micro-optimization: input vector for poly::get_variables()

[tueda]

poly::get_variables() does not need a temporary copy of the input vector containing pointers to function arguments. Passing it by reference rather than by value avoids a heap allocation and a memory copy.

[coveralls]

coverage: 49.988% (+0.002%) from 49.986%
when pulling 506e5d9 on tueda:improve-get_variables
into 61ecfdd on vermaseren:master.

[tueda]

I've reserved capacity for vectors passed to poly::get_variables() when they are known to have a fixed size. Without reserving capacity, for example,

vector<WORD *> e;
e.push_back(a);
e.push_back(b);
poly::get_variables(BHEAD e, false, false);

calls new and delete twice each, because typically the capacity grows as 0, 1, 2, 4, ..., so a reallocation is needed.

If we can use C++11, then this could be rewritten with std::array and by templatizing:

    template <typename Container>
    static void get_variables (PHEAD const Container &, bool, bool);

(Optionally, with_arghead and bool sort_vars can be converted to template parameters, which allows more compiler optimization.)

[jodavies]

Could you measure any improvement from these in poly benches? #297 could go in with this also in principle?

[tueda]

The improvement in speed is negligible. So, practically, the difference is only in writing code in a more proper style.

  Revision: v5.0.0-beta.1-71-g61ecfdd (master)
  Command being timed: "/home/tueda/work/form/build/sources/form ratfuntest.frm"
  n = 10                                            Mean (          SD)            Min            Max
  User time (seconds):                            14.139 (       0.028)          14.08          14.17
  System time (seconds):                           0.508 (       0.021)           0.48           0.55

  Revision: v5.0.0-beta.1-71-g61ecfdd (master)
  Command being timed: "/home/tueda/work/form/build/sources/form test-mbox1l.frm"
  n = 10                                            Mean (          SD)            Min            Max
  User time (seconds):                            42.063 (       0.018)          42.04           42.1
  System time (seconds):                           0.030 (       0.011)           0.01           0.05

vs.

  Revision: v5.0.0-beta.1-73-g506e5d9 (improve-get_variables)
  Command being timed: "/home/tueda/work/form/build/sources/form ratfuntest.frm"
  n = 10                                            Mean (          SD)            Min            Max
  User time (seconds):                            14.170 (       0.028)          14.13          14.21
  System time (seconds):                           0.513 (       0.025)           0.47           0.56

  Revision: v5.0.0-beta.1-73-g506e5d9 (improve-get_variables)
  Command being timed: "/home/tueda/work/form/build/sources/form test-mbox1l.frm"
  n = 10                                            Mean (          SD)            Min            Max
  User time (seconds):                            42.142 (       0.021)          42.11          42.17
  System time (seconds):                           0.024 (       0.010)           0.01           0.04