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.
1 file changed
