On Thu, 20 Jun 2019 at 21:26, Kacvinsky, Tom <Tom.Kacvinsky@xxxxxxxxxx> wrote: > > Hi, > > > -----Original Message----- > > From: Jonathan Wakely <jwakely.gcc@xxxxxxxxx> > > Sent: Thursday, June 20, 2019 11:54 AM > > To: Kacvinsky, Tom <Tom.Kacvinsky@xxxxxxxxxx> > > Cc: gcc-help@xxxxxxxxxxx > > Subject: Re: libatomic, 32-bit object code vs. 64-bit object, GCC 8.3.0 > > > > On Thu, 20 Jun 2019 at 16:19, Kacvinsky, Tom <Tom.Kacvinsky@xxxxxxxxxx> > > wrote: > > > > > > Hi, > > > > > > I have built a 32-bit GCC 8.3.0 (no multi-lib support) on RHEL 5 and a > > > 64-bit GCC 8.3.0 (again, no multi-lib support) on CentOS 5.11. What I > > > find interesting is that when I go to compile a program that will need > > > atomic locks (a Boost template class pulled in via a header file), the > > > 32-bit object code requires the library -latomic, but 64-bit object code does > > not require that. > > > > > > Looking over "info gcc" I see a multitude of options for atomic > > > operations. The take away for me sis that for architectures that have > > > an instruction set that has support for atomic operations, libatomic is not > > necessary, but for other architectures, it is. > > > > Right. Sort of. It depends what kind of variables you're using the atomic > > operations with. > > > > libatomic can still be needed on x86_64, but only for types larger than 64-bits. > > > > OK, for our purposes we are using integral types of 64-bits wide or less. > > > On i386 you need libatomic even for 32-bit integers, as it has no atomic > > operations. i586 has 64-bit atomics though. > > > > So if you configured your 32-bit compiler for i386, not i586 or later, then by > > default it will want to use libatomic for all atomic ops. > > If by configuring GCC for i386 you mean having i386 in the build triplet (used in the > the --build option), then yes indeed I compiled my 32-bit GCC for i386. Not sure if > we should target i586 (or higher). Right. You can either configure --with-arch=i586 to set the default -march value, or equivalently use i586 in the triplet. Either way, that only sets the default, you can still change it per-compilation using -march=i586 or -march=whatever, but if you originally built for i386 then the C++ runtime library (libstdc++) will be built for i386 and so will assume no atomics, and so probably require libatomic anyway. > All of that said, it's not a problem to use libatomic, I just wanted to understand why > our 32-bit product needs libatomic but our 64-bit product does not. > > Thanks for the help. You're welcome.
