From 339c2b886a6fe5741774a506c258a8c4adf90f1c Mon Sep 17 00:00:00 2001
From: Tim Blechmann <tim@klingt.org>
Date: Fri, 27 Dec 2024 12:07:04 +0800
Subject: [PATCH] cmSystemTools: Add RandomNumber method that avoid re-seeding
 from OS

When profiling Qt builds on macos, about 2.2% of a `cmake` invocation
was spent reading from `/dev/urandom`.  Use a (thread)local rng to
mitigate this cost, particularly in `cmGeneratedFileStreamBase::Open`.
---
 Source/CTest/cmCTestMultiProcessHandler.cxx |  2 +-
 Source/cmCoreTryCompile.cxx                 |  2 +-
 Source/cmGeneratedFileStream.cxx            |  2 +-
 Source/cmSystemTools.cxx                    | 25 +++++++++++++++++++++
 Source/cmSystemTools.h                      |  3 ++-
 Source/cmake.cxx                            |  2 +-
 6 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/Source/CTest/cmCTestMultiProcessHandler.cxx b/Source/CTest/cmCTestMultiProcessHandler.cxx
index 4ef47e7314..f771a1568e 100644
--- a/Source/CTest/cmCTestMultiProcessHandler.cxx
+++ b/Source/CTest/cmCTestMultiProcessHandler.cxx
@@ -723,7 +723,7 @@ void cmCTestMultiProcessHandler::StartNextTestsOnTimer()
   // Wait between 1 and 5 seconds before trying again.
   unsigned int const milliseconds = this->FakeLoadForTesting
     ? 10
-    : (cmSystemTools::RandomSeed() % 5 + 1) * 1000;
+    : (cmSystemTools::RandomNumber() % 5 + 1) * 1000;
   this->StartNextTestsOnTimer_.start(
     [](uv_timer_t* timer) {
       uv_timer_stop(timer);
diff --git a/Source/cmCoreTryCompile.cxx b/Source/cmCoreTryCompile.cxx
index 9674bf2bbe..a8fd0e0929 100644
--- a/Source/cmCoreTryCompile.cxx
+++ b/Source/cmCoreTryCompile.cxx
@@ -359,7 +359,7 @@ cm::optional<cmTryCompileResult> cmCoreTryCompile::TryCompileCode(
        of the same executable name (some filesystems fail on that).  */
     char targetNameBuf[64];
     snprintf(targetNameBuf, sizeof(targetNameBuf), "cmTC_%05x",
-             cmSystemTools::RandomSeed() & 0xFFFFF);
+             cmSystemTools::RandomNumber() & 0xFFFFF);
     targetName = targetNameBuf;
   }
 
diff --git a/Source/cmGeneratedFileStream.cxx b/Source/cmGeneratedFileStream.cxx
index 37a2be4a75..30f8a30296 100644
--- a/Source/cmGeneratedFileStream.cxx
+++ b/Source/cmGeneratedFileStream.cxx
@@ -139,7 +139,7 @@ void cmGeneratedFileStreamBase::Open(std::string const& name)
   } else {
     char buf[64];
     snprintf(buf, sizeof(buf), "tmp%05x",
-             cmSystemTools::RandomSeed() & 0xFFFFF);
+             cmSystemTools::RandomNumber() & 0xFFFFF);
     this->TempName += buf;
   }
 
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index cac7605b2a..6bc04f6b91 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -18,6 +18,14 @@
 #  define _DARWIN_C_SOURCE
 #endif
 
+#ifndef __has_feature
+#  define __has_feature(x) 0
+#endif
+
+#if !defined(__clang__) || __has_feature(cxx_thread_local)
+#  define CM_HAVE_THREAD_LOCAL
+#endif
+
 #include "cmSystemTools.h"
 
 #include <iterator>
@@ -79,10 +87,15 @@
 #include <functional>
 #include <iostream>
 #include <memory>
+#include <random>
 #include <sstream>
 #include <utility>
 #include <vector>
 
+#ifndef CM_HAVE_THREAD_LOCAL
+#  include <mutex>
+#endif
+
 #include <fcntl.h>
 
 #include "cmsys/Directory.hxx"
@@ -2810,6 +2823,18 @@ unsigned int cmSystemTools::RandomSeed()
 #endif
 }
 
+unsigned int cmSystemTools::RandomNumber()
+{
+#ifndef CM_HAVE_THREAD_LOCAL
+  static std::mutex gen_mutex;
+  std::lock_guard<std::mutex> gen_mutex_lock(gen_mutex);
+#else
+  thread_local
+#endif
+  static std::mt19937 gen{ cmSystemTools::RandomSeed() };
+  return static_cast<unsigned int>(gen());
+}
+
 namespace {
 std::string InitLogicalWorkingDirectory()
 {
diff --git a/Source/cmSystemTools.h b/Source/cmSystemTools.h
index dd6170b4bd..13f2572e5d 100644
--- a/Source/cmSystemTools.h
+++ b/Source/cmSystemTools.h
@@ -540,8 +540,9 @@ public:
 
   static void EnsureStdPipes();
 
-  /** Random seed generation.  */
+  /** Random number generation.  */
   static unsigned int RandomSeed();
+  static unsigned int RandomNumber();
 
   /** Find the directory containing CMake executables.  */
   static void FindCMakeResources(const char* argv0);
diff --git a/Source/cmake.cxx b/Source/cmake.cxx
index 5b4d58f72c..a41b53874a 100644
--- a/Source/cmake.cxx
+++ b/Source/cmake.cxx
@@ -211,7 +211,7 @@ bool cmakeCheckStampFile(const std::string& stampName)
   // The build system is up to date.  The stamp file has been removed
   // by the VS IDE due to a "rebuild" request.  Restore it atomically.
   std::ostringstream stampTempStream;
-  stampTempStream << stampName << ".tmp" << cmSystemTools::RandomSeed();
+  stampTempStream << stampName << ".tmp" << cmSystemTools::RandomNumber();
   std::string stampTemp = stampTempStream.str();
   {
     // TODO: Teach cmGeneratedFileStream to use a random temp file (with