From: Rosen Penev <rosenp@gmail.com>
To: musl@lists.openwall.com
Subject: [PATCH] a_ctz_32: Instead of a subtraction by 31, use an xor
Date: Thu, 7 Nov 2019 18:39:54 -0800 [thread overview]
Message-ID: <20191108023955.4402-1-rosenp@gmail.com> (raw)
This reduces produced assembly from a mov and sub to a single xor.
Tested with godbolt: https://godbolt.org/z/Qz-Qr5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
src/internal/atomic.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/internal/atomic.h b/src/internal/atomic.h
index f938879b..196b3fb1 100644
--- a/src/internal/atomic.h
+++ b/src/internal/atomic.h
@@ -256,7 +256,7 @@ static inline void a_crash()
static inline int a_ctz_32(uint32_t x)
{
#ifdef a_clz_32
- return 31-a_clz_32(x&-x);
+ return a_clz_32(x&-x)^31;
#else
static const char debruijn32[32] = {
0, 1, 23, 2, 29, 24, 19, 3, 30, 27, 25, 11, 20, 8, 4, 13,
--
2.23.0
next reply other threads:[~2019-11-08 2:39 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-08 2:39 Rosen Penev [this message]
2019-12-12 15:10 ` Alexander Monakov
2019-12-12 15:17 ` Rich Felker
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