What the heck?

gistfile1.txt

➜  test  clang -O2 -o pow2 pow2.c
➜  test  ./pow2 36000000
1000000 => old=0.015580; new=0.008266 [new faster by 0.007314]
1500000 => old=0.018460; new=0.011828 [new faster by 0.006632]
2000000 => old=0.027408; new=0.016054 [new faster by 0.011354]
2500000 => old=0.027412; new=0.024791 [new faster by 0.002621]
3000000 => old=0.036629; new=0.024436 [new faster by 0.012193]
3500000 => old=0.048103; new=0.032976 [new faster by 0.015127]
4000000 => old=0.049787; new=0.037055 [new faster by 0.012732]
4500000 => old=0.065063; new=0.044097 [new faster by 0.020966]
5000000 => old=0.065990; new=0.050475 [new faster by 0.015515]
5500000 => old=0.080379; new=0.054399 [new faster by 0.025980]
6000000 => old=0.082577; new=0.061412 [new faster by 0.021165]
6500000 => old=0.098540; new=0.066543 [new faster by 0.031997]
7000000 => old=0.107789; new=0.063299 [new faster by 0.044490]
7500000 => old=0.096774; new=0.075163 [new faster by 0.021611]
8000000 => old=0.108336; new=0.074936 [new faster by 0.033400]
8500000 => old=0.117378; new=0.076655 [new faster by 0.040723]
9000000 => old=0.128703; new=0.076933 [new faster by 0.051770]
9500000 => old=0.122862; new=0.101043 [new faster by 0.021819]
10000000 => old=0.157738; new=0.107936 [new faster by 0.049802]
10500000 => old=0.152995; new=0.115077 [new faster by 0.037918]
11000000 => old=0.144275; new=0.109170 [new faster by 0.035105]
11500000 => old=0.150648; new=0.115991 [new faster by 0.034657]
12000000 => old=0.149821; new=0.120618 [new faster by 0.029203]
12500000 => old=0.160725; new=0.129542 [new faster by 0.031183]
13000000 => old=0.172176; new=0.127184 [new faster by 0.044992]
13500000 => old=0.189031; new=0.133414 [new faster by 0.055617]
14000000 => old=0.180719; new=0.126753 [new faster by 0.053966]
14500000 => old=0.154266; new=0.146342 [new faster by 0.007924]
15000000 => old=0.181697; new=0.153723 [new faster by 0.027974]
15500000 => old=0.199289; new=0.151586 [new faster by 0.047703]
16000000 => old=0.213118; new=0.155925 [new faster by 0.057193]
16500000 => old=0.195801; new=0.156066 [new faster by 0.039735]
17000000 => old=0.198415; new=0.158540 [new faster by 0.039875]
17500000 => old=0.237873; new=0.169121 [new faster by 0.068752]
18000000 => old=0.226076; new=0.179466 [new faster by 0.046610]
18500000 => old=0.247722; new=0.184660 [new faster by 0.063062]
19000000 => old=0.265786; new=0.203819 [new faster by 0.061967]
19500000 => old=0.249065; new=0.195782 [new faster by 0.053283]
20000000 => old=0.259865; new=0.203469 [new faster by 0.056396]
20500000 => old=0.271278; new=0.207270 [new faster by 0.064008]
21000000 => old=0.278296; new=0.204981 [new faster by 0.073315]
21500000 => old=0.301317; new=0.225279 [new faster by 0.076038]
22000000 => old=0.296840; new=0.263050 [new faster by 0.033790]
22500000 => old=0.305177; new=0.234838 [new faster by 0.070339]
23000000 => old=0.318652; new=0.242081 [new faster by 0.076571]
23500000 => old=0.334581; new=0.253680 [new faster by 0.080901]
24000000 => old=0.343642; new=0.252057 [new faster by 0.091585]
24500000 => old=0.322568; new=0.245789 [new faster by 0.076779]
25000000 => old=0.330123; new=0.240147 [new faster by 0.089976]
25500000 => old=0.342316; new=0.276150 [new faster by 0.066166]
26000000 => old=0.360430; new=0.271383 [new faster by 0.089047]
26500000 => old=0.388001; new=0.285873 [new faster by 0.102128]
27000000 => old=0.377464; new=0.274242 [new faster by 0.103222]
27500000 => old=0.399054; new=0.324653 [new faster by 0.074401]
28000000 => old=0.414425; new=0.308021 [new faster by 0.106404]
28500000 => old=0.449715; new=0.323772 [new faster by 0.125943]
29000000 => old=0.447367; new=0.334729 [new faster by 0.112638]
29500000 => old=0.453269; new=0.333012 [new faster by 0.120257]
30000000 => old=0.466704; new=0.347743 [new faster by 0.118961]
30500000 => old=0.474047; new=0.355513 [new faster by 0.118534]
31000000 => old=0.426458; new=0.333172 [new faster by 0.093286]
31500000 => old=0.523885; new=0.357293 [new faster by 0.166592]
32000000 => old=0.450361; new=0.345432 [new faster by 0.104929]
32500000 => old=0.488458; new=0.352029 [new faster by 0.136429]
33000000 => old=0.451655; new=0.332754 [new faster by 0.118901]
33500000 => old=0.467955; new=0.373742 [new faster by 0.094213]
34000000 => old=0.503356; new=0.357131 [new faster by 0.146225]
34500000 => old=0.507329; new=0.356231 [new faster by 0.151098]
35000000 => old=0.474974; new=0.389110 [new faster by 0.085864]
35500000 => old=0.516450; new=0.365703 [new faster by 0.150747]
36000000 => old=0.463270; new=0.360197 [new faster by 0.103073]