https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70889
Bug ID: 70889
Summary: memory reordering across loads tagged as
memory_order_seq_cst
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: lucenadeveloper at gmail dot com
Target Milestone: ---
Based on my stackoverflow question here,
http://stackoverflow.com/questions/36958372/gcc-reordering-up-across-load-with-memory-order-seq-cst-is-this-allowed,
the RTL optimization stage reorders a nonatomic load up across a load tagged
with memory_order_seq_cst. This violates some synchronizes-with relationships
that should be established between reader and writer threads and breaks many
synchronization primitives such as seqlocks.
Further, this only occurs if the load in question is under 123 bytes. Anything
>= 123 does not get reordered.
