RFR: 8351949: RISC-V: Cleanup and enable store-load peephole for membars [v6]

[Robbin Ehn]

> Hi please consider.
> 
> |RVWMO| Patched|
> | ---------- | ---------- |
> |fence iorw,iorw| fence iorw,ow|
> |sw  t4,120(t2) | sw  t4,120(t2) |
> |fence ow,ir | unnecessary_membar_volatile_rvwmo |
> | sw  t6,128(t2) // Non-volatile | sw  t6,128(t2) // Non-volatile |
> |fence iorw,ow | fence iorw,ow|
> |sw  t5,124(t2) |sw  t5,124(t2) |
> 
> |TSO                                | Patched|
> | ---------- | ---------- |
> | lw	a4,120(t2) | lw	a6,120(t2) |
> | sw	a0,124(t2) | sw	t6,124(t2) |
> | fence	iorw,iorw | unnecessary_membar_volatile_tso |
> | sw	t4,120(t2) | sw	t4,120(t2) |
> | fence	ow,ir | unnecessary_membar_volatile_tso |
> | sw	t6,128(t2) | sw	t5,128(t2) |
> | sw	t5,124(t2) // Non-volatile| sw	a1,124(t2)  // Non-volatile |
> | fence	iorw,iorw  | unnecessary_membar_volatile_tso |
> |...  | ... |
> | sw	a3,120(t2) | sw	a0,120(t2) |
> | fence	ow,ir | fence	ow,ir |
> | lw	a7,124(t2) | lw	a5,124(t2) |
> 
> For the specific rvwmo volatile store + store + volatile store is around 30% faster on VF2.
> 
> The patch do:
> - Separate ztso and rvwmo in ad by using UseZtso predicate.
> - Match all that requires the same membar.
> - Make fence/fencei protected as they shouldn't be using directly.
> - Increased cost of membars to VOLATILE_REF_COST.
> - Added a real_empty pipe.
> - Change to pipe_slow on TSO (as x86).
> 
> Note that C2-rv64 is now superior to gcc/clang regrading fencing:
> https://godbolt.org/z/6E3YTP15j
> 
> Testing jcstress, tier1 and manually reading the generated assembly.
> Doing additional testing, but RFR it now as it may need some consideration.
> 
> /Robbin

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

  format comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24035/files
  - new: https://git.openjdk.org/jdk/pull/24035/files/e0e4fff3..cb184209

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24035&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24035&range=04-05

  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/24035.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24035/head:pull/24035

PR: https://git.openjdk.org/jdk/pull/24035