Re: [ptxdist] Go in ptxdist

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

Hi,

On Thu, Oct 06, 2016 at 11:17:40AM +0200, Michael Olbrich wrote:
> This is still not clear. 'use VFPv3' does not imply armhf ABI. You can mix
> code that uses VFPv3 with code that only uses ARMv5 features, as long as
> both use the same ABI (armel). But you cannot mix armel and armhf code.

To determine the ABI, I built 3 different Go hello world binaries and
looked at the headers.

First I tried with file and readelf but that did not help much.
Only the entry points differ between GOARM=5 and GOARM=7.

1: env GOOS=linux GOARCH=arm GOARM=5 go build hello.go
file: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
readelf -h | grep Entry: 0x71620
readelf -h | grep Flags: 0x5000002, Version5 EABI, <unknown>

2: env GOOS=linux GOARCH=arm GOARM=6 go build hello.go
file: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
readelf -h | grep Entry: 0x693cc
readelf -h | grep Flags: 0x5000002, Version5 EABI, <unknown>

3: env GOOS=linux GOARCH=arm GOARM=7 go build hello.go
file: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
readelf -h | grep Entry: 0x69300
readelf -h | grep Flags: 0x5000002, Version5 EABI, <unknown>

--

Marc wrote that in armel, floating point values are passed via normal
32-bit registers (r0, r1, .. ?) instead of VFP registers (s0, s1, .. /
d0, d1, .. ?)

So I disassembled the hello world binaries..

All contain single- and double-precision VFP floating point register
accesses like the following:
112fc:      ed932a00        vldr    s4, [r3]

But the first binary, built with GOARM=5, always has a branch before
floating point instructions or floating point registers are used.
It looks like this:
112fc:      eb01823f        bl      71c00 <_sfloat>

After the jump to _sfloat there are no VFP register accesses, only
regular integer registers and non-VFP instructions.

The second and third one, built with GOARM=6 and 7 do not even have
_sfloat and seem to use VFP registers unconditionally.

--

So, does this mean that they use armel EABI for GOARM=5 to stay
compatible with softfloat but take advantage of VFP if it is available?
(Similar to the GCC option -mfpu=softfp?)

And armhf for GOARM=6 and GOARM=7 where they do not have a softfloat
fallback and also use ARMv7 instructions like dmb with GOARM=7.
(The latter does not occur in my basic hello world example)

If you want to have a look at the disassemblies, I uploaded them here:

Search for runtime.f32hash to see the differences I spoke of.
And take a look at line 100968 of the disassembly hello_goarm5.S to see
the _sfloat instructions.

GOARM=5
https://gist.githubusercontent.com/clemensg/66d44f4149f0f4b38810a95125369de1/raw/d931d0ce89cde08569abce939953270a71d1e560/hello_goarm5.S

GOARM=6
https://gist.githubusercontent.com/clemensg/72deef3935eb4a6549172ab8c8d54848/raw/a8b32d0811848c3a352a0a77cdcf863da88aba68/hello_goarm6.S

GOARM=7
https://gist.githubusercontent.com/clemensg/14a5b1d606c974bc5f2890f02afedcc7/raw/4bc269e93eb21486b8dfcef69f9f695c65a59093/hello_goarm7.S

> I think we can rely on a host Go compiler to build 'our' Go compiler. We do
> the same thing with the C compiler and similar things. I think we may want
> to add it to 'ptxdist setup' or something like that, so that the user can
> specify a defined version.

OK, sounds good.

Clemens

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