I think I understand this issue now.
In the various toom functions, we suppress tests for recursive calls
which cannot happen when each function is invoked for the intended
range. These things are controlled by the relative TOOM threshold.
This makes tune/speed measurements look bad, but perform
GMP 4.3:
shell$ ./speed -p1 -s100-1 -f10 mpn_toom3_mul_n
overhead 0.2 secs, precision 1 units of 3.13e-10 secs, CPU
freq 3200.00 MHz
mpn_toom3_mul_n
100 0.05181
1000 0.000169392
1 0.005313959
100.159352000
GMP repo:
shell$ ./speed -p