[llvm-branch-commits] [lld] [lld][MachO] Tail merge strings (PR #161262)
Ellis Hoag via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Mon Sep 29 12:22:56 PDT 2025
https://github.com/ellishg created https://github.com/llvm/llvm-project/pull/161262
TODO: Measure size win
TODO: Measure performance
Depends on https://github.com/llvm/llvm-project/pull/161253.
>From 4742cca2ec71b6651f76c12b9d4aea2706cec02c Mon Sep 17 00:00:00 2001
From: Ellis Hoag <ellishoag at meta.com>
Date: Mon, 29 Sep 2025 12:21:49 -0700
Subject: [PATCH] [lld][MachO] Tail merge strings
---
lld/MachO/SyntheticSections.cpp | 58 +++++++++++++++++++-
lld/test/MachO/cstring-dedup.s | 3 +-
lld/test/MachO/cstring-tailmerge.s | 85 ++++++++++++++++++++++++++++++
3 files changed, 143 insertions(+), 3 deletions(-)
create mode 100644 lld/test/MachO/cstring-tailmerge.s
diff --git a/lld/MachO/SyntheticSections.cpp b/lld/MachO/SyntheticSections.cpp
index 903ba78a27c75..460a0b5a16ab0 100644
--- a/lld/MachO/SyntheticSections.cpp
+++ b/lld/MachO/SyntheticSections.cpp
@@ -1746,6 +1746,7 @@ void CStringSection::finalizeContents() {
void DeduplicatedCStringSection::finalizeContents() {
// Find the largest alignment required for each string.
DenseMap<CachedHashStringRef, Align> strToAlignment;
+ std::vector<CachedHashStringRef> deduplicatedStrs;
for (const CStringInputSection *isec : inputs) {
for (const auto &[i, piece] : llvm::enumerate(isec->pieces)) {
if (!piece.live)
@@ -1754,17 +1755,57 @@ void DeduplicatedCStringSection::finalizeContents() {
assert(isec->align != 0);
auto align = getStringPieceAlignment(isec, piece);
auto [it, wasInserted] = strToAlignment.try_emplace(s, align);
+ if (wasInserted)
+ deduplicatedStrs.push_back(s);
if (!wasInserted && it->second < align)
it->second = align;
}
}
+ // Like lexigraphical sort, except we read strings in reverse and take the
+ // longest string first
+ // TODO: We could improve performance by implementing our own sort that avoids
+ // comparing characters we know to be the same. See
+ // StringTableBuilder::multikeySort() for details
+ llvm::sort(deduplicatedStrs, [](const auto &left, const auto &right) {
+ for (const auto &[leftChar, rightChar] :
+ llvm::zip(llvm::reverse(left.val()), llvm::reverse(right.val()))) {
+ if (leftChar == rightChar)
+ continue;
+ return leftChar < rightChar;
+ }
+ return left.size() > right.size();
+ });
+ std::optional<CachedHashStringRef> mergeCandidate;
+ DenseMap<CachedHashStringRef, std::pair<CachedHashStringRef, uint64_t>>
+ tailMergeMap;
+ for (auto &s : deduplicatedStrs) {
+ if (!mergeCandidate || !mergeCandidate->val().ends_with(s.val())) {
+ mergeCandidate = s;
+ continue;
+ }
+ uint64_t tailOffset = mergeCandidate->size() - s.size();
+ // TODO: If the tail offset is incompatible with this string's alignment, we
+ // might be able to find another superstring with a compatible tail offset.
