RFR: 8332689: RISC-V: Use load instead of trampolines [v14]

[Fei Yang]

On Thu, 20 Jun 2024 11:24:24 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi all, please consider!
>> 
>> Today we do JAL to **dest** if **dest** is in reach (+/- 1 MB).
>> Using a very small application or running very short time we have fast patchable calls.
>> But any normal application running longer will increase the code size and code chrun/fragmentation.
>> So whatever or not you get hot fast calls rely on luck.
>> 
>> To be patchable and get code cache reach we also emit a stub trampoline which we can point the JAL to.
>> This would be the common case for a patchable call.
>> 
>> Code stream:
>> JAL <trampo>
>> Stubs:
>> AUIPC
>> LD
>> JALR
>> <DEST>
>> 
>> 
>> On some CPUs L1D and L1I can't contain the same cache line, which means the tramopline stub can bounce from L1I->L1D->L1I, which is expensive.
>> Even if you don't have that problem having a call to a jump is not the fastest way.
>> Loading the address avoids the pitsfalls of cmodx.
>> 
>> This patch suggest to solve the problems with trampolines, we take small penalty in the naive case of JAL to **dest**,
>> and instead do by default:
>> 
>> Code stream:
>> AUIPC
>> LD
>> JALR
>> Stubs:
>> <DEST>
>> 
>> An experimental option for turning trampolines back on exists.
>> 
>> It should be possible to enhanced this with the WIP [Zjid](https://github.com/riscv/riscv-j-extension) by changing the JALR to JAL and nop out the auipc+ld (as the current proposal of Zjid forces the I-fetcher to fetch instruction in order (meaning we will avoid a lot issues which arm has)) when in reach and vice-versa.
>> 
>> Numbers from VF2 (I have done them a few times, they are always overall in favor of this patch):
>> 
>> fop                                        (msec)    2239       |  2128       =  0.950424
>> h2                                         (msec)    18660      |  16594      =  0.889282
>> jython                                     (msec)    22022      |  21925      =  0.995595
>> luindex                                    (msec)    2866       |  2842       =  0.991626
>> lusearch                                   (msec)    4108       |  4311       =  1.04942
>> lusearch-fix                               (msec)    4406       |  4116       =  0.934181
>> pmd                                        (msec)    5976       |  5897       =  0.98678
>> jython                                     (msec)    22022      |  21925      =  0.995595
>> Avg:                                       0.974112                              
>> fop(xcomp)                                 (msec)    2721       |  2714       =  0.997427
>> h2(xcomp) ...
>
> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:
> 
>  - Minor review comments
>  - Merge branch 'master' into 8332689
>  - To be pushed
>  - Merge branch 'master' into 8332689
>  - Review comments, removed dead code.
>  - Merge branch 'master' into 8332689
>  - Merge branch 'master' into 8332689
>  - Merge branch 'master' into 8332689
>  - Merge branch 'master' into 8332689
>  - Merge branch 'master' into 8332689
>  - ... and 12 more: https://git.openjdk.org/jdk/compare/d7dad50a...e47f2454

src/hotspot/cpu/riscv/compiledIC_riscv.cpp line 74:

> 72:   // Somewhat pessimistically, we count 4 instructions here (although
> 73:   // there are only 3) because we sometimes emit an alignment nop.
> 74:   // Trampoline stubs are always word aligned.

Seems the code comment needs update to reflect this change.

src/hotspot/cpu/riscv/riscv.ad line 1244:

> 1242:     return 1 * NativeInstruction::instruction_size; // jal
> 1243:   }
> 1244:   return 3 * NativeInstruction::instruction_size; // auipc + ld + jalr

Question: As we will only patch the address in the stub, do we still need the handling in compute_padding (`CallStaticJavaDirectNode::compute_padding` & `CallDynamicJavaDirectNode::compute_padding`) when `UseTrampolines` is false?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19453#discussion_r1648491039
PR Review Comment: https://git.openjdk.org/jdk/pull/19453#discussion_r1648542886