On Thu, 24 Mar 2016 00:42:32 +0100 Rasmus Villemoes <[email protected]>
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!
