RFR: 8367692: RISC-V: Align post call nop

[Fei Yang]

On Wed, 24 Sep 2025 11:52:20 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Hi please, consider.
> 
> As ziccif require instructions to natural aligned to be atomic the 4 byte post call nop must be aligned.
> But I don't want to add a c.nop(2b) to align the nop(4b) which means the jal(r) must also be algined.
> As we have no utility to aligned the end of an instruction sequence the call it self is aligned and uses only 4 byte instructions. Only in the case where we could use an two c-instruction we may loose space.
> 
> Thanks, Robbin

Hi, Thanks for finding this. Nice fix! I only have several minor comments.

src/hotspot/cpu/riscv/assembler_riscv.hpp line 859:

> 857:     return insn;
> 858:   }
> 859: 

I am not sure but is it better to place this with friends `encode_jal` and `encode_jalr` at line 928? These all return an instruction encoding instead of writing the code cache and thus are different from other assember routines.

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 359:

> 357: void MacroAssembler::post_call_nop() {
> 358:   assert(!in_compressible_scope(), "Must be");
> 359:   assert_alignment(pc());

Does the first assertion make sense here? Seems to me the second one will just suffice.

src/hotspot/cpu/riscv/sharedRuntime_riscv.cpp line 1007:

> 1005:     __ bnez(c_rarg2, call_thaw);
> 1006: 
> 1007:     address tr_call;

Can you rename this to `call_pc` while you are on it? `tr_call` is named after trampoline call, but we don't have that now.

src/hotspot/cpu/riscv/sharedRuntime_riscv.cpp line 1042:

> 1040:   __ bnez(c_rarg2, call_thaw);
> 1041: 
> 1042:   address tr_call;

Similar here.

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PR Review: https://git.openjdk.org/jdk/pull/27467#pullrequestreview-3263439361
PR Review Comment: https://git.openjdk.org/jdk/pull/27467#discussion_r2377446188
PR Review Comment: https://git.openjdk.org/jdk/pull/27467#discussion_r2376159064
PR Review Comment: https://git.openjdk.org/jdk/pull/27467#discussion_r2377533241
PR Review Comment: https://git.openjdk.org/jdk/pull/27467#discussion_r2377533438