[master] RFR: 8303813: [Lilliput] (AArch64) Use tbz instead of tst and br in load_klass()

[Paul Hohensee]

On Wed, 8 Mar 2023 09:43:02 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> In the implementation of load_klass(), we have a test-and-branch idiom:
> 
> 
> tst(dst, markWord::monitor_value);
> br(Assembler::EQ, fast);
> 
> 
> We can make this better:
> 
> `tbz(dst, exact_log2(markWord::monitor_value), fast);
> `
> This is not only smaller and perhaps a little faster, it also has the advantage that it doesn't touch the condition flags. This is relevant in at least one place: in verify_oop() the condition flag is alive across the load_klass() call and one jtreg test (runtime/CheckUnhandledOops/TestVerifyOops.java) is failing because of that.
> 
> Testing:
>  - [x] tier1

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 4266:

> 4264:   // Check if we can take the (common) fast path, if obj is unlocked.
> 4265:   ldr(dst, Address(src, oopDesc::mark_offset_in_bytes()));
> 4266:   tbz(dst, exact_log2(markWord::monitor_value), fast);

This is a bit fragile because monitor_value must be a power of 2 for it to work.

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PR: https://git.openjdk.org/lilliput/pull/78