From: Michael Davidson <md@google.com>
To: Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
David Miller <davem@davemloft.net>,
Matthew Wilcox <mawilcox@microsoft.com>,
Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org, Michael Davidson <md@google.com>
Subject: [PATCH] lib/int_sqrt.c: optimize for small argument values
Date: Thu, 19 Oct 2017 13:31:37 -0700 [thread overview]
Message-ID: <20171019203137.131042-1-md@google.com> (raw)
int_sqrt() currently takes approximately constant time
regardless of the value of the argument. By using the
magnitude of the operand to set the initial conditions
for the calculation the cost becomes proportional to
log2 of the argument with the worst case behavior not
being measurably slower than it currently is.
Signed-off-by: Michael Davidson <md@google.com>
---
lib/int_sqrt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
index 1ef4cc344977..8394b0dcecd4 100644
--- a/lib/int_sqrt.c
+++ b/lib/int_sqrt.c
@@ -21,7 +21,7 @@ unsigned long int_sqrt(unsigned long x)
if (x <= 1)
return x;
- m = 1UL << (BITS_PER_LONG - 2);
+ m = 1UL << (__fls(x) & ~1UL);
while (m != 0) {
b = y + m;
y >>= 1;
--
2.15.0.rc1.287.g2b38de12cc-goog
next reply other threads:[~2017-10-19 20:32 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-19 20:31 Michael Davidson [this message]
2017-10-19 20:42 ` Kees Cook
2017-10-19 20:56 ` Jason A. Donenfeld
2017-10-19 21:04 ` Kees Cook
2017-10-20 6:13 ` Joe Perches
2017-10-20 14:18 ` Kees Cook
2017-10-20 16:22 ` Peter Zijlstra
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