Description of problem:
vec_vmul{e,o}u{b,h} functions return incorrect results, because they compile down to instructions that differ in signedness, e.g. vec_vmuleub function compiles down to the vmulesb instruction. This also applies to generic vec_mule and vec_mulo functions when they are used with a pair of unsigned char or unsigned short arguments.


Version-Release number of selected component (if applicable):
gcc-7.0.1-0.10.fc26


How reproducible:
always


Steps to Reproduce:
1. $ cat reproducer.c
#include <limits.h>
#include <assert.h>
#include <altivec.h>

int main(void)
{
    vector unsigned char v0 = {1, 0, 0, 0, 0, 0, 0, 0};
    vector unsigned char v1 = {UCHAR_MAX, 0, 0, 0, 0, 0, 0, 0};

    vector unsigned short res = vec_vmuleub(v0, v1);

    assert(res[0] == (unsigned short)v0[0] * (unsigned short)v1[0]);
}

2. $ gcc -maltivec -mabi=altivec reproducer.c -o reproducer

3. $ ./reproducer 
reproducer: reproducer.c:12: main: Assertion `res[0] == (unsigned short)v0[0] * (unsigned short)v1[0]' failed.
Aborted (core dumped)


Actual results:
assertion fails


Expected results:
assertion succeeds


Additional info:
This affects build of libjpeg-turbo, making it generate broken images.