RFR: 8340241: RISC-V: Returns mispredicted [v4]

[Robbin Ehn]

> Hi, please consider.
> 
> RISC-V don't have dedicated call/ret instructions.
> Instead the registers used in the jal/jalr instructions determine if this is a JUMP or CALL/RET.
> The cpu have a return-address stack where it stores return addresses for prediction.
> There are two possible calling conventions: x1 and x5 (or using both for co-routines).
> This stack is updated according this table (from unpriv manual, 2.5.1. Unconditional Jumps) for JALR:
> 
> | rd is x1/x5 | rs1 is x1/x5 | rd=rs1 | RAS action
> | ------------- | ------------- | ------------- |------------- |
> |No | No | — | None|
> |No | Yes | — | Pop|
> |Yes | No | — | Push|
> |Yes | Yes | No | Pop, then push|
> |Yes | Yes | Yes | Push|
> 
> And additionally:
> "A JAL instruction should push the return address onto a return-address stack (RAS) only when rd is 'x1' or x5."
> 
> As the JDK is using x5/(t0) as main scratch all plains jumps are actually calls and calls are co-routine calls (push and pop).
> This causes performance issues as the predictions is often wrong.
> 
> Average time for 10 best iterations (VF2):
> | Benchmark | Baseline (ms) | RAS fixed (ms) | Diff |
> |-- | -- | -- | -- |
> |future-genetic | 22126.6 | 20461.8 | -7.52%|
> |akka-uct | 97119.6 | 97498 | 0.39%|
> |movie-lens | 82359.3 | 81009.2 | -1.64%|
> |scala-doku | 29246.1 | 24518.6 | -16.16%|
> |chi-square | 10207.3 | 10624.9 | 4.09%|
> |fj-kmeans | 55127.9 | 56169.1 | 1.89%|
> |finagle-http | 24845 | 24891.9 | 0.19%|
> |reactors | 97473.9 | 96655.5 | -0.84%|
> |dec-tree | 8322.99 | 8243.11 | -0.96%|
> |naive-bayes | 79249.1 | 76851.9 | -3.02%|
> |als | 52678 | 51245.9 | -2.72%|
> |par-mnemonics | 52237.4 | 53149.8 | 1.75%|
> |scala-kmeans | 2990.88 | 2992.14 | 0.04%|
> |philosophers | 9156.9 | 7754.5 | -15.32%|
> |log-regression | 7621.65 | 7540.85 | -1.06%|
> |gauss-mix | 9835.7 | 9396.25 | -4.47%|
> |mnemonics | 73087.3 | 69426.6 | -5.01%|
> |dotty | 10970.9 | 10719.1 | -2.30%|
> |finagle-chirper | 23386.1 | 23630.3 | 1.04%|
> |recursive fibonacci | 7338.56 | 5369.83 | **-26.83%**|
> 
> For some of workloads, e.g. call to small function in a loop, it really matters.
> 
> This patch blacklist x5(/t0) for JAL/JALR as we only use x1 calling convention.
> And changes all jumps to use x6(/t1) instead of x5(/t0).
> This patch was incrementally done, i.e. the first change removed the default t0.
> I visited all places makings jumps, to make sure t1 was available.
> Then changed to default t1 and removed argument in many cases.
> 
> Other approaches was tested, e.g. completely switch t0 <-> t1.
> This was much harder and more intr...

Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:

  Use x9, comment update

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21406/files
  - new: https://git.openjdk.org/jdk/pull/21406/files/c8d8fdc8..1bab8119

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21406&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21406&range=02-03

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21406.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21406/head:pull/21406

PR: https://git.openjdk.org/jdk/pull/21406