RFR: 8270086: ARM32-softfp: Do not load CONSTANT_double using the condy helper methods in the interpreter

[Aleksey Shipilev]

On Tue, 13 Jul 2021 13:17:58 GMT, Christoph Göttschkes <cgo at openjdk.org> wrote:

> Hi,
> 
> please review this fix for the template interpreter for ARM32-softfp.
> 
> With the introduction of condy, the interpreter started to load double constants using the condy_helper, which is unnecessary. Also, the [resolve_ldc](https://github.com/openjdk/jdk/blob/8973867fb9568a3a527b763c9ce10cebdfb306d0/src/hotspot/share/interpreter/interpreterRuntime.cpp#L148) implementation of the interpreter runtime might not be designed to load constant values. It should only load strings and condy.
> 
> https://github.com/openjdk/jdk/blob/2ee56fd1cf4acd634e19cc77b7c9a6858e1c315a/src/hotspot/cpu/arm/templateTable_arm.cpp#L515-L560
> 
> I refactored the ldc2_w bytecode imlpementation. Now, the ifdef between soft and hard float covers both types, CONSTANT_Long and CONSTANT_Double. I did this, because for the softfp, we can use a conditional cmp, since double and long are both placed on the stack in the same way. The same is also done for CONSTANT_Integer and CONSTANT_Float in the ldc bytecode implementation. Also, I think it is easier to follow the code this way.
> 
> Old ldc2_w implementation (before condy was implemented for ARM32):
> 
> https://github.com/openjdk/jdk/blob/7101b28dc3903be27cb46a00781f4d74f81f1114/src/hotspot/cpu/arm/templateTable_arm.cpp#L497-L537
> 
> Testing: ARMv7-A (hardfp) hotspot tier1, ARMv5TE (softfp) hotspot tier1

AFAICS, the real problem is that "double" path is not implemented for "!__ABI_HARD__" path, so it slides right into condy helper, right? Which calls for fixing that part, and that part only. This also avoids touching the "long" path and relying on the "ltos" being fine for doubles. Other parts of template interpreter deal with this already, see `TemplateTable::dload`.

AFAIU, `ABI_HARD` and `SOFTFP` are defining different things. `SOFTFP` defines whether to use hardware instructions or software emulation to deal with FP. Whereas `ABI_HARD` covers the _ABI_ parts, i.e. which registers to use to pass FP values between calls. Here we don't call anything, but only do loads. This reasoning seems to be consistent with what `TemplateTable::dload` does. Seeing how `-mfloat-abi` takes three values, I can see `ABI_HARD` and `SOFTFP` might be not mutually exclusive?

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PR: https://git.openjdk.java.net/jdk/pull/4767