From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Cc: Daniel Cashman <dcashman@android.com>, Jon Hunter <jonathanh@nvidia.com>,
        linux-doc@vger.kernel.org, catalin.marinas@arm.com,
        will.deacon@arm.com, linux-mm@kvack.org, hpa@zytor.com,
        mingo@kernel.org, aarcange@redhat.com, linux@arm.linux.org.uk,
        kirill.shutemov@linux.intel.com, corbet@lwn.net, xypron.glpk@gmx.de,
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        bp@suse.de, nnk@google.com, dzickus@redhat.com, keescook@chromium.org,
        jpoimboe@redhat.com, tglx@linutronix.de, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, salyzyn@android.com,
        ebiederm@xmission.com, jeffv@google.com, n-horiguchi@ah.jp.nec.com,
        dcashman@google.com
Subject: Re: [PATCH v5 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.
Date: Tue, 08 Dec 2015 11:03:51 +0100
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On Monday 07 December 2015 10:26:34 Daniel Cashman wrote:
> > Ideally we'd remove the #ifdef around the mmap_rnd_compat_bits declaration
> > and change this code to use
> > 
> >       if (IS_ENABLED(CONFIG_COMPAT) && test_thread_flag(TIF_32BIT))
> > 
> That would result in "undefined reference to mmap_rnd_compat_bits" in
> the not-defined case, no?

No. The compiler eliminates all code paths that it knows are unused.
The IS_ENABLED() macro is designed to let the compiler figure this out.

	Arnd
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