RFR: 8320448: Accelerate IndexOf using AVX2 [v33]

[Scott Gibbons]

On Fri, 24 May 2024 06:31:40 GMT, Daniel Jeliński <djelinski at openjdk.org> wrote:

>> It may, but I believe the movq is shorter (although maybe not to r15).  I'll do some experimentation.
>
> the RIP-relative lea should have a shorter encoding. I think something like `lea(r15, ExternalAddress(small_jump_table))` should produce it (untested)

Just did the experiment and it turns out that `mov64(r15, (int64_t)small_jump_table)` and `lea(r15, ExternalAddress(small_jump_table))` produce exactly the same code:

`0x00007fffe463d68b:  49 bf a0 d5 63 e4 ff 7f 00 00   movabs r15,0x7fffe463d5a0`

The code in `MacroAssembler` for `lea` calls `mov_literal64` with no check for whether it can be ip-relative.

I tried doing it myself via `leaq(r15, Address(rip, (int64_t)small_jump_table - (int64_t)(__ pc())))` but there is no definition in `register_x86.hpp` for register `rip`.  So I'm not sure exactly how to produce RIP-relative addressing.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16753#discussion_r1613560044