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. 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.

2. 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.

3. 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.

4. 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.

5. 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).