… source is volatile
Current optimization: When we copy a->b and then b->c, we might as well
copy a->c instead of b->c (then b might be dead and go away).
*Except* if a is a volatile location (might be clobbered by a call).
In that case, we really do want to copy a immediately, because there
might be a call before we can do the a->c copy.
User calls can't happen in between, because the rule matches up the
memory states. But calls inserted for memory barriers, particularly
(I guess we could introduce a register-calling-convention version
of runtime.typedmemmove, but that seems a bigger change than this one.)
Trust: Keith Randall <firstname.lastname@example.org>
Trust: Josh Bleecher Snyder <email@example.com>
Run-TryBot: Keith Randall <firstname.lastname@example.org>
TryBot-Result: Go Bot <email@example.com>
Reviewed-by: Josh Bleecher Snyder <firstname.lastname@example.org>
(cherry picked from commit 304f769)