
.asciz "v24@0:8i16i20"
97+
98+
.section __TEXT,__objc_methname,cstring_literals
99+
l_OBJC_METH_VAR_NAME_.1: ; @OBJC_METH_VAR_NAME_.1
100+
.asciz "error:"
101+
102+
.section __TEXT,__objc_methtype,cstring_literals
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l_OBJC_METH_VAR_TYPE_.2: ; @OBJC_METH_VAR_TYPE_.2
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.asciz "v20@0:8i16"
105+
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.section __DATA,__objc_const
107+
.p2align 3, 0x0 ; @"_OBJC_$_INSTANCE_METHODS_Foo"
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__OBJC_$_INSTANCE_METHODS_Foo:
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.long 24 ; 0x18
110+
.long 2 ; 0x2
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.quad l_OBJC_METH_VAR_NAME_
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.quad l_OBJC_METH_VAR_TYPE_
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.quad "-[Foo withBar:error:]"
114+
.quad l_OBJC_METH_VAR_NAME_.1
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.quad l_OBJC_METH_VAR_TYPE_.2
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.quad "-[Foo error:]"
117+
118+
.p2align 3, 0x0 ; @"_OBJC_CLASS_RO_$_Foo"
119+
__OBJC_CLASS_RO_$_Foo:
120+
.long 2 ; 0x2
121+
.long 0 ; 0x0
122+
.long 0 ; 0x0
123+
.space 4
124+
.quad 0
125+
.quad l_OBJC_CLASS_NAME_
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.quad __OBJC_$_INSTANCE_METHODS_Foo
127+
.quad 0
128+
.quad 0
129+
.quad 0
130+
.quad 0
131+
132+
.globl __objc_empty_cache ; @_objc_empty_cache
133+
.zerofill __DATA,__common,__objc_empty_cache,8,3
134+
.section __DATA,__objc_classlist,regular,no_dead_strip
135+
.p2align 3, 0x0 ; @"OBJC_LABEL_CLASS_$"
136+
l_OBJC_LABEL_CLASS_$:
137+
.quad _OBJC_CLASS_$_Foo
138+
139+
.section __DATA,__objc_imageinfo,regular,no_dead_strip
140+
L_OBJC_IMAGE_INFO:
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.long 0
142+
.long 64
143+
144+
.subsections_via_symbols

