On 8 Sep 2017, at 21:09, Mateusz Guzik <m...@freebsd.org> wrote:
>
> Author: mjg
> Date: Fri Sep 8 20:09:14 2017
> New Revision: 323329
> URL: https://svnweb.freebsd.org/changeset/base/323329
>
> Log:
> Allow __builtin_memset instead of bzero for small buffers of known size
This change seems redundant, because modern compilers already do this
optimisation. For example:
#include <strings.h>
char buf[42];
void bz(void)
{
bzero(buf, 42);
}
With clang 4.0 on x86 compiles to:
pushq %rbp
movq %rsp, %rbp
xorps %xmm0, %xmm0
movups %xmm0, buf+26(%rip)
movaps %xmm0, buf+16(%rip)
movaps %xmm0, buf(%rip)
popq %rbp
retq
On AArch64, it compiles to:
adrp x8, buf
add x8, x8, :lo12:buf
strh wzr, [x8, #40]
stp xzr, xzr, [x8, #24]
stp xzr, xzr, [x8, #8]
str xzr, [x8]
ret
Neither contains a call, both have inlined the zeroing. This change is
strictly worse, because the compiler has some carefully tuned heuristics that
are set per target for when to inline the memset / bzero and when to call the
function. These are based on both the size and the alignment, including
whether the target supports misaligned accesses and whether misaligned accesses
are cheap. None of this is captured by this change.
In the kernel, this optimisation is disabled by -ffreestanding, however
__builtin_memset will be turned into a memset call if the size is not constant
or if the memset call would be more efficient (as determined by the
aforementioned heuristics). Simply using __builtin_memset in all cases should
give better code, and is more likely to be forward compatible with future ISAs
where the arbitrary constant picked in this patch may or may not be optimal.
David
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