https://llvm.org/bugs/show_bug.cgi?id=30570
Bug ID: 30570
Summary: Memory model spec allows for tearing between same-size
accesses
Product: new-bugs
Version: unspecified
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
At the moment, the LLVM memory model specification allows for tearing. This is
correct for mixed-size or unaligned access, for example if mutexes are used for
large accesses, and atomic instructions are used for small accesses.
Unfortunately, the spec as it currently stands allows for tearing even when the
accesses are all of the same size.
The problem is that in
http://llvm.org/docs/LangRef.html#memory-model-for-concurrent-operations, there
is no guarantee that each of the W_byte operations observed by one R come from
the same W, even when all accesses are to the same address range.
Adding a new sentence to the specification would fix this, something like "If
an atomic read R can only see atomic writes to the same address range, then
there must be a single write W such that each R_byte in R chooses a W_byte from
W."
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