RFR: 8310939: [c1] The visibility of write-volatile requires membar_volatile instead of membar

[Andrew Haley]

On 7/19/23 21:39, Dean Long wrote:
> It's confusing to me to rename the post-barrier but not the pre-barrier.  Instead of changing membar_release + membar to
> membar_release + membar_volatile, something like volatile_write_pre_barrier + volatile_write_post_barrier in the shared code makes more sense to me.

I know that this patch is now withdrawn, but I'd like to make a point.

Please don't use terms like volatile_write_post_barrier for StoreLoad
if you can possibly avoid it, because that's only one way to implement
Java's volatile.

The assumption that volatile write is implemented by release; write; StoreLoad
or similar permeates HotSpot, and is a pain for architectures like AArch64
which that don't do volatile that way.

[ Comment for people reading this who don't know how AArch64 does work.

AArch64 uses an instruction that acts something like

   release; store   (called LDAR)

for a volatile write, and

   <StoreLoad if needed> ; load ; acquire   (called STLR)

for a volatile read.

Note: this isn't the full story, and you have to look at the definition
of things like barrier-ordered-before to know all the details. ]