[PATCH v7 0/9] ARM: VDSO

On Mon, Jun 30, 2014 at 10:56:49PM +0100, Andy Lutomirski wrote:

On 06/30/2014 08:42 AM, Nathan Lynch wrote:

quoted

On 06/30/2014 09:40 AM, Will Deacon wrote:

quoted

  - If we use your code to build an arm64 vdso, how much slower is it than
    the handcrafted asm we currently have?

Someone with access to 64-bit hardware (i.e. not me) would have to find
this out, but I would not be surprised if the C code could be tightened
up -- I have not spent much time on optimizing it.

I can't comment on ARM specifically, but the only assembly in the x86
vclock_gettime is an asm volatile (""), which causes gcc to emit more
optimal code.  I doubt I could shave off more than a cycle or two by
rewriting it in assembly.

The arm64 version avoids the stack completely (we have lots of registers),
so I'd expect a significant win over a C version. However, I'd be interested
to see a comparison as the C version is clearly more maintainable.

Will