Re: [PATCH 2/2] include/linux: apply __malloc attribute

[Andrew Morton]

On Thu, 24 Mar 2016 00:42:32 +0100 Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> 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!