RFR: 8302976: C2 intrinsification of Float.floatToFloat16 and Float.float16ToFloat yields different result than the interpreter [v4]

[Vladimir Ivanov]

On Wed, 8 Mar 2023 05:17:53 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Implemented `Float.floatToFloat16` and `Float.float16ToFloat` intrinsics in Interpreter and C1 compiler to produce the same results as C2 intrinsics on x64, Aarch64 and RISC-V - all platforms where C2 intrinsics for these Java methods were implemented originally.
>> 
>> Replaced `SharedRuntime::f2hf()` and `hf2f()` C runtime functions with calls to runtime stubs which use the same HW instructions as C2 intrinsics. Only for 64-bit x64 because 32-bit x86 stub does not work: result is passed through FPU register and NaN values become different from C2 intrinsic. This runtime stub is only used to calculate constant values during C2 compilation and can be skipped.
>> 
>> I added new tests based on Tobias's `TestAll.java` And copied `jdk/lang/Float/Binary16Conversion*.java` tests to run them with `-Xcomp` to make sure code is compiled by C1 or C2. I modified `Binary16ConversionNaN.java` to compare results from Interpreter, C1 and C2.
>> 
>> Tested tier1-5, Xcomp, stress
>
> Vladimir Kozlov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove RISC-V port code for float16 intrinsics

Overall, looks good. Minor comments/suggestions follow.

src/hotspot/share/opto/convertnode.cpp line 171:

> 169:   if (t == Type::TOP) return Type::TOP;
> 170:   if (t == Type::FLOAT) return TypeInt::SHORT;
> 171:   if (StubRoutines::f2hf() == nullptr) return bottom_type();

What's the purpose of this check? My understanding is ConvF2HF/ConvHF2F require intrinsification and on platforms where stubs are absent, intrinsification is disabled.

src/hotspot/share/opto/convertnode.cpp line 244:

> 242: 
> 243:   const TypeInt *ti = t->is_int();
> 244:   if (ti->is_con()) {

I find it confusing that `ConvHF2FNode::Value()` has `is_con()` check, but `ConvF2HFNode::Value()`doesn't. I'd prefer to see both implementations unified.

src/hotspot/share/runtime/sharedRuntime.cpp line 451:

> 449:   assert(StubRoutines::f2hf() != nullptr, "floatToFloat16 intrinsic is not supported on this platform");
> 450:   typedef jshort (*f2hf_stub_t)(jfloat x);
> 451:   return ((f2hf_stub_t)StubRoutines::f2hf())(x);

What's the point of keeping the wrappers around? The stubs can be called directly, can't they?

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PR: https://git.openjdk.org/jdk/pull/12869