Add threshold for fft_small which is set during configuration

[albinahlback]

Also use preprocessor instead of compiler in the configuration process to speed things up.

Solves #1790 and #1789.

On Skylake, I get the following timings on Skylake with fft_small/profile/p-fft_small_vs_gmp.c:

mpn_mul_n vs flint_mpn_mul_n
n =  900: 1.06
n =  936: 0.90
n =  973: 0.92
n = 1011: 0.87
n = 1051: 0.90
n = 1093: 0.93
n = 1136: 0.95
n = 1181: 1.04
n = 1228: 1.05
n = 1277: 1.07
n = 1328: 1.09
n = 1381: 0.81
n = 1436: 0.84
n = 1493: 0.86
n = 1552: 1.18
n = 1614: 1.22
n = 1678: 1.01
n = 1745: 1.28
n = 1814: 1.33
n = 1886: 1.01
n = 1961: 1.06
n = 2039: 1.04
n = 2120: 1.07
n = 2204: 1.14
n = 2292: 1.16
n = 2383: 1.38
n = 2478: 1.39
n = 2577: 1.41
n = 2680: 1.44
n = 2787: 1.03
n = 2898: 1.08
n = 3013: 1.08
n = 3133: 1.57
n = 3258: 1.60
n = 3388: 1.63
n = 3523: 1.64
n = 3663: 1.71
n = 3809: 1.26
n = 3961: 1.58

And zooming in on the range 1500-1600, we see something wierd:

mpn_mul_n vs flint_mpn_mul_n

n = 1500: 0.86
n = 1505: 0.90
n = 1510: 0.89
n = 1515: 0.89
n = 1520: 0.89
n = 1525: 0.89
n = 1530: 0.89
n = 1535: 0.89
n = 1540: 1.17
n = 1545: 1.19
n = 1550: 1.19
n = 1555: 1.20
n = 1560: 1.20
n = 1565: 1.20
n = 1570: 1.22
n = 1575: 1.22
n = 1580: 1.21
n = 1585: 1.22
n = 1590: 1.22
n = 1595: 1.22
n = 1600: 1.22

Hence, I set the threshold for CPUs with fast vroundpd to 400, and for slow to 1540.

[fredrik-johansson]

Please make sure that make profile works on machines that don't have this function.

[albinahlback]

Thanks for observing this.

[fredrik-johansson]

Is the squaring threshold really 2x on Intel too?

[albinahlback]

Unfortunately it is exactly 2x on my machine... We should really try to look into if we can improve this.

[albinahlback]

mpn_sqr vs fft_small

n = 1400: 0.69
n = 1470: 0.71
n = 1543: 1.17
n = 1620: 1.20
n = 1701: 0.90
n = 1786: 1.26
n = 1875: 0.89
n = 1968: 0.92
n = 2066: 0.93
n = 2169: 0.97
n = 2277: 1.01
n = 2390: 1.29
n = 2509: 1.31
n = 2634: 1.33
n = 2765: 0.86
n = 2903: 0.91
n = 3048: 0.95
n = 3200: 1.56
n = 3360: 1.60
n = 3528: 1.66
n = 3704: 1.72
n = 3889: 1.20

And zooming in:

mpn_sqr vs fft_small

n = 2900: 0.89
n = 2910: 0.90
n = 2920: 0.90
n = 2930: 0.90
n = 2940: 0.90
n = 2950: 0.93
n = 2960: 0.91
n = 2970: 0.91
n = 2980: 0.92
n = 2990: 0.91
n = 3000: 0.93
n = 3010: 0.91
n = 3020: 0.91
n = 3030: 0.91
n = 3040: 0.93
n = 3050: 0.95
n = 3060: 0.92
n = 3070: 0.89
n = 3080: 1.01
n = 3090: 1.42
n = 3100: 1.45
n = 3110: 1.44
n = 3120: 1.45
n = 3130: 1.46
n = 3140: 1.45
n = 3150: 1.45
n = 3160: 1.46
n = 3170: 1.47
n = 3180: 1.47
n = 3190: 1.47
n = 3200: 1.47

[albinahlback]

One could argue that the peaks in performance surrounding these valleys are worth it, but I'd rather have smooth performance. Either way, this should be fixed.