Date: Sat, 31 Aug 2013 12:34:13 +0100 From: David Chisnall <theraven@FreeBSD.org> To: Ed Schouten <ed@80386.nl> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: svn commit: r255092 - in head: lib/libcompiler_rt sys/arm/arm Message-ID: <34C4386C-25D8-449C-8E53-5A0597FEEF7A@FreeBSD.org> In-Reply-To: <CAJOYFBB63WuWXRCTtj4VbN1G3WBaQ9P6uhWwLDqWJxFNwjztug@mail.gmail.com> References: <201308310850.r7V8ojQX022383@svn.freebsd.org> <CAJOYFBB63WuWXRCTtj4VbN1G3WBaQ9P6uhWwLDqWJxFNwjztug@mail.gmail.com>
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On 31 Aug 2013, at 12:30, Ed Schouten <ed@80386.nl> wrote: > 1. Fix LLVM/Clang. >=20 > Never ever let LLVM/Clang emit calls to __sync_*. You can easily > implement the __sync_* interface on top of __atomic_*. What baffles > me, is that the calls to __sync_* are emitted by LLVM > (SelectionDAGLegalize::ExpandAtomic), while Clang is responsible for > emitting the __atomic_* calls (CodeGenFunction::EmitAtomicExpr). >=20 > All of this should have been pushed down into LLVM in the first place. > It is currently hard to implement your own programming language on top > of LLVM that is capable of doing atomic operations the same way > C11/C++11 does them. You either have to use the __sync_* intrinsics or > duplicate all the code that emits the libcalls. >=20 > I've noticed that due to this separation of doing __sync_* in LLVM and > __atomic_* in Clang, the behaviour of Clang can sometimes be > incredibly counter-intuitive. For example, both LLVM and Clang need a > similar piece of code to determine whether hardware atomics are > available. I've noticed that if this logic is not identical, Clang > does some really weird things. If you call __atomic_* functions and > Clang thinks we have hardware atomics, but LLVM thinks we do not, we > may end up emitting calls to __sync_* instead. >=20 > Furthermore, the duplication of code between EmitAtomicExpr, > EmitAtomicStore and EmitAtomicLoad leads to the problem that > EmitAtomicExpr properly emits power-of-two-sized calls for most > primitive datatypes, while EmitAtomicStore and EmitAtomicLoad do not. I completely agree with all of this. There are lots of things in the = layering in LLVM that I don't like that place too much target-specific = knowledge in the front ends. =20 > 2. Fix the consumers. >=20 > Right now there are only two pieces of code in our tree (libc++ and > libcxxrt) that emit calls to __sync_* unconditionally. All the other > code either uses <machine/atomic.h> or <stdatomic.h>. These pieces of > code could have been ported to use C11 atomics with a similar amount > of effort. >=20 > For libcxxrt you could even argue that this should have done in the > first place, because it already used __atomic_* calls in certain > source files. >=20 > Example patch for libcxxrt: >=20 > http://80386.nl/pub/libcxxrt.txt libcxxrt, in particular, has the constraint that it must also compile = with gcc and Path64, so patches of this nature need careful testing. = Although this would fix the issue in the tree, a number of ports = explicitly call the __sync_* builtins, including some that have USE_GCC = set, so even fixing clang would not address this. David
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