[llvm-branch-commits] [lld] [lld-macho] Build the export trie by edges, not bytes (NFC) (PR #234857)

[Jez Ng via llvm-branch-commits]

https://github.com/int3 created https://github.com/llvm/llvm-project/pull/234857

Emit singleton suffixes directly and scan common prefixes in one pass,
avoiding repeated byte-wise partitioning. Preserve radix traversal,
node allocation order, and terminal encoding. This speeds up builds that
emit large export tries.

Wall time with [LinkerBench](https://github.com/int3/macho-linker-bench),
#233399 → this PR, Release, assertions off. Median ± 95% CI over at least 20
interleaved runs per side.

M4 Mac mini (10 cores, 16 GB):

| Link | Exports | Default threads | 1 thread |
| --- | ---: | ---: | ---: |
| rust-analyzer (debug) | 248,607 | 650.3 ± 1.2 → 625.4 ± 1.9 ms (−3.8%) | 951.7 ± 0.9 → 870.3 ± 1.7 ms (−8.6%) |
| SourceKit-LSP | 105,359 | 604.2 ± 2.3 → 605.4 ± 1.6 ms | 774.5 ± 1.3 → 767.5 ± 0.6 ms (−0.9%) |
| Firefox for iOS | 82,489 | 552.6 ± 0.9 → 552.0 ± 1.6 ms | 651.8 ± 1.3 → 646.5 ± 1.3 ms (−0.8%) |
| libLLVM | 55,996 | 365.3 ± 1.0 → 365.7 ± 1.0 ms | |

rust-analyzer (release), Telegram and Firefox for macOS are unchanged (1.00×).

Ryzen 7 7700 (Linux), default threads, all 15 LinkerBench workloads:
rust-analyzer (debug) goes from 1.241 ± 0.011 to 1.158 ± 0.006 s (−6.7%);
the other 14 workloads are within noise of 1.00×.

Assisted by Opus 5.5

>From 603b2ae7280c71933444f70744840a2326a7b556 Mon Sep 17 00:00:00 2001
From: Jez Ng <jezreel at gmail.com>
Date: Mon, 19 Oct 2026 11:05:22 +0200
Subject: [PATCH 1/2] [lld-macho] Build the export trie by edges, not bytes
 (NFC)

Emit singleton suffixes directly and scan common prefixes in one pass,
avoiding repeated byte-wise partitioning. Preserve radix traversal,
node allocation order, and terminal encoding.

On a Ryzen 7 7700, a rust-analyzer debug link with 248,607 exports
changes from 2079 to 1948 ms at one thread and 1269 to 1185 ms at
eight threads (median wall times over four interleaved pairs).
---
 lld/MachO/ExportTrie.cpp | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/lld/MachO/ExportTrie.cpp b/lld/MachO/ExportTrie.cpp
index 4a9e14c3d920d..55235219fb3b1 100644
--- a/lld/MachO/ExportTrie.cpp
+++ b/lld/MachO/ExportTrie.cpp
@@ -226,6 +226,19 @@ void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
   if (vec.empty())
     return;
 
+  // A singleton needs just one edge for its remaining suffix.
+  if (vec.size() == 1) {
+    const Symbol *sym = vec.front();
+    StringRef suffix = sym->getName().drop_front(lastPos);
+    if (!suffix.empty()) {
+      TrieNode *newNode = makeNode();
+      node->edges.emplace_back(suffix, newNode);
+      node = newNode;
+    }
+    node->info = ExportInfo(*sym, imageBase);
+    return;
+  }
+
   // Partition items so that items in [0, i) are less than the pivot,
   // [i, j) are the same as the pivot, and [j, vec.size()) are greater than
   // the pivot.
@@ -264,6 +277,20 @@ void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
     // sortAndBuild(vec.slice(i, j - i), node, lastPos, pos + 1);
     vec = vec.slice(i, j - i);
     ++pos;
+    // Skip the rest of a common prefix without partitioning at every byte.
+    if (vec.size() > 1) {
+      StringRef firstName = vec.front()->getName();
+      size_t commonEnd = firstName.size();
+      for (const Symbol *sym : vec.drop_front()) {
+        StringRef name = sym->getName();
+        commonEnd = std::min(commonEnd, name.size());
+        size_t commonPos = pos;
+        while (commonPos < commonEnd && firstName[commonPos] == name[commonPos])
+          ++commonPos;
+        commonEnd = commonPos;
+      }
+      pos = commonEnd;
+    }
     goto tailcall;
   }
 }

>From 470f33ec44ca83a72cb3378bdeaaf56bbc2dc467 Mon Sep 17 00:00:00 2001
From: Jez Ng <me at jezng.com>
Date: Mon, 19 Oct 2026 04:02:38 -0700
Subject: [PATCH 2/2] Improve comments in ExportTrie.cpp

Refactor comments for clarity and optimization explanation.
---
 lld/MachO/ExportTrie.cpp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lld/MachO/ExportTrie.cpp b/lld/MachO/ExportTrie.cpp
index 55235219fb3b1..2caf0f7ede6bd 100644
--- a/lld/MachO/ExportTrie.cpp
+++ b/lld/MachO/ExportTrie.cpp
@@ -226,7 +226,7 @@ void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
   if (vec.empty())
     return;
 
-  // A singleton needs just one edge for its remaining suffix.
+  // Create singletons in one go instead of doing a tail call per character.
   if (vec.size() == 1) {
     const Symbol *sym = vec.front();
     StringRef suffix = sym->getName().drop_front(lastPos);
@@ -277,7 +277,8 @@ void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
     // sortAndBuild(vec.slice(i, j - i), node, lastPos, pos + 1);
     vec = vec.slice(i, j - i);
     ++pos;
-    // Skip the rest of a common prefix without partitioning at every byte.
+    // Skip the rest of a common prefix instead of doing a tail call per
+    // character.
     if (vec.size() > 1) {
       StringRef firstName = vec.front()->getName();
       size_t commonEnd = firstName.size();