I suppose the compiler may be able to replace your first loop by
value=sinf(a[9999]);
or may do nothing there, since you don't use value.

You could check (e.g. by saving .asm) to see whether that loop produces an svml library call or a single evaluation, if even that.

Hi!

Compiler actually does eliminate "dead code" of sinf loop, because sinf results are used nowhere.
Look at the generated asm:

=============================================================
call DWORD PTR __imp__QueryPerformanceCounter@4

.B1.6:
lea eax, DWORD PTR [esp+16]
push eax
call DWORD PTR __imp__QueryPerformanceCounter@4

.B1.7:
lea eax, DWORD PTR [esp+24]
push eax
call DWORD PTR __imp__QueryPerformanceCounter@4

.B1.8:
lea edx, DWORD PTR [esp+40]
lea eax, DWORD PTR [esp+40040]
push eax
push edx
push 10000
call _vsSin

.B1.17:
add esp, 12

.B1.9:
lea eax, DWORD PTR [esp+32]
push eax
call DWORD PTR __imp__QueryPerformanceCounter@4
=============================================================

As one can see there is no sinf loop between first two QueryPerformanceCounter calls.
To avoid such situation in future use one of two (or combination) methods:
1) compile your timing routine with optimization disabled - /Od compiler switch
2) emulate timed function results usage. For example, just print sinf values like:

======================================================
QueryPerformanceCounter((LARGE_INTEGER*)&time1);
for (int i=0;i b[i]=sinf(a[i]);
QueryPerformanceCounter((LARGE_INTEGER*)&time2);

QueryPerformanceCounter((LARGE_INTEGER*)&time3);
vsSin(n,a,b);
QueryPerformanceCounter((LARGE_INTEGER*)&time4);

for(i=0; i < n; i++)
{
printf("%f ", b[i]);
}
======================================================

By the way your timing results almost agree with actual VML performance (see vml notes).

Another one hint for accuracte timing - repeat your timing procedure several times (10-20).
And take the best result of them.

======================================================
besttime = INT_MAX;
curtime = 0;

for(int repeat = 0; repeat < 15; repeat++)
{
QueryPerformanceCounter((LARGE_INTEGER*)&time3);
vsSin(n,a,b);
QueryPerformanceCounter((LARGE_INTEGER*)&time4);
curtime = time4 - time3;
if(curtime < besttime)
besttime = curtime;
}

printf("time: %d
",besttime);
======================================================

This hint will help you to avoid two issues - "cold cach
e" effect and operation system impact to performance measuring.

The best regards and good luck!

Andrey K.