+ // The difficulty is how to do this efficiently
+ const auto &align = strToAlignment.at(s);
+ if (!isAligned(align, tailOffset))
+ continue;
+ auto &mergeCandidateAlign = strToAlignment[*mergeCandidate];
+ if (align > mergeCandidateAlign)
+ mergeCandidateAlign = align;
+ tailMergeMap.try_emplace(s, *mergeCandidate, tailOffset);
+ }
+
// Sort the strings for performance and compression size win, and then
// assign an offset for each string and save it to the corresponding
// StringPieces for easy access.
for (auto &[isec, i] : priorityBuilder.buildCStringPriorities(inputs)) {
auto &piece = isec->pieces[i];
auto s = isec->getCachedHashStringRef(i);
+ // Skip tail merged strings until their superstring offsets are resolved
+ if (tailMergeMap.count(s))
+ continue;
auto [it, wasInserted] = stringOffsetMap.try_emplace(s, /*placeholder*/ 0);
if (wasInserted) {
// Avoid computing the offset until we are sure we will need to
@@ -1776,8 +1817,23 @@ void DeduplicatedCStringSection::finalizeContents() {
// only need to assign the offset.
piece.outSecOff = it->second;
}
- for (CStringInputSection *isec : inputs)
+ for (CStringInputSection *isec : inputs) {
+ for (const auto &[i, piece] : llvm::enumerate(isec->pieces)) {
+ if (!piece.live)
+ continue;
+ auto s = isec->getCachedHashStringRef(i);
+ auto it = tailMergeMap.find(s);
+ if (it == tailMergeMap.end())
+ continue;
+ const auto &[superString, tailOffset] = it->second;
+ assert(!tailMergeMap.count(superString));
+ auto &outSecOff = stringOffsetMap[s];
+ outSecOff = stringOffsetMap.at(superString) + tailOffset;
+ piece.outSecOff = outSecOff;
+ assert(isAligned(strToAlignment.at(s), piece.outSecOff));
+ }
isec->isFinal = true;
+ }
}
void DeduplicatedCStringSection::writeTo(uint8_t *buf) const {
diff --git a/lld/test/MachO/cstring-dedup.s b/lld/test/MachO/cstring-dedup.s
index a4b15f26afff0..0a42b3d6fcff3 100644
--- a/lld/test/MachO/cstring-dedup.s
+++ b/lld/test/MachO/cstring-dedup.s
@@ -8,11 +8,10 @@
# RUN: llvm-objdump --macho --section="__DATA,ptrs" --syms %t/test | FileCheck %s
# RUN: llvm-readobj --section-headers %t/test | FileCheck %s --check-prefix=HEADER
-## Make sure we only have 3 deduplicated strings in __cstring.
+## Make sure we only have 2 deduplicated strings in __cstring.
# STR: Contents of (__TEXT,__cstring) section
# STR: {{[[:xdigit:]]+}} foo
# STR: {{[[:xdigit:]]+}} barbaz
-# STR: {{[[:xdigit:]]+}} {{$}}
## Make sure both symbol and section relocations point to the right thing.
# CHECK: Contents of (__DATA,ptrs) section
diff --git a/lld/test/MachO/cstring-tailmerge.s b/lld/test/MachO/cstring-tailmerge.s
new file mode 100644
index 0000000000000..83d2810a78139
--- /dev/null
+++ b/lld/test/MachO/cstring-tailmerge.s
@@ -0,0 +1,85 @@
+# REQUIRES: x86
+# RUN: rm -rf %t; split-file %s %t
+
+# RUN: sed "s/<ALIGN>/0/g" %t/align.s.template > %t/align-1.s
+# RUN: sed "s/<ALIGN>/1/g" %t/align.s.template > %t/align-2.s
+# RUN: sed "s/<ALIGN>/2/g" %t/align.s.template > %t/align-4.s
