[lld] [lld][MachO] Follow-up to use madvise() for threaded file page-in. (PR #157917)

[John Holdsworth via llvm-commits]

https://github.com/johnno1962 updated https://github.com/llvm/llvm-project/pull/157917

>From dacbca8992e3fdc61839f0d7aea9ffa88c81f575 Mon Sep 17 00:00:00 2001
From: John Holdsworth <github at johnholdsworth.com>
Date: Wed, 10 Sep 2025 20:03:03 +0200
Subject: [PATCH] Switch to use madvise() to page-in files.

---
 lld/MachO/Driver.cpp | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 3db638e1ead96..eba674879d4f9 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -53,6 +53,8 @@
 #include "llvm/TextAPI/Architecture.h"
 #include "llvm/TextAPI/PackedVersion.h"
 
+#include <sys/mman.h>
+
 using namespace llvm;
 using namespace llvm::MachO;
 using namespace llvm::object;
@@ -334,11 +336,10 @@ class SerialBackgroundQueue {
 // This code forces the page-ins on multiple threads so
 // the process is not stalled waiting on disk buffer i/o.
 void multiThreadedPageInBackground(DeferredFiles &deferred) {
-  static const size_t pageSize = Process::getPageSizeEstimate();
   static const size_t largeArchive = 10 * 1024 * 1024;
 #ifndef NDEBUG
   using namespace std::chrono;
-  std::atomic_int numDeferedFilesTouched = 0;
+  std::atomic_int numDeferedFilesAdvised = 0;
   static std::atomic_uint64_t totalBytes = 0;
   auto t0 = high_resolution_clock::now();
 #endif
@@ -349,13 +350,18 @@ void multiThreadedPageInBackground(DeferredFiles &deferred) {
       return;
 #ifndef NDEBUG
     totalBytes += buff.size();
-    numDeferedFilesTouched += 1;
+    numDeferedFilesAdvised += 1;
 #endif
 
+#if _WIN32
     // Reference all file's mmap'd pages to load them into memory.
     for (const char *page = buff.data(), *end = page + buff.size(); page < end;
          page += pageSize)
       LLVM_ATTRIBUTE_UNUSED volatile char t = *page;
+#else
+    // Advise that mmap'd files should be loaded into memory.
+    madvise((void *)buff.data(), buff.size(), MADV_WILLNEED);
+#endif
   };
 #if LLVM_ENABLE_THREADS
   { // Create scope for waiting for the taskGroup
@@ -376,7 +382,7 @@ void multiThreadedPageInBackground(DeferredFiles &deferred) {
   auto dt = high_resolution_clock::now() - t0;
   if (Process::GetEnv("LLD_MULTI_THREAD_PAGE"))
     llvm::dbgs() << "multiThreadedPageIn " << totalBytes << "/"
-                 << numDeferedFilesTouched << "/" << deferred.size() << "/"
+                 << numDeferedFilesAdvised << "/" << deferred.size() << "/"
                  << duration_cast<milliseconds>(dt).count() / 1000. << "\n";
 #endif
 }