Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

gccgo from gcc 7

What operating system and processor architecture are you using (go env)?

Ubuntu 16.04 ppc64le

What did you do?

We have a build team attempting to build gccgo from the gcc-7 branch using upstream glibc which will be 2.26.

What did you expect to see?

Successful build.

What did you see instead?

Runtime errors when using the built gccgo. This happens when attempting to do:
go version

panic: runtime error: index out of range

Through debugging it was found that the problem occurs in
libgo/go/runtime/signal_unix.go:85
Inside function initseg is this loop:

 for i := uint32(0); i < _NSIG; i++ {
                t := &sigtable[i]        <----- error occurs here because len(sigtable) and _NSIG don't match
                if t.flags == 0 || t.flags&_SigDefault != 0 {
                        continue
                } 
                fwdSig[i] = getsig(i)

                if !sigInstallGoHandler(i) {
                        // Even if we are not installing a signal handler,
                        // set SA_ONSTACK if necessary.
                        if fwdSig[i] != _SIG_DFL && fwdSig[i] != _SIG_IGN {
                                setsigstack(i)
                        }
                        continue
                }

                t.flags |= _SigHandling
                setsig(i, getSigtramp())
        }

Further debugging shows that the sigtab.go file is generated incorrectly. The value of _NSIG ends up being 65 but there are only 34 entries put into the sigtable.

The reason for this is this line in mksigtab.sh:

nsig=grep 'const _*NSIG = [0-9]*$' gen-sysinfo.go | sed -e 's/.* = \([0-9]*\)/\1/'

But _NSIG in gen-sysinfo.go now looks like this so doesn't match:

const __NSIG = (___SIGRTMAX + 1)

The glibc patch that causes the issue is https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=8082d91e1c449e0cb137468b731004a5e605c8c6.

@ianlancetaylor If you have a workaround to suggest, that would be appreciated.