On 4/21/21 10:16 PM, Jakub Jelinek wrote: > On Wed, Apr 21, 2021 at 08:53:54PM +0100, Jonathan Wakely wrote: >>>> What would be IMHO a good idea would be to use configure test for >>>> #include <thread> >>>> int t = std::thread::hardware_concurrency (); >>>> and in that case use that as a fallback to the previous implementation, >>>> that will be strictly an improvement. >>> >>> That does not make sense to me. Original motivation was removal of the >>> complicated >>> implementation with a C++ standard function. > > The reason it is larger and more complicated is that it answers a different > question than std::thread::hardware_concurrency(). > hardware_concurrency() says how many threads the hw has available, if we > wanted just that, you could just keep the sysconf call. > The current implementation answers the question how many threads can this > process and its children be scheduled on. > Consider some NUMA box with hundreds of threads, but where it might > be desirable to keep specific compilations limited to some cores and do > other compilations or whatever else on other cores. > > Sure, hardware_concurrency() answer is better than no information about the > concurrency at all. But I don't really see the costs of the current > implementation, it is < 100 lines of code that have been written once and > it is unlikely they will need changing - at this point the interfaces are > fairly stable, haven't been changed on the glibc side for years. It is true > that when it was introduced in glibc, there have been 2 big changes to it > soon.
All right, I understand that hardware_concurrency doesn't match exactly our needs. Thus I'm going to install the following patch that removes the FIXME. Thanks for help with that, Martin > > Jakub >
>From dbe87a4229126829e0a912e9928c82b8dde7b51c Mon Sep 17 00:00:00 2001 From: Martin Liska <mli...@suse.cz> Date: Thu, 22 Apr 2021 09:14:28 +0200 Subject: [PATCH] Remove not feasible FIXME gcc/ChangeLog: * lto-wrapper.c: Remove FIXME about usage of hardware_concurrency. The function is not on par with what we have now. --- gcc/lto-wrapper.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/gcc/lto-wrapper.c b/gcc/lto-wrapper.c index 0b626d7c811..49894e4fc61 100644 --- a/gcc/lto-wrapper.c +++ b/gcc/lto-wrapper.c @@ -1304,8 +1304,6 @@ init_num_threads (void) #endif } -/* FIXME: once using -std=c++11, we can use std::thread::hardware_concurrency. */ - /* Test and return reason why a jobserver cannot be detected. */ static const char * -- 2.31.1
