linux-kernel - Re: [GIT PULL] x86/build changes for v4.17

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Date:   Thu, 5 Apr 2018 19:02:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     James Y Knight <jyknight@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chandler Carruth <chandlerc@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Guenter Roeck <groeck@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17

On Thu, Apr 5, 2018 at 3:51 PM, James Y Knight <jyknight@...gle.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