cmd/link: ppc64 (big endian) cgo errors

[laboger]

I've built golang from master with the patches from issue 11184 to get external linking to work with ppc64le. That seems to work well on ppc64le with external linking and cgo.

In cmd/dist/build.go, linux/ppc64le is in the cgoEnabled map but linux/ppc64 is not. So the build of golang with the latest patches on ppc64 does not quite work with cgo.

If I build the golang toolchain on ppc64 with CGO_ENABLED=1, I first get this error:

cannot use dynamic imports with -d flag

I made a change to cmd/link/internal/ppc64/obj.go to get rid of this message, but then hit this error:
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
......
runtime/cgo(.opd): unexpected relocation type 307
.......
and then too many errors

I can see that relocation type 51 is R_PPC64_TOC and that is not handled by the code. I tried adding those defines but not sure what should be generated for this relocation type. If there are suggestions on what to do I can try them out.

[laboger]

I've been doing this on a RHEL7.2 system.

[mwhudson]

Those errors suggest that you're not actually linking externally. Try
adding -ldflags=-linkmode=external to your go tool invocation.

I'm pretty sure that things still won't work though.

On 10 November 2015 at 09:25, laboger notifications@github.com wrote:

I've been doing this on a RHEL7.2 system.

—
Reply to this email directly or view it on GitHub
#13192 (comment).

[laboger]

These errors happen during the build of golang on ppc64 if I build with CGO_ENABLED=1.
They don't happen if CGO_ENABLED is not set.

Under ##### Building packages and commands for linux/ppc64.
.....
cmd/pprof
net/rpc/jsonrpc

cmd/trace

/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/net.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
/home/boger/golang/gitsrc/latest/go/pkg/linux_ppc64/runtime/cgo.a(_all.o): unknown relocation type 51; compiled without -fpic?
runtime/cgo(.opd): unexpected relocation type 307
runtime/cgo(.opd): unexpected relocation type 307
/home/boger/golang/gitsrc/latest/go/pkg/tool/linux_ppc64/link: too many errors

[mwhudson]

@laboger so you're saying that the patches from issue 11184 break building go on ppc64? that's bad, I'll make sure to fix that. I'm not promising to make external linking work on ppc64 though (which is what I thought this was about).

[laboger]

I'm sorry that I wasn't clear. I'm not saying your patches for 11184 break the build of ppc64. The above errors occur if I build on ppc64 and set CGO_ENABLED=1. The errors are the same or similar if I try to do the same build without your patches on ppc64.

You are correct, the point of this issue is to document the errors that I've seen with external linking on ppc64. I used your patches in my testing just because they are the latest changes I'm aware of for linking on Power. I understand you are not promising to make it work for ppc64, but I'm trying to understand how much is missing from external linking for ppc64.

[laboger]

This issue should not affect the merging of your patches from 11184. They work great and I know of no problem with them. They are for ppc64le, this issue is ppc64.

[ianlancetaylor]

Just to make sure we're all on the same page, these errors are from internal linking, which is what the toolchain does when building the tools. Presumably something has changed causing the C compiler to generate R_PPC64_TOC relocs. The code in cmd/link/internal/ppc64/asm.go needs to handle R_POWER_TOC, by resolving it to the value of symtoc. The code in cmd/link/internal/ld/ldelf.go needs to recognize R_PPC64_TOC, setting the size to 8.

[mwhudson]

FWIW, I'm not sure that cgo has ever worked on ppc64 BE (ping @aclements @minux), so it might not be C compiler changes to blame.

[minux]

I don't think cgo works on BE (I have an old patch set to enable external linking, but it needs major updating after Michael's changes.) Austin's internal linking work is for ppc64le. What's the version of gcc?

[laboger]

I have some old patches that Minux gave me back on May to make external linking work. There are only a few ppc64 BE specific changes that aren't in Michael's. I will try adding them and see if that helps. Perhaps something in his patches prevents the generation of R_PPC64_TOC.

My RHEL 7.2 ppc64 BE machine defaults to gcc 4.8.5.

[laboger]

Here are the CLs that I am aware of related to ppc64 BE linking which match the patches I had previously used from Minux:
https://go-review.googlesource.com/#/c/9677
change to obj.go no longer needed due to MHDs change
https://go-review.googlesource.com/#/c/9676
change to tls_ppc64x.s no longer needed due to MHDs change in TLS handling
I applied the other changes from this CL.
https://go-review.googlesource.com/#/c/9673
change to obj.go no longer needed

The file asm.go from 9677 and 9673 is a question because I see there are conflicting changes now with what's upstream.

I tried to add support to handle R_PPC64_TOC based on Ian's comments above but was unsure what to put in adddynrel for the R_PPC64_TOC case.

And then after making those changes, I see errors like this:

TOC-relative relocation in object without .TOC.

I don't see where a .TOC. object would get generated so it would be found when calling Linkrlookup.

I'm not sure what the next step is to make this work. I agree with previous comments that when CGO_ENABLED=1 on ppc64 BE this has probably never worked. I can see that gcc generates .opd sections with the R_PPC64_TOC relocation type and that happens even with older versions of gcc. I found this .opd section in some of the _all.o files that are generated during the golang build that are generating the error message about the missing relocation type.

[rsc]

We're not going to get to this for Go 1.6.