Re: [ptxdist] Go in ptxdist

[Clemens Gruber <clemens.gruber@pqgruber.com>]

On Tue, Oct 11, 2016 at 06:18:26PM +0200, Clemens Gruber wrote:
> Hi,
> 
> On Fri, Oct 07, 2016 at 08:37:55PM +0200, Uwe Kleine-König wrote:
> > I only took a quick look, but there is a return in _sfloat and after
> > that the VFP instructions are executed. And there are no conditional
> > instructions that would prevent a return. So unless I'm missing
> > something the ARMv5 code depends on kernel support.
> 
> They are doing something fancy here: They emulate the floating point
> instructions and then overwrite the LR to avoid returning to those
> hardfloat instructions.

s/LR/PC/g

After the emulation, they increase the program counter to skip the
hardfloat instructions after returning from _sfloat.
https://github.com/golang/go/blob/master/src/runtime/softfloat_arm.go#L648

> 
> See stepflt, _sfloat2 and sfloat2 in:
> https://github.com/golang/go/blob/master/src/runtime/softfloat_arm.go
> 
> As well as _sfloat and _sfloatpanic in:
> https://github.com/golang/go/blob/master/src/runtime/vlop_arm.s
> 
> 
> So, we could start with letting the user choose the GOARM value manually
> with explanations when to use which one:
> GOARM=5 (>=ARMv5 && <=ARMv7, armel with softfloat emulation)
> GOARM=6 (>=ARMv6 && <=ARMv7, armhf with VFPv1, but runs on VFPv2&3 too)
> GOARM=7 (==ARMv7, armhf with VFPv3 only)
> 
> Note:
> GOARM=5 works everywhere from ARMv5 to ARMv7 and does not require a VFP
> unit and no kernel emulation, because all floating point calls are
> emulated within Go itself. The linker inserts the call to _sfloat in
> those binaries. See the links above for the implementation.
> 
> 
> I think we could determine which GOARM version to use from both
> PTXCONF_GNU_TARGET and PTXCONF_CROSS_GCC_CONFIG_EXTRA in
> selected_toolchain/ptxconfig.
> I'd definitely prefer the autodetection, because it is very confusing at
> first which GOARM value is correct and why.
> 
> Bonus feature: Test compile a simple Go program which calls into a C
> library from the project, e.g. glibc. Problem is, we can't execute it.
> But at least we would have "some" problems detected by the linker. (?)
> 
> Thanks,
> Clemens


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