x86: partial register stall in cvtsi2sd in big loops but not in small loops

I'm testing floating point code generation of some compiler. The code in question calculates a simple sum:

    double sum = 0.0;
    for (int i = 1; i <= n; i++) {
        sum += 1.0 / i;
    }

I get two versions of this code, a simple one:

L017:   cvtsi2sd xmm1, eax
        add     eax, byte 1
        movsd   xmm2, qword [rel L007]
        divsd   xmm2, xmm1
        addsd   xmm0, xmm2
        cmp     edx, eax 
        jge     short L017

and one with the loop unrolled:

L017:   cvtsi2sd xmm1, ecx
        movsd   xmm2, qword [rel L008]
        divsd   xmm2, xmm1
        addsd   xmm2, xmm0
        lea     edx, [rcx+1H]
        add     ecx, byte 2
        cvtsi2sd xmm0, edx
        movsd   xmm1, qword [rel L008]
        divsd   xmm1, xmm0
        addsd   xmm2, xmm1
        movapd  xmm0, xmm2
        cmp     eax, ecx
        jnz     short L017

The unrolled one runs much longer then the simple one: 4s vs 1.6 s when n=1000000000. The problem is resolved by clearing the register before cvtsi2sd :

L017:   pxor xmm1, xmm1
        cvtsi2sd xmm1, ecx
        movsd   xmm2, qword [rel L008]
...

The new unrolled version runs as fast as the simple one. OK, this is a partial register stall, although I'd never expected it to be so bad. There are however two questions:

1. Why does it not affect the simple version? It looks like tight loops do not suffer from this stall.

2. What makes cvtsi2sd so much different from other scalar FP instructions, such as addsd? They also affect only part of an xmm register (in non-VEX mode, and this is the mode used).

I observed this on three processors:

• i7-6820HQ (Skylake)
• i7-8665U (Whiskey Lake)
• Xeon Gold 5120 (Skylake)