linux-kernel - Re: [PATCH 2/2] include/linux: apply _

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Message-Id: <20160324153639.bb996d7bf5a585dfb46740b7@linux-foundation.org>
Date:	Thu, 24 Mar 2016 15:36:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/2] include/linux: apply __malloc attribute

On Thu, 24 Mar 2016 00:42:32 +0100 Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> Attach the malloc attribute to a few allocation functions. This helps
> gcc generate better code by telling it that the return value doesn't
> alias any existing pointers (which is even more valuable given the
> pessimizations implied by -fno-strict-aliasing).
> 
> A simple example of what this allows gcc to do can be seen by looking
> at the last part of drm_atomic_helper_plane_reset:
> 
> 	plane->state = kzalloc(sizeof(*plane->state), GFP_KERNEL);
> 
> 	if (plane->state) {
> 		plane->state->plane = plane;
> 		plane->state->rotation = BIT(DRM_ROTATE_0);
> 	}
> 
> which compiles to
> 
>     e8 99 bf d6 ff          callq  ffffffff8116d540 <kmem_cache_alloc_trace>
>     48 85 c0                test   %rax,%rax
>     48 89 83 40 02 00 00    mov    %rax,0x240(%rbx)
>     74 11                   je     ffffffff814015c4 <drm_atomic_helper_plane_reset+0x64>
>     48 89 18                mov    %rbx,(%rax)
>     48 8b 83 40 02 00 00    mov    0x240(%rbx),%rax [*]
>     c7 40 40 01 00 00 00    movl   $0x1,0x40(%rax)
> 
> With this patch applied, the instruction at [*] is elided, since the
> store to plane->state->plane is known to not alter the value of
> plane->state.

Shaves 6 bytes off my 1MB i386 defconfig vmlinux.  Winner!