Bug 37152

It's not hard to reproduce the problem with a somewhat smaller example. Here is my take:

$ cat var.c

int helper_var = 42;

$ cat func.c

extern int helper_var;

int helper_func() {
    return helper_var;
}

$ gcc -flto -fPIC -O2 -g -c -o func.o func.c
$ gcc -flto -fPIC -O2 -g -c -o var.o var.c
$ gcc -flto -shared -o libhelper-lto.so func.o var.o
collect2: fatal error: ld terminated with signal 11 [Segmentation fault], core dumped
compilation terminated.
/usr/bin/ld.default: /tmp/.private/iv/ccGL82XI.ltrans0.ltrans.o: relocation R_MIPS_HI16 against `__gnu_local_gp' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld.default: BFD (GNU Binutils) 2.31.1.20181202 assertion fail elfxx-mips.c:3566

Adding -fPIC or removing -flto to the last commend helps and produces a working shared library.

With this example at hand, I was able to determine that the relocation in question is generated because -mno-shared is passed to collect2:

$ gcc -v -flto -shared -o libhelper-lto.so func.o var.o                                                                                                                                                                   
[... skipping the boring part ...]
COLLECT_GCC_OPTIONS='-v' '-flto' '-shared' '-o' 'libhelper-lto.so' '-march=mips32r2' '-mfpxx' '-mllsc' '-mno-lxc1-sxc1' '-mno-madd4' '-mips32r2' '-mno-shared' '-EL' '-mabi=32'
 /usr/libexec/gcc/mipsel-alt-linux/8/collect2 -plugin /usr/libexec/gcc/mipsel-alt-linux/8/liblto_plugin.so -plugin-opt=/usr/libexec/gcc/mipsel-alt-linux/8/lto-wrapper -plugin-opt=-fresolution=/tmp/.private/iv/ccgFMx4f.res -plugin-opt=-pass-
through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -flto --build-id --no-add-needed --eh-frame-hdr --as-needed -EL -mips32r2 -shared -melf32ltsmip 
-o libhelper-lto.so /usr/lib/gcc/mipsel-alt-linux/8/../../../../lib/crti.o /usr/lib/gcc/mipsel-alt-linux/8/crtbeginS.o -L/usr/lib/gcc/mipsel-alt-linux/8 -L/usr/lib/gcc/mipsel-alt-linux/8/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/us
r/lib/gcc/mipsel-alt-linux/8/../../.. func.o var.o -lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc --push-state --as-needed -lgcc_s --pop-state /usr/lib/gcc/mipsel-alt-linux/8/crtendS.o /usr/lib/gcc/mipsel-alt-linux/8/../../../
../lib/crtn.o
collect2: fatal error: ld terminated with signal 11 [Segmentation fault], core dumped
compilation terminated.
[...]

'-mno-shared'  is a mipsel-specific option that enables certain optimizations which are not valid for postion-independent code of any kind. It is added by the gcc driver as a part of DRIVER_SPECS (see NO_SHARED_SPECS) here:

http://git.altlinux.org/gears/g/gcc8.git?a=blob;f=gcc/gcc/config/mips/gnu-user.h;h=22097e285bf2645d0b96877fa1de17831b8cfde1#l103

I'm not entirely convinced that this should not be addressed on the other side -- that is, by adding the required options to the linking commend. man gcc(1) for both -shared an -pie says:

> For predictable results, you must also specify the same set
> of options used for compilation (-fpie, -fPIE, or model suboptions)
> when you specify this linker option.

But I think that consistency across platforms is also valuable, so I'm suggesting the following patch:

http://git.altlinux.org/people/iv/packages/gcc8.git?a=blob;f=alt/alt-mips-shared-implies-mshared.patch;h=e853da1ee7c47037cf27a0cc60a25f949ac3ae1b

Here is the full change:

http://git.altlinux.org/people/iv/packages/gcc8.git?a=commitdiff;h=ce841dcf74343fd634b4308c2de7c8ea3a81024e

With this change, I can't reproduce the problem anymore, and gcc apparently build working systemd and python3 with LTO enabled (task #33910 on gyle-secondary).

Since the change entirely mips-specific, I think we can safely add it to gcc8 in Sisyphus. I can create a task. WDYT?