[lld-macho] Simplify DeduplicatedCStringSection::finalizeContents. NFC

[MaskRay]

Summary

Tail merge is slow and of low value. With regular string deduplication, we can
just use the return value of StringTableBuilder::add.

There is no noticeable performance increase because without deduplication
__cstring is quite small (7.6MiB for chromium_framework).

---

I tried an ELF port style parallel algorithm (https://reviews.llvm.org/P8275), but did not see an improvement
when linking chromium_framework.

[int3]

Could you include in the commit message the size % change (edit: and the change in link time too) when linking chromium_framework?

[MaskRay]

In D117273#3244787, @int3 wrote:

Could you include in the commit message the size % change when linking chromium_framework?

There is no observable difference, likely because the string section size is small (__cstring 007a31a1) without --deduplicate-literals.

ld64 does not concatenate debug info so the biggest bottleneck (.debug_str) on ELF is not a problem for Mach-O...

lld/MachO/SyntheticSections.cpp

 // huge: using 16-byte alignment (vs no alignment) is only a 0.5% size overhead
 // when linking chromium_framework on x86_64.
 DeduplicatedCStringSection::DeduplicatedCStringSection()
     : builder(StringTableBuilder::RAW, /*Alignment=*/16) {}
 
 void DeduplicatedCStringSection::finalizeContents() {
   // Add all string pieces to the string table builder to create section
   // contents.
-  for (const CStringInputSection *isec : inputs)
+  for (CStringInputSection *isec : inputs) {
     for (size_t i = 0, e = isec->pieces.size(); i != e; ++i)
       if (isec->pieces[i].live)
-        builder.add(isec->getCachedHashStringRef(i));
-
-  // Fix the string table content. After this, the contents will never change.
-  builder.finalizeInOrder();
-
-  // finalize() fixed tail-optimized strings, so we can now get
-  // offsets of strings. Get an offset for each string and save it
-  // to a corresponding SectionPiece for easy access.
-  for (CStringInputSection *isec : inputs) {
-    for (size_t i = 0, e = isec->pieces.size(); i != e; ++i) {
-      if (!isec->pieces[i].live)
-        continue;
         isec->pieces[i].outSecOff =
-          builder.getOffset(isec->getCachedHashStringRef(i));
-      isec->isFinal = true;
-    }
-  }
+            builder.add(isec->getCachedHashStringRef(i));
+
+  builder.finalizeInOrder();
 }
 
 // This section is actually emitted as __TEXT,__const by ld64, but clang may
 // emit input sections of that name, and LLD doesn't currently support mixing
 // synthetic and concat-type OutputSections. To work around this, I've given
 // our merged-literals section a different name.
 WordLiteralSection::WordLiteralSection()
     : SyntheticSection(segment_names::text, section_names::literals) {