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Vector task group minutes for 2020/8/21


Krste Asanovic
 

Date: 2020/8/21
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~12
Current issues on github: https://github.com/riscv/riscv-v-spec

Issues discussed:

#501 Ordering of element loads

Discussion was around vector memory access ordering and interaction
with global memory consistency model. The original proposal for
vector memory ordering was that it behaved like a scalar loop over the
elements, however this would imply that a vector load that accessed
the same address would have to access that address in element order.
This only affects stride-0 loads and indexed gathers. The proposal
was to relax indexed gathers to always be unordered, and to use vl=1
if the program required ordering between loop iterations.

Another dicussion was on access to ordered and/or non-idempotent
memory regions, where initiating memory accessed out-of-order would
not give expected semantics, and restarting instructions once this was
detected could be problematic for implementations without load buffers
or renamed registers, and when having to maintain
precise-to-the-element exception semantics. One proposal was to
weaken precise-to-element semantics and forbid overlap of source index
and destination vector on gathers.

Implementing debug data watchpoints with unordered accesses also is
problematic if watchpoints have to be triggered in element. A
proposal was to allow watchpoints to be reported out of element order.

Currently, strided segment stores where segments overlap are defined
to occur in element order. Discussion was around loosening this
constraint to out-of-order writes, or to trap if the overlap was
detected.