runtime: mips32 soft float point support

[juliandroid]

I have built Golang by using the master branch and I have realized that there is no soft point support for the new mips32 target. That means the core like MIPS32 74Kc is not supported due to lack of FPU.

I have wrote an email to Vladimir Stefanovic, asking him to confirm the soft-float point support issue and his response was that in the near future there are no plans soft-point support to be implemented, but that might change later.

It would be great to have a ticket here to keep track the development of the issue and hopefully one day this issue to be resolved.

[bradfitz]

/cc @vstefanovic @cherrymui

[cherrymui]

see also #14635.

[cherrymui]

There are softfloat code for ARM that we can reuse. What still need to be done is MIPS instruction decoding.

[vstefanovic]

@juliandroid
Yes, and after you quite a few other people asked about soft float support, so looks like the future has almost arrived.
If the code wasn't in feature freeze state, the patch would be submitted relatively soon.

[juliandroid]

@vstefanovic
That was fast, the issue comes with twist :) Thanks for working on it!
Can you please leave the commit id (gerrit) here, once you complete the soft-float support? I'm willing to try it even before the merge in the master.

[vstefanovic]

@juliandroid
Sure, I'll add a link.

[minux]

After discussion with @cherrymui on this issue, we think it's probably a good idea to always enable soft float for mips and mips64 (for both endianness) so that we don't need to add a GOMIPS knob. We can make _sfloat return immediately without interpreting the following FPU instructions if the machine does have an FPU. The rationale is that most MIPS machines do not have an FPU, this is unlike the ARM situation where if we do drop ARMv6 support, almost all supported machine will have an FPU. Of course, we should base our final decision on both binary size and execution time overhead for this. If we decide to add a $GOMIPS knob to mirror the existing $GOARM, we should also consider add support for the legacy 32-bit MIPS instruction sets (i.e. MIPS I/II) so that our 32-bit port also runs on Loongson 2.

[petar-jovanovic]

The rationale is that most MIPS machines do not have an
FPU

I have to disagree. Majority of modern MIPS boards do have FP
unit. There is a long list of legacy boards that indeed lack FPU
support, but relatively modern MIPS boards all have it.
$GOMIPS is important also to make a distinction between MIPS32r1/r2/r6 and
MIPS64r2/r6 (and MIPS3 as a legacy) that we plan to support through Go.

[aep]

@vstefanovic any progress? need any help with this?

[vstefanovic]

@aep
Thanks, this will be submitted after go 1.8 is released, since it was too late for 1.8.