cmd/compile: incomplete relocation support on ppc64le

[nehaljwani]

Since 2c4f389c I am unable to compile go with CGO_ENABLED=1 on ppc64le with GCC 10.3.0 and binutils 2.36.1

I see errors like:

/path/to/go/pkg/linux_ppc64le/a/b/c/d.a(_x???.o): unknown relocation type <>; compiled without -fpic?

Where <> are numbers: 31, 60, 119, 120. I am not sure if I am reading this correctly, but these correspond to:
R_PPC_PLT16_HA, R_PPC64_PLT16_LO_DS, R_POWERPC_PLTSEQ, R_POWERPC_PLTCALL and references to these are missing in:

src/cmd/link/internal/loadelf/ldelf.go
src/cmd/link/internal/ppc64/asm.go
src/debug/elf/elf.go

[mengzhuo]

@golang/ppc64 @laboger

[pmur]

Hi @nehaljwani, thank you for reporting. Would you be able to share the gcc flags used to compile the C code? A simplified test case would help me. I suspect it just needs to make libc call with the compiler options used on the project.

I am still digging through the linked CI logs trying to understand how the linked utility is being compiled.

A trivial workaround is to explicitly request external linking on ppc64le. This was the behavior prior 1.18 with any cgo usage. I will try to implement these relocs. They are fairly new (introduced with ELFv2 1.5), but I suspect we need to handle with newer GCC.

[pmur]

@nehaljwani is one of the CFLAG options -fno-plt, or similar option to optimize library calls in C/C++? I can reproduce a similar error using this option.

[nehaljwani]

Hello @pmur, we use:

CGO_CFLAGS=-ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O3 -pipe

[gopherbot]

Change https://go.dev/cl/412715 mentions this issue: cmd/link: optimize PPC64 inline plt sequences if local

[gopherbot]

Change https://go.dev/cl/412714 mentions this issue: debug/elf, cmd/link: support -fno-plt compiled gcc objects on ppc64le