linux-kernel - Re: [PATCH v5 3/4] arm64: mm: support ARCH_MMAP_RND

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Date:	Tue, 08 Dec 2015 11:03:51 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Daniel Cashman <dcashman@...roid.com>,
	Jon Hunter <jonathanh@...dia.com>, linux-doc@...r.kernel.org,
	catalin.marinas@....com, will.deacon@....com, linux-mm@...ck.org,
	hpa@...or.com, mingo@...nel.org, aarcange@...hat.com,
	linux@....linux.org.uk, kirill.shutemov@...ux.intel.com,
	corbet@....net, xypron.glpk@....de, x86@...nel.org,
	hecmargi@....es, mgorman@...e.de, rientjes@...gle.com, bp@...e.de,
	nnk@...gle.com, dzickus@...hat.com, keescook@...omium.org,
	jpoimboe@...hat.com, tglx@...utronix.de, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, salyzyn@...roid.com,
	ebiederm@...ssion.com, jeffv@...gle.com, n-horiguchi@...jp.nec.com,
	dcashman@...gle.com
Subject: Re: [PATCH v5 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

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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