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/first.s -o %t/first.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/align-1.s -o %t/align-1.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/align-2.s -o %t/align-2.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/align-4.s -o %t/align-4.o
+
+# RUN: %lld -dylib --deduplicate-strings %t/first.o %t/align-1.o -o %t/align-1
+# RUN: llvm-objdump --macho --section="__TEXT,__cstring" --syms %t/align-1 | FileCheck %s --check-prefixes=CHECK,ALIGN1
+
+# RUN: %lld -dylib --deduplicate-strings %t/first.o %t/align-2.o -o %t/align-2
+# RUN: llvm-objdump --macho --section="__TEXT,__cstring" --syms %t/align-2 | FileCheck %s --check-prefixes=CHECK,ALIGN2
+
+# RUN: %lld -dylib --deduplicate-strings %t/first.o %t/align-4.o -o %t/align-4
+# RUN: llvm-objdump --macho --section="__TEXT,__cstring" --syms %t/align-4 | FileCheck %s --check-prefixes=CHECK,ALIGN4
+
+# CHECK: Contents of (__TEXT,__cstring) section
+# CHECK: [[#%.16x,START:]] get awkward offset{{$}}
+
+# ALIGN1: [[#%.16x,START+19]] myotherlongstr{{$}}
+# ALIGN1: [[#%.16x,START+19+15]] otherstr{{$}}
+
+# ALIGN2: [[#%.16x,START+20]] myotherlongstr{{$}}
+# ALIGN2: [[#%.16x,START+20+16]] longstr{{$}}
+# ALIGN2: [[#%.16x,START+20+16+8]] otherstr{{$}}
+# ALIGN2: [[#%.16x,START+20+16+8+10]] str{{$}}
+
+# ALIGN4: [[#%.16x,START+20]] myotherlongstr{{$}}
+# ALIGN4: [[#%.16x,START+20+16]] otherlongstr{{$}}
+# ALIGN4: [[#%.16x,START+20+16+16]] longstr{{$}}
+# ALIGN4: [[#%.16x,START+20+16+16+8]] otherstr{{$}}
+# ALIGN4: [[#%.16x,START+20+16+16+8+12]] str{{$}}
+
+# CHECK: SYMBOL TABLE:
+
+# ALIGN1: [[#%.16x,START+19]] l O __TEXT,__cstring _myotherlongstr
+# ALIGN1: [[#%.16x,START+21]] l O __TEXT,__cstring _otherlongstr
+# ALIGN1: [[#%.16x,START+26]] l O __TEXT,__cstring _longstr
+# ALIGN1: [[#%.16x,START+34]] l O __TEXT,__cstring _otherstr
+# ALIGN1: [[#%.16x,START+39]] l O __TEXT,__cstring _str
+
+# ALIGN2: [[#%.16x,START+20]] l O __TEXT,__cstring _myotherlongstr
+# ALIGN2: [[#%.16x,START+20+2]] l O __TEXT,__cstring _otherlongstr
+# ALIGN2: [[#%.16x,START+20+16]] l O __TEXT,__cstring _longstr
+# ALIGN2: [[#%.16x,START+20+16+8]] l O __TEXT,__cstring _otherstr
+# ALIGN2: [[#%.16x,START+20+16+8+10]] l O __TEXT,__cstring _str
+
+# ALIGN4: [[#%.16x,START+20]] l O __TEXT,__cstring _myotherlongstr
+# ALIGN4: [[#%.16x,START+20+16]] l O __TEXT,__cstring _otherlongstr
+# ALIGN4: [[#%.16x,START+20+16+16]] l O __TEXT,__cstring _longstr
+# ALIGN4: [[#%.16x,START+20+16+16+8]] l O __TEXT,__cstring _otherstr
+# ALIGN4: [[#%.16x,START+20+16+16+8+12]] l O __TEXT,__cstring _str
+
+#--- first.s
+.cstring
+.p2align 2
+.asciz "get awkward offset" # length = 19
+
+#--- align.s.template
+.cstring
+
+.p2align <ALIGN>
+ _myotherlongstr:
+.asciz "myotherlongstr" # length = 15
+
+.p2align <ALIGN>
+ _otherlongstr:
+.asciz "otherlongstr" # length = 13, tail offset = 2
+
+.p2align <ALIGN>
+ _longstr:
+.asciz "longstr" # length = 8, tail offset = 7
+
+.p2align <ALIGN>
+ _otherstr:
+.asciz "otherstr" # length = 9
+
+.p2align <ALIGN>
+ _str:
+.asciz "str" # length = 4, tail offset = 5
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