Some questions about the gcc __sync intrinsics

Hi,

I have been comparing the stock gcc 5.2 and the Linaro 5.2 (Linaro GCC 5.2-2015.11-1) and have noticed a difference with the __sync intrinsics.

Here is the simple test case

--- cut here --- int add_int(int add_value, int *dest) { return __sync_add_and_fetch(dest, add_value); } --- cut here ---

Compiling with the stock gcc 5.2 (-S -O3) I get

--------- add_int: .L2: ldaxr w2, [x1] add w2, w2, w0 stlxr w3, w2, [x1] cbnz w3, .L2 mov w0, w2 ret ---------

Wheras with Linaro gcc 5.2 I get

--------- add_int: .L2: ldxr w2, [x1] add w2, w2, w0 stlxr w3, w2, [x1] cbnz w3, .L2 dmb ish mov w0, w2 ret ---------

Why the extra (unnecessary?) memory barrier?

Also, is it worthwhile putting a prfm before the ldaxr. EG

add_int: prfm pst1strm, [x1] .L2: ldaxr w2, [x1]

See the following thread

http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/355996.html

All the best, Ed