Comment 0 for bug 1883984

In porting software to guest Ubuntu 18.04 and 20.04 VMs for S/390x, I discovered
that some of my own numerical programs, and also a GNU configure script for at
least one package with CC=clang, would cause an instant crash of the VM, sometimes
also destroying recently opened files, and producing long strings of NUL characters
in /var/log/syslog in the S/390 guest O/S.

Further detective work narrowed the cause of the crash down to a single IBM S/390
instruction: sqxbr (128-bit IEEE 754 square root). Here is a one-line program
that when compiled and run on a VM hosted on QEMUcc emulator version 4.2.0
(Debian 1:4.2-3ubuntu6.1) [hosted on Ubuntu 20.04 on a Dell Precision 7920
workstation with an Intel Xeon Platinum 8253 CPU], and also on QEMU emulator
version 5.0.0, reproducibly produces a VM crash under qemu-system-s390x.

% cat bug-sqrtl-one-line.c
int main(void) { volatile long double x, r; x = 4.0L; __asm__ __volatile__("sqxbr %0, %1" : "=f" (r) : "f" (x)); return (0);}

% cc bug-sqrtl-one-line.c && ./a.out
Segmentation fault (core dumped)

The problem code may be the function float128_sqrt() defined in qemu-5.0.0/fpu/softfloat.c
starting at line 7619. I have NOT attempted to run the qemu-system-s390x executable
under a debugger. However, I observe that S/390 is the only CPU family that I know of,
except possibly for a Fujitsu SPARC-64, that has a 128-bit square root in hardware.
Thus, this instruction bug may not have been seen before.