[lld] [ELF] Reuse SHT_GROUP selection verdicts in initializeSections. NFC (PR #207437)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/207437

For each SHT_GROUP section, the parallel initializeSections re-derives
what the serial parse() already decided
(https://reviews.llvm.org/D130810).

Record the kept group section indices during parse() so that we can save
the work (xxh3 string hash and DenseMap lookup) in the parallel
initializeSections().

In a clang-relassert --threads=8 benchmark, "Initialize sections"
decreases from 39.0 to 30.6.


>From 3944d0947bb41a1fe0caae22a63c444444ab8bfb Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Fri, 3 Jul 2026 09:43:41 -0700
Subject: [PATCH] [ELF] Reuse SHT_GROUP selection verdicts in
 initializeSections. NFC

For each SHT_GROUP section, the parallel initializeSections re-derives
what the serial parse() already decided
(https://reviews.llvm.org/D130810).

Record the kept group section indices during parse() so that we can save
the work (xxh3 string hash and DenseMap lookup) in the parallel
initializeSections().

In a clang-relassert --threads=8 benchmark, "Initialize sections"
decreases from 39.0 to 30.6.
---
 lld/ELF/InputFiles.cpp | 17 +++++++++--------
 lld/ELF/InputFiles.h   |  4 ++++
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 2f544f0fe0958..102bd384220a5 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -604,6 +604,7 @@ template <class ELFT> void ObjFile<ELFT>::parse(bool ignoreComdats) {
                            .try_emplace(CachedHashStringRef(signature), this)
                            .second;
       if (keepGroup) {
+        keptGroups.push_back(i);
         if (!ctx.arg.resolveGroups)
           sections[i] = createInputSection(
               i, sec, check(obj.getSectionName(sec, shstrtab)));
@@ -769,6 +770,7 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
   StringRef shstrtab = CHECK2(obj.getSectionStringTable(objSections), this);
   uint64_t size = objSections.size();
   SmallVector<ArrayRef<Elf_Word>, 0> selectedGroups;
+  size_t keptIdx = 0;
   AArch64BuildAttrSubsections aarch64BAsubSections;
   bool hasAArch64BuildAttributes = false;
   for (size_t i = 0; i != size; ++i) {
@@ -826,14 +828,13 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
     case SHT_GROUP: {
       if (!ctx.arg.relocatable)
         sections[i] = &InputSection::discarded;
-      StringRef signature =
-          cantFail(this->getELFSyms<ELFT>()[sec.sh_info].getName(stringTable));
-      ArrayRef<Elf_Word> entries =
-          cantFail(obj.template getSectionContentsAsArray<Elf_Word>(sec));
-      if ((entries[0] & GRP_COMDAT) == 0 || ignoreComdats ||
-          ctx.symtab->comdatGroups.find(CachedHashStringRef(signature))
-                  ->second == this)
-        selectedGroups.push_back(entries);
+      // Use the verdict parse() recorded for this group instead of repeating
+      // the signature hashing and comdatGroups lookup.
+      while (keptIdx != keptGroups.size() && keptGroups[keptIdx] < i)
+        ++keptIdx;
+      if (keptIdx != keptGroups.size() && keptGroups[keptIdx] == i)
+        selectedGroups.push_back(
+            cantFail(obj.template getSectionContentsAsArray<Elf_Word>(sec)));
       break;
     }
     case SHT_SYMTAB_SHNDX:
diff --git a/lld/ELF/InputFiles.h b/lld/ELF/InputFiles.h
index aef599102ecfc..0ded9b2fa38e2 100644
--- a/lld/ELF/InputFiles.h
+++ b/lld/ELF/InputFiles.h
@@ -315,6 +315,10 @@ template <class ELFT> class ObjFile : public ELFFileBase {
   // The following variable contains the contents of .symtab_shndx.
   // If the section does not exist (which is common), the array is empty.
   ArrayRef<Elf_Word> shndxTable;
+
+  // Section indices of kept SHT_GROUP sections, recorded by parse() in
+  // ascending order, to be used by the parallel initializeSections().
+  SmallVector<uint32_t, 0> keptGroups;
 };
 
 class BitcodeFile : public InputFile {