[PATCH][AArch64][1/5] Improve immediate generation
Wilco Dijkstra
wdijkstr@arm.com
Wed Sep 2 12:35:00 GMT 2015
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.
ChangeLog:
2015-09-02 Wilco Dijkstra <wdijkstr@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
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