GCC 6.3.1 doesn't autovectorize without -ffinite-math-only - gcc

Question

I would like to understand why GCC does not autovectorize the following loop, unless I pass the -ffinite-math-only. As to my understanding and the GCC manual the optimization requires the -funsafe-math-optimizations
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This is because NEON
hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.
In particular, the flag enables the compiler to assume associative math, so that it can first accumulate with 4 partial sums. The code seems pretty straight forward
template<typename SumType = double>
class UipLineResult {
public:
SumType sqsum;
SumType dcsum;
float pkp;
float pkn;
public:
UipLineResult() {
clear();
}
void clear() {
sqsum = 0;
dcsum = 0;
pkp = -std::numeric_limits<float>::max();
pkn = +std::numeric_limits<float>::max();
}
};
Loop that is not vectorized
static void addSamplesLine(const float* ss, UipLineResult<>* line) {
UipLineResult<float> intermediate;
for(int idx = 0; idx < 120; idx++) {
float s = ss[idx];
intermediate.sqsum += s * s;
intermediate.dcsum += s;
intermediate.pkp = intermediate.pkp < s ? s : intermediate.pkp;
intermediate.pkn = intermediate.pkn > s ? s : intermediate.pkn;
}
line->addIntermediate(&intermediate);
}
For example, the squared addition look like
intermediate.sqsum += s * s;
107da: ee47 6aa7 vmla.f32 s13, s15, s15
With -ffinite-math-only this becomes
intermediate.sqsum += s * s;
1054c: ef40 6df0 vmla.f32 q11, q8, q8
Compiler flags
-funsafe-math-optimizations -ffinite-math-only -mcpu=cortex-a9 -mfpu=neon