‎lld/test/MachO/cstring-tailmerge.s‎

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@@ -0,0 +1,85 @@
1+
; REQUIRES: aarch64
2+
; RUN: rm -rf %t && split-file %s %t
3+
4+
; RUN: sed "s/<ALIGN>/0/g" %t/align.s.template > %t/align-1.s
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; RUN: sed "s/<ALIGN>/1/g" %t/align.s.template > %t/align-2.s
6+
; RUN: sed "s/<ALIGN>/2/g" %t/align.s.template > %t/align-4.s
7+
8+
; RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/first.s -o %t/first.o
9+
; RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/align-1.s -o %t/align-1.o
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; RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/align-2.s -o %t/align-2.o
11+
; RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/align-4.s -o %t/align-4.o
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13+
; RUN: %lld -dylib -arch arm64 --tail-merge-strings %t/first.o %t/align-1.o -o %t/align-1
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; RUN: llvm-objdump --macho --section="__TEXT,__cstring" --syms %t/align-1 | FileCheck %s --check-prefixes=CHECK,ALIGN1
15+
16+
; RUN: %lld -dylib -arch arm64 --tail-merge-strings %t/first.o %t/align-2.o -o %t/align-2
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; RUN: llvm-objdump --macho --section="__TEXT,__cstring" --syms %t/align-2 | FileCheck %s --check-prefixes=CHECK,ALIGN2
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19+
; RUN: %lld -dylib -arch arm64 --tail-merge-strings %t/first.o %t/align-4.o -o %t/align-4
20+
; RUN: llvm-objdump --macho --section="__TEXT,__cstring" --syms %t/align-4 | FileCheck %s --check-prefixes=CHECK,ALIGN4
21+
22+
; CHECK: Contents of (__TEXT,__cstring) section
23+
; CHECK: [[#%.16x,START:]] get awkward offset{{$}}
24+
25+
; ALIGN1: [[#%.16x,START+19]] myotherlongstr{{$}}
26+
; ALIGN1: [[#%.16x,START+19+15]] otherstr{{$}}
27+
28+
; ALIGN2: [[#%.16x,START+20]] myotherlongstr{{$}}
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; ALIGN2: [[#%.16x,START+20+16]] longstr{{$}}
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; ALIGN2: [[#%.16x,START+20+16+8]] otherstr{{$}}
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; ALIGN2: [[#%.16x,START+20+16+8+10]] str{{$}}
32+
33+
; ALIGN4: [[#%.16x,START+20]] myotherlongstr{{$}}
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; ALIGN4: [[#%.16x,START+20+16]] otherlongstr{{$}}
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; ALIGN4: [[#%.16x,START+20+16+16]] longstr{{$}}
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; ALIGN4: [[#%.16x,START+20+16+16+8]] otherstr{{$}}
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; ALIGN4: [[#%.16x,START+20+16+16+8+12]] str{{$}}
38+
39+
; CHECK: SYMBOL TABLE:
40+
41+
; ALIGN1: [[#%.16x,START+19]] l O __TEXT,__cstring _myotherlongstr
42+
; ALIGN1: [[#%.16x,START+21]] l O __TEXT,__cstring _otherlongstr
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; ALIGN1: [[#%.16x,START+26]] l O __TEXT,__cstring _longstr
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; ALIGN1: [[#%.16x,START+34]] l O __TEXT,__cstring _otherstr
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; ALIGN1: [[#%.16x,START+39]] l O __TEXT,__cstring _str
46+
47+
; ALIGN2: [[#%.16x,START+20]] l O __TEXT,__cstring _myotherlongstr
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; ALIGN2: [[#%.16x,START+20+2]] l O __TEXT,__cstring _otherlongstr
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; ALIGN2: [[#%.16x,START+20+16]] l O __TEXT,__cstring _longstr
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; ALIGN2: [[#%.16x,START+20+16+8]] l O __TEXT,__cstring _otherstr
51+
; ALIGN2: [[#%.16x,START+20+16+8+10]] l O __TEXT,__cstring _str
52+
53+
; ALIGN4: [[#%.16x,START+20]] l O __TEXT,__cstring _myotherlongstr
54+
; ALIGN4: [[#%.16x,START+20+16]] l O __TEXT,__cstring _otherlongstr
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; ALIGN4: [[#%.16x,START+20+16+16]] l O __TEXT,__cstring _longstr
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; ALIGN4: [[#%.16x,START+20+16+16+8]] l O __TEXT,__cstring _otherstr
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; ALIGN4: [[#%.16x,START+20+16+16+8+12]] l O __TEXT,__cstring _str
58+
59+
;--- first.s
60+
.cstring
61+
.p2align 2
62+
.asciz "get awkward offset" ; length = 19
63+
64+
;--- align.s.template
65+
.cstring
66+
67+
.p2align <ALIGN>
68+
_myotherlongstr:
69+
.asciz "myotherlongstr" ; length = 15
70+
71+
.p2align <ALIGN>
72+
_otherlongstr:
73+
.asciz "otherlongstr" ; length = 13, tail offset = 2
74+
75+
.p2align <ALIGN>
76+
_longstr:
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.asciz "longstr" ; length = 8, tail offset = 7
78+
79+
.p2align <ALIGN>
80+
_otherstr:
81+
.asciz "otherstr" ; length = 9
82+
83+
.p2align <ALIGN>
84+
_str:
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.asciz "str" ; length = 4, tail offset = 5

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