ppc64le: possible SIMD issue casting double to int

Host environment

• Operating system: Debian 11 "bullseye"
• OS/kernel version: 5.15.0-0.bpo.2-amd64
• Architecture: x86_64
• QEMU flavor: qemu-system-ppc64le
• QEMU version: 6.1.0
• QEMU command line:

qemu-system-ppc64le test.img

Emulated/Virtualized environment

• Operating system: Debian unstable "sid"
• OS/kernel version: 5.15.0-3-powerpc64le
• Architecture: ppc64le

Description of problem

Working with numpy in a ppc64le VM, I ran into a strange double -to casting issue, specifically when casting an array of 1.0 values to 1 values. The numpy folks guided me to a small reproducible test case.

The attached convert.c creates double and int arrays of length 1 <= n <= 16. The double array is filled with the value 1.0, and both arrays are passed to a function that converts the value.

With -O2, output is as expected (truncated here):

i =  1:   1
i =  2:   1 1
i =  3:   1 1 1
i =  4:   1 1 1 1
i =  5:   1 1 1 1 1
i =  6:   1 1 1 1 1 1

With -O3, all values that fit into blocks of four become zero:

i =  1:   1
i =  2:   1 1
i =  3:   1 1 1
i =  4:   0 0 0 0
i =  5:   0 0 0 0 1
i =  6:   0 0 0 0 1 1

I tested this with executables compiled on a physical ppc64le host, where the issue is not reproducible.

Steps to reproduce

1. gcc -O2 -o convert convert.c && ./convert
2. gcc -O3 -o convert convert.c && ./convert