Curious slowdown in Toom-3

[Torbjorn Granlund]

GMP 4.3:

shell$ ./speed -p10000 -s100-100000000 -f10 mpn_toom3_mul_n
overhead 0.000000002 secs, precision 10000 units of 3.13e-10 secs, CPU
freq 3200.00 MHz
        mpn_toom3_mul_n
100       0.000005181
1000      0.000169392
10000     0.005313959
100000    0.159352000

GMP repo:

shell$ ./speed -p10000 -s100-100000000 -f10 mpn_toom33_mul
overhead 0.000000002 secs, precision 10000 units of 3.13e-10 secs, CPU
freq 3200.00 MHz
        mpn_toom33_mul
100       0.000005110
1000      0.000191381
10000     0.007476515
100000    0.287884000

(GMP 5.0, 5.1, and the repo perform similarly.)

-- 
Torbjörn