[PATCH 2/3] riscv: word-at-a-time: improve find_zero() without Zbb

Jisheng Zhang
2026-01-13T12:43:18Z

Previous commit improved the find_zero() performance for !RISCV_ISA_ZBB.
What about RISCV_ISA_ZBB=y but the HW doesn't support Zbb? We have the
same heavy generic fls64() issue.

Let's improve this situation by checking Zbb extension and fall back
to generic count_masked_bytes() if Zbb isn't supported.

To remove non-necessary zero bits couting on RV32, we also replace the
'fls64(mask) >> 3' with '!mask ? 0 : ((__fls(mask) + 1) >> 3);'

We will get similar performance improvement as previous commit for
RISCV_ISA_ZBB=y but HW doesn't support Zbb.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
---
arch/riscv/include/asm/word-at-a-time.h | 29 ++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/word-at-a-time.h b/arch/riscv/include/asm/word-at-a-time.h
index 0c8a9b337f93..ca3d30741ed1 100644
--- a/arch/riscv/include/asm/word-at-a-time.h
+++ b/arch/riscv/include/asm/word-at-a-time.h
@@ -42,9 +42,36 @@ static inline unsigned long create_zero_mask(unsigned long bits)
return bits >> 7;
}

+#ifdef CONFIG_64BIT
+/*
+ * Jan Achrenius on G+: microoptimized version of
+ * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
+ * that works for the bytemasks without having to
+ * mask them first.
+ */
+static inline long count_masked_bytes(unsigned long mask)
+{
+ return mask*0x0001020304050608ul >> 56;
+}
+
+#else /* 32-bit case */
+
+/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
+static inline long count_masked_bytes(long mask)
+{
+ /* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
+ long a = (0x0ff0001+mask) >> 23;
+ /* Fix the 1 for 00 case */
+ return a & mask;
+}
+#endif
+
static inline unsigned long find_zero(unsigned long mask)
{
- return fls64(mask) >> 3;
+ if (riscv_has_extension_likely(RISCV_ISA_EXT_ZBB))
+ return !mask ? 0 : ((__fls(mask) + 1) >> 3);
+
+ return count_masked_bytes(mask);
}

/* The mask we created is directly usable as a bytemask */
--
2.51.0