Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <CAA2zVHohAdM3v4WVsePPbCB1iC6OFOW9=Pxiqe=YYUTkxUNcwQ@mail.gmail.com>
References: <20180404093007.GI4082@hirez.programming.kicks-ass.net>
 <20180404191724.GF87376@google.com> <CAK8P3a1VFWYBymd3AeG1DwW++tCGsP9+vw6t8XSGHby2jmQPqA@mail.gmail.com>
 <20180404205848.GG87376@google.com> <CAK8P3a2tf3v6seq=OeyBakZ1ZXYzzL=tY=aHvKYv-GtD7sWeTA@mail.gmail.com>
 <20180404214639.GH87376@google.com> <CA+55aFwp8vRmf5p09nbunp6DfO+YvvWLj=FsU8KSW0ZwF6u80w@mail.gmail.com>
 <20180404221744.GI87376@google.com> <CA+55aFz+RcyPOTs3WM-FcwoQtgaotcBVqswma7qZ-3O2Fv__9Q@mail.gmail.com>
 <20180404233111.GJ87376@google.com> <20180405072017.GN4043@hirez.programming.kicks-ass.net>
 <CAA2zVHraWtmmW1yFvVH+kUbih5wxiDLuM=wArnQanrWN9NF=Xw@mail.gmail.com>
 <CA+55aFwrZU-VPa9HShikNDkvuOvFXtO2vw8sDeRRTCU_hV7dWA@mail.gmail.com>
 <CAA2zVHrF+teb3SwigH_sDqkWfKDg4itq4TkefYXUXw2RF9+4qw@mail.gmail.com>
 <CA+55aFz3PUE1mSM+yFeb55ca3EZAYHHqTyHjktiW1FVfK-tP9A@mail.gmail.com> <CAA2zVHohAdM3v4WVsePPbCB1iC6OFOW9=Pxiqe=YYUTkxUNcwQ@mail.gmail.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 5 Apr 2018 19:02:11 -0700
Message-ID: <CA+55aFxjR5=0uQ7JVUs=aZp0frtBZorAvsPeXk6GYbQJZ_VU+Q@mail.gmail.com>
Subject: Re: [GIT PULL] x86/build changes for v4.17
To:     James Y Knight <jyknight@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Matthias Kaehlcke <mka@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chandler Carruth <chandlerc@google.com>,
        Stephen Hines <srhines@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@google.com>,
        Guenter Roeck <groeck@chromium.org>,
        Greg Hackmann <ghackmann@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Apr 5, 2018 at 3:51 PM, James Y Knight <jyknight@google.com> wrote:
>
> Unfortunately, that behavior is required by the standard, it's not up to
> compiler optimization to change.

I actually mis-read your example - in your case it obviously does pass
the array itself down to the call, and yes, it obviously needs to be
allocated.

I had a test-case at one point where gcc avoided the stack allocation
entirely for a regular array, but not for a VLA (of the same constant
size) because the VLA logic is apparently different enough - even when
the size of the array is a compile-time constant.

We had that issue because we had a lot of trouble coming up with a
"max()" macro that was still an I-C-E (and we had a number of array
sizes that used "max()"). So all the array sizes were compile-time
constants, they just weren't traditional C arrays.

But now I can't recreate the thing. Maybe I had screwed up in my
test-case somehow.

               Linus