Adding Zalasr

[mehnadnerd]

Adding load-acquire/store-release. Note they are written here as lb. for the load-acquire byte (so lb.aq and lb.aqrl), I'm not sure that will work but it passes the tests here.

[codecov-commenter]

Codecov Report

Merging #201 (ce0312a) into master (19e3ddc) will not change coverage.
The diff coverage is n/a.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   92.83%   92.83%           
=======================================
  Files           3        3           
  Lines         642      642           
=======================================
  Hits          596      596           
  Misses         46       46           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[aswaterman]

I get the strategy, but I can guarantee it'll cause some confusion :-) Let's see if it works out in Spike, Binutils, etc.

[mehnadnerd]

I've updated it so it now uses lb.aq and sb.rl as the "base" names. I've got it working in Spike (https://github.com/mehnadnerd/riscv-isa-sim/tree/zalasr). I'm planning to do LLVM support next.

[aswaterman]

Thanks, Brendan. I’ll take a look at the Spike PR later this week.

[aswaterman]

Thinking about this some more, maybe consider adding back the lb. versions with aq and rl as args, but as pseudo-ops? Implementations will want to decode them that way and, by happenstance, treat the other reserved aq/rl combinations the same way that LR/SC currently do. Having these generic versions in the decode table facilitates that implementation style.

[mehnadnerd]

Hmm, I think that will still have the confusion you mentioned. I'm also worried that people might choose to ignore the acquire bit for load-acquire, which would ruin compatibility for a future load-release.

[aswaterman]

Fair enough. Implementers are obviously still free to do what they will with a reserved encoding.