Speed up (+ 2 2) by a factor of 10
Improve arithmetic performance by avoiding bignums until needed. Also, simplify bignum memory management, fixing some unlikely leaks. This patch improved the performance of (+ 2 2) by a factor of ten on a simple microbenchmark computing (+ x 2), byte-compiled, with x a local variable initialized to 2 via means the byte compiler could not predict: performance improved from 135 to 13 ns. The platform was Fedora 28 x86-64, AMD Phenom II X4 910e. Performance also improved 0.6% on ‘make compile-always’. * src/bignum.c (init_bignum_once): New function. * src/emacs.c (main): Use it. * src/bignum.c (mpz): New global var. (make_integer_mpz): Rename from make_integer. All uses changed. * src/bignum.c (double_to_bignum, make_bignum_bits) (make_bignum, make_bigint, make_biguint, make_integer_mpz): * src/data.c (bignum_arith_driver, Frem, Flogcount, Fash) (expt_integer, Fadd1, Fsub1, Flognot): * src/floatfns.c (Fabs, rounding_driver, rounddiv_q): * src/fns.c (Fnthcdr): Use mpz rather than mpz_initting and mpz_clearing private temporaries. * src/bignum.h (bignum_integer): New function. * src/data.c (Frem, Fmod, Fash, expt_integer): * src/floatfns.c (rounding_driver): Use it to simplify code. * src/data.c (FIXNUMS_FIT_IN_LONG, free_mpz_value): Remove. All uses removed. (floating_point_op): New function. (floatop_arith_driver): New function, with much of the guts of the old float_arith_driver. (float_arith_driver): Use it. (floatop_arith_driver, arith_driver): Simplify by assuming NARGS is at least 2. All callers changed. (float_arith_driver): New arg, containing the partly converted value of the next arg. Reorder args for consistency. All uses changed. (bignum_arith_driver): New function. (arith_driver): Use it. Do fixnum-only integer calculations in intmax_t instead of mpz_t, when they fit. Break out mpz_t calculations into bignum_arith_driver. (Fquo): Use floatop_arith_driver instead of float_arith_driver, since the op is known to be valid. (Flogcount, Fash): Simplify by coalescing bignum and fixnum code. (Fadd1, Fsub1): Simplify by using make_int.
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