A Tour of NTL: Tips for Getting the Best Performance out of NTL

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A Tour of NTL: Tips for Getting the Best Performance out of NTL

1. Build NTL using GMP as the long integer package. This is extremely important, as the GMP implementation of long integer arithmetic is much faster than the default implementation. Go here for details.

2. On many machines that optionally offer 64-bit integer arithmetic (recent Mac OSX machines, for instance), you should compile using `gcc` with the option `-m64` to get the full benefit. To do this, pass `"CFLAGS=-O2 -m64"` to the `configure` script (note the use of quotes). If you are using NTL with GMP on such a machine, you must do this to get compatible code. Note, however, that 64-bit is becoming the default, so this may not be necessary.

3. On Sparcs, pass the argument `"CFLAGS=-O2 -mcpu=v8"` to the `configure` script. On more recent, 64-bit sparcs, pass `"CFLAGS=-O2 -mcpu=v9 -m64"` to get the full instruction set and 64-bit code.

4. Make sure you run the configuration wizard when you install NTL. This is the default behaviour in the makefile in the Unix distribution, so don't change this; in the Windows distribution, there is unfortunately no easy way to run the wizard.

5. In time-critical code, avoid creating unnecessary temporary objects. For example, instead of

   ZZ InnerProduct(const ZZ *a, const ZZ *b, long n) { long i; ZZ res; for (i = 0; i < n; i++) res += a[i] * b[i]; return res; }

   write this as

   ZZ InnerProduct(const ZZ *a, const ZZ *b, long n) { long i; ZZ res, t; for (i = 0; i < n; i++) { mul(t, a[i], b[i]); add(res, res, t); } return res; }

   The first version of `InnerProduct` creates and destroys a temporary object, holding the value `a[i]*b[i]`, in every loop iteration. The second does not.

6. If you use the class `ZZ_p`, try to avoid switching the modulus too often, as this can be a rather expensive operation. If you must switch the modulus often, use the class `ZZ_pContext` to save the information associated with the modulus (see ZZ_p.txt).