linux-kernel - Re: [PATCH] lib: bitrev.c micro-optimization

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Tue, 29 Apr 2008 08:50:03 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Harvey Harrison <harvey.harrison@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib: bitrev.c micro-optimization



Not very good. Why do it in memory, when doing it in a register is much 
clearer?

This generates one byte longer code for me, but avoids the partial memory 
stalls and looks more obvious. It is also portable, unlike your version 
(you need to use a union to avoid aliasing issues - admittedly we disable 
the gcc alias code anyway because of other kernel code, but still).

		Linus

---
 lib/bitrev.c |   10 ++++------
 1 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/lib/bitrev.c b/lib/bitrev.c
index 989aff7..e0cb44e 100644
--- a/lib/bitrev.c
+++ b/lib/bitrev.c
@@ -42,17 +42,15 @@ const u8 byte_rev_table[256] = {
 };
 EXPORT_SYMBOL_GPL(byte_rev_table);
 
-static __always_inline u16 bitrev16(u16 x)
-{
-	return (bitrev8(x & 0xff) << 8) | bitