[PATCH v2 2/3] riscv: word-at-a-time: improve find_zero() without Zbb

[Jisheng Zhang]

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 at kernel.org>
---
 arch/riscv/include/asm/word-at-a-time.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/word-at-a-time.h b/arch/riscv/include/asm/word-at-a-time.h
index 1dcac57a73dd..f2d16c1eea8d 100644
--- a/arch/riscv/include/asm/word-at-a-time.h
+++ b/arch/riscv/include/asm/word-at-a-time.h
@@ -65,8 +65,9 @@ static inline long count_masked_bytes(long mask)
 
 static inline unsigned long find_zero(unsigned long mask)
 {
-	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB))
-		return fls64(mask) >> 3;
+	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB) &&
+	    riscv_has_extension_likely(RISCV_ISA_EXT_ZBB))
+		return !mask ? 0 : ((__fls(mask) + 1) >> 3);
 
 	return count_masked_bytes(mask);
 }
-- 
2.53.0