RFR: 8326306: RISC-V: Re-structure MASM calls and jumps [v5]

[Fei Yang]

On Mon, 6 May 2024 08:36:07 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi, please consider.
>> 
>> We have code that directly use the asm for call/jumps instead masm.
>> Our masm have a bit odd naming, and we don't use 'proper' pseudoinstructions/mnemonics.
>> Suggested by [riscv-asm-manual](https://github.com/riscv-non-isa/riscv-asm-manual/tree/master)
>> 
>> j offset	jal x0, offset	Jump
>> jal offset	jal x1, offset	Jump and link
>> jr rs	        jalr x0, rs, 0	Jump register
>> jalr rs	        jalr x1, rs, 0	Jump and link register
>> ret	        jalr x0, x1, 0	Return from subroutine
>> call offset	auipc x1, offset[31:12]; jalr x1, x1, offset[11:0]	Call far-away subroutine	
>> tail offset	auipc x6, offset[31:12]; jalr x0, x6, offset[11:0]	Tail call far-away subroutine
>> 
>> But these can only be implemented like this if you have small enough application.
>> The fallback of these is to use GOT (your C compiler should place a copy of GOT every 2G so it's always reachable).
>> We don't have GOT, instead we materialize, so there is still differences between these and ours.
>> 
>> This patch:
>> - Tries to follow these suggested mappings as good we can.
>> - Make sure all jumps/calls go through MASM. (so we get control and can easily change for sites using a certain calling convention)
>> - To avoid confusion between MASM public/private methods and ASM methods and the mnemonics there are some renaming.
>>   E.g. the mnemonics jal means call offset, as we can't use that so there is no 'jal'.
>> - I enabled c.j, but right now we never generate it.
>> - As always the macro does no good and are legacy from when code base did not use templates. (also the x-macros screws up my IDE (vim+rtags))
>> 
>> I started down this path due to I have followup patch on top of this which removes trampoline in favor for load-n-jump.
>> (WIP: https://github.com/robehn/jdk/compare/jal-fixes...robehn:jdk:load-n-link?expand=1)
>> While looking into our calls it was a bit confusing, this helps. 
>> 
>> Done a couple of t1-3 slightly different version of this patch, and as part of the followup, no issues found. (VF2, qemu, LP4)
>> Re-running tests, had some last minute changes.
>> 
>> Thanks, Robbin
>
> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
> 
>  - VM leaf should use li
>  - Merge branch 'master' into jal-fixes
>  - Merge branch 'master' into jal-fixes
>  - Merge branch 'master' into jal-fixes
>  - Corrected method name
>  - Missed a ws
>  - JALR

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 938:

> 936:   int32_t offset = 0;
> 937:   la(temp, dest, offset);
> 938:   Assembler::jalr(x1, temp, offset);

Hi, did you check the possible impact on performance of this change too?

old: call() => mv(tmp, adr) => li(tmp, adr)
new: call() => la(tmp, adr) => auipc/movptr(tmp, adr)

It's OK if we have auipc emitted by the new call(), which should not be worse then the old one. But the new call() could emit a fixed-sized movptr depending on the `is_32bit_offset_from_codecache` check which should be slower than a li(tmp, adr).

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PR Review Comment: https://git.openjdk.org/jdk/pull/18942#discussion_r1591114954