[lld] ELF: Switch to parallelSort for RELR relocations. (PR #138370)

[Peter Collingbourne via llvm-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/138370

For firefox-x64 one of the more time consuming parts
of finalizeSections() was the call to llvm::sort in
RelrSection::updateAllocSize(). Switching that to use parallelSort
yielded the following improvement on firefox-x64 with ldflags -S on
an Apple M2 Ultra:

```
    N           Min           Max        Median           Avg        Stddev
x 512     1.1446024     1.2462944     1.1918706     1.1929871      0.016145
+ 512     1.1142867     1.2350058     1.1858642     1.1858839   0.016219708
Difference at 95.0% confidence
	-0.00710318 +/- 0.00198234
	-0.595412% +/- 0.166166%
	(Student's t, pooled s = 0.0161824)
```


>From 173ed5860651c37c8601926a5fe13fa609272cbe Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <pcc at google.com>
Date: Fri, 2 May 2025 17:26:17 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.6-beta.1
---
 lld/ELF/SyntheticSections.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index 2531227cb99b7..eceb297dbfc0d 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -2111,7 +2111,7 @@ template <class ELFT> bool RelrSection<ELFT>::updateAllocSize(Ctx &ctx) {
   std::unique_ptr<uint64_t[]> offsets(new uint64_t[relocs.size()]);
   for (auto [i, r] : llvm::enumerate(relocs))
     offsets[i] = r.getOffset();
-  llvm::sort(offsets.get(), offsets.get() + relocs.size());
+  llvm::parallelSort(offsets.get(), offsets.get() + relocs.size());
 
   // For each leading relocation, find following ones that can be folded
   // as a bitmap and fold them.