On Wed, Sep 02, 2015 at 01:34:48PM +0100, Wilco Dijkstra wrote:
> This patch reimplements aarch64_bitmask_imm using bitwise arithmetic rather
> than a slow binary search. The algorithm searches for a sequence of set bits.
> If there are no more set bits and not all bits are set, it is a valid mask.
> Otherwise it determines the distance to the next set bit and checks the mask
> is repeated across the full 64 bits. Native performance is 5-6x faster on
> typical queries.
>
> No change in generated code, passes GCC regression/bootstrap.
OK.
Thanks,
James
> ChangeLog:
> 2015-09-02 Wilco Dijkstra <wdijk...@arm.com>
>
> * gcc/config/aarch64/aarch64.c (aarch64_bitmask_imm):
> Reimplement using faster algorithm.
>
> ---
> gcc/config/aarch64/aarch64.c | 62
> +++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 53 insertions(+), 9 deletions(-)
>
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index c666dce..ba1b77e 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -3301,19 +3301,63 @@ aarch64_movw_imm (HOST_WIDE_INT val, machine_mode
> mode)
> || (val & (((HOST_WIDE_INT) 0xffff) << 16)) == val);
> }
>
> +/* Multipliers for repeating bitmasks of width 32, 16, 8, 4, and 2. */
> +
> +static const unsigned HOST_WIDE_INT bitmask_imm_mul[] =
> + {
> + 0x0000000100000001ull,
> + 0x0001000100010001ull,
> + 0x0101010101010101ull,
> + 0x1111111111111111ull,
> + 0x5555555555555555ull,
> + };
> +
>
> /* Return true if val is a valid bitmask immediate. */
> +
> bool
> -aarch64_bitmask_imm (HOST_WIDE_INT val, machine_mode mode)
> +aarch64_bitmask_imm (HOST_WIDE_INT val_in, machine_mode mode)
> {
> - if (GET_MODE_SIZE (mode) < 8)
> - {
> - /* Replicate bit pattern. */
> - val &= (HOST_WIDE_INT) 0xffffffff;
> - val |= val << 32;
> - }
> - return bsearch (&val, aarch64_bitmasks, AARCH64_NUM_BITMASKS,
> - sizeof (aarch64_bitmasks[0]), aarch64_bitmasks_cmp) != NULL;
> + unsigned HOST_WIDE_INT val, tmp, mask, first_one, next_one;
> + int bits;
> +
> + /* Check for a single sequence of one bits and return quickly if so.
> + The special cases of all ones and all zeroes returns false. */
> + val = (unsigned HOST_WIDE_INT) val_in;
> + tmp = val + (val & -val);
> +
> + if (tmp == (tmp & -tmp))
> + return (val + 1) > 1;
> +
> + /* Replicate 32-bit immediates so we can treat them as 64-bit. */
> + if (mode == SImode)
> + val = (val << 32) | (val & 0xffffffff);
> +
> + /* Invert if the immediate doesn't start with a zero bit - this means we
> + only need to search for sequences of one bits. */
> + if (val & 1)
> + val = ~val;
> +
> + /* Find the first set bit and set tmp to val with the first sequence of one
> + bits removed. Return success if there is a single sequence of ones. */
> + first_one = val & -val;
> + tmp = val & (val + first_one);
> +
> + if (tmp == 0)
> + return true;
> +
> + /* Find the next set bit and compute the difference in bit position. */
> + next_one = tmp & -tmp;
> + bits = clz_hwi (first_one) - clz_hwi (next_one);
> + mask = val ^ tmp;
> +
> + /* Check the bit position difference is a power of 2, and that the first
> + sequence of one bits fits within 'bits' bits. */
> + if ((mask >> bits) != 0 || bits != (bits & -bits))
> + return false;
> +
> + /* Check the sequence of one bits is repeated 64/bits times. */
> + return val == mask * bitmask_imm_mul[__builtin_clz (bits) - 26];
> }
>
>
> --
> 1.8.3
>
>
