Re: [ptxdist] Go in ptxdist

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

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.

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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