Re: [PATCH] powerpc: workaround clang codegen bug in dcbz

From: Arnd Bergmann <arnd@arndb.de>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: kbuild test robot <lkp@intel.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Paul Mackerras <paulus@samba.org>,
	Nathan Chancellor <natechancellor@gmail.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz
Date: Tue, 30 Jul 2019 16:30:29 +0200	[thread overview]
Message-ID: <CAK8P3a2755_6xq453C2AePLW8BeQk_Jg=HfjB_F-zyVMnQDfdg@mail.gmail.com> (raw)
In-Reply-To: <20190730134856.GO31406@gate.crashing.org>

On Tue, Jul 30, 2019 at 3:49 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Tue, Jul 30, 2019 at 09:34:28AM +0200, Arnd Bergmann wrote:
> > Upon a second look, I think the issue is that the "Z" is an input argument
> > when it should be an output. clang decides that it can make a copy of the
> > input and pass that into the inline asm. This is not the most efficient
> > way, but it seems entirely correct according to the constraints.
>
> Most dcb* (and all icb*) do not change the memory pointed to.  The
> memory is an input here, logically as well, and that is obvious.

Ah, right. I had only thought of dcbz here, but you are right that using
an output makes little sense for the others.

readl() is another example where powerpc currently uses "Z" for an
input, which illustrates this even better.

> > Changing it to an output "=Z" constraint seems to make it work:
> >
> > https://godbolt.org/z/FwEqHf
> >
> > Clang still doesn't use the optimum form, but it passes the correct pointer.
>
> As I said many times already, LLVM does not seem to treat all asm
> operands as lvalues.  That is a bug.  And it is critical for memory
> operands for example, as should be obvious if you look at at for a few
> seconds (you pass *that* memory, not a copy of it).  The thing you pass
> has an identity.  It's an lvalue.  This is true for *all* inline asm
> operands, not just output operands and memory operands, but it is most
> obvious there.

From experimentation, I would guess that llvm handles "m" correctly, but
not "Z". See https://godbolt.org/z/uqfDx_ for another example.

> Or, LLVM might have a bug elsewhere.
>
> Either way, the asm is fine, and it has worked fine in GCC since
> forever.  Changing this constraint to be an output constraint would
> just be obfuscation (we could change *all* operands to *everything* to
> be inout ("+") constraints, and it won't affect correctness, just the
> reader's sanity).

I would still argue that for dcbz specifically, an output makes more
sense than an input, but as you say that does not solve the others.

        Arnd