expansion: Improve double-word modulo by certain constant divisors [PR97459]

As discussed in the PR, e.g. on x86_64 (both -m32 and -m64) there is no
double-word modulo and so we expand it to a __{,u}mod[dt]i3 call.
For certain constant divisors we can do better.  E.g. consider
32-bit word-size, 0x100000000ULL % 3 == 1, so we can use partly the Hacker's
delight modulo by summing digits approach and optimize
unsigned long long foo (unsigned long long x) { return x % 3; }
as
unsigned long long foo (unsigned long long x) {
  unsigned int sum, carry;
  carry = __builtin_add_overflow ((unsigned int) x, (unsigned int) (x >> 32), &sum);
  sum += carry;
  return sum % 3;
}
Similarly, 0x10000000ULL % 5 == 1 (note, 1 << 28), so
unsigned long long bar (unsigned long long x) { return x % 5; }
as
unsigned long long bar (unsigned long long x) {
  unsigned int sum = x & ((1 << 28) - 1);
  sum += (x >> 28) & ((1 << 28) - 1);
  sum += (x >> 56);
  return sum % 5;
}
etc.
And we can do also signed modulo,
long long baz (long long x) { return x % 5; }
as
long long baz (long long x) {
  unsigned int sum = x & ((1 << 28) - 1);
  sum += ((unsigned long long) x >> 28) & ((1 << 28) - 1);
  sum += ((unsigned long long) x >> 56);
  /* Sum adjustment for negative x.  */
  sum += (x >> 63) & 3;
  unsigned int rem = sum % 5;
  /* And finally adjust it to the right interval for negative values.  */
  return (int) (rem + ((x >> 63) & -4));
}

2020-11-30  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/97459
	* internal-fn.h (expand_addsub_overflow): Declare.
	* internal-fn.c (expand_addsub_overflow): No longer static.
	* optabs.c (expand_doubleword_mod): New function.
	(expand_binop): Optimize double-word mod with constant divisor.

	* gcc.dg/pr97459-1.c: New test.
	* gcc.dg/pr97459-2.c: New test.

0 files changed, 0 insertions, 0 deletions