gdb: fix libstdc++ assert caused by invalid use of std::clamp

After this commit:

  commit 33ae45434d0ab1f7de365b9140ad4e4ffc34b8a2
  Date:   Mon Dec 4 14:23:17 2023 +0000

      gdb: Enable early init of thread pool size

I am now seeing this assert from libstdc++:

  /usr/include/c++/9/bits/stl_algo.h:3715: constexpr const _Tp& std::clamp(const _Tp&, const _Tp&, const _Tp&) [with _Tp = int]: Assertion '!(__hi < __lo)' failed.

This may only be visible because I compile with:

  -D_GLIBCXX_DEBUG=1 -D_GLIBCXX_DEBUG_PEDANTIC=1

but I haven't checked.  The issue the assert is highlighting is real,
and is caused by this block of code:

  if (n_threads < 0)
    {
      const int hardware_threads = std::thread::hardware_concurrency ();
      /* Testing in #29959 indicates that parallel efficiency drops between
         n_threads=5 to 8.  Therefore, clamp the default value to 8 to avoid an
         excessive number of threads in the pool on many-core systems.  */
      const int throttle = 8;
      n_threads = std::clamp (hardware_threads, hardware_threads, throttle);
    }

The arguments to std::clamp are VALUE, LOW, HIGH, but in the above, if
we have more than 8 hardware threads available the LOW will be greater
than the HIGH, which is triggering the assert I see above.

I believe std::clamp is the wrong tool to use here.  Instead std::min
would be a better choice; we want the smaller value of
HARDWARE_THREADS or THROTTLE.  If h/w threads is 2, then we want 2,
but if h/w threads is 16 we want 8, this is what std::min gives us.

After this commit, I no longer see the assert.

diff --git a/gdb/maint.c b/gdb/maint.c
index 2e6754c95581dc96e1c27c7cb7b900556b53bc26..68b70bf403d325d0b9d253137192a55562cccd9a 100644 (file)
--- a/gdb/maint.c
+++ b/gdb/maint.c
@@ -855,11 +855,12 @@ update_thread_pool_size ()
   if (n_threads < 0)
     {
       const int hardware_threads = std::thread::hardware_concurrency ();
-      /* Testing in #29959 indicates that parallel efficiency drops between
-        n_threads=5 to 8.  Therefore, clamp the default value to 8 to avoid an
-        excessive number of threads in the pool on many-core systems.  */
-      const int throttle = 8;
-      n_threads = std::clamp (hardware_threads, hardware_threads, throttle);
+      /* Testing in PR gdb/29959 indicates that parallel efficiency drops
+        between n_threads=5 to 8.  Therefore, use no more than 8 threads
+        to avoid an excessive number of threads in the pool on many-core
+        systems.  */
+      const int max_thread_count = 8;
+      n_threads = std::min (hardware_threads, max_thread_count);
     }
 
   gdb::thread_pool::g_thread_pool->set_thread_count (n_threads);