Kasperd asked in a comment of the accepted answer:
The Java and C examples use quite different register names. Are both example using the AMD64 ISA?
xor edx, edx
xor eax, eax
.L2:
mov ecx, edx
imul ecx, edx
add edx, 1
lea eax, [rax+rcx*2]
cmp edx, 1000000000
jne .L2
I don't have enough reputation to answer this in the comments, but these are the same ISA. It's worth pointing out that the GCC version uses 32-bit integer logic and the JVM compiled version uses 64-bit integer logic internally.
R8 to R15 are just new X86_64 registers. EAX to EDX are the lower parts of the RAX to RDX general purpose registers. The important part in the answer is that the GCC version is not unrolled. It simply executes one round of the loop per actual machine code loop. While the JVM version has 16 rounds of the loop in one physical loop (based on rustyx answer, I did not reinterpret the assembly). This is one of the reasons why there are more registers being used since the loop body is actually 16 times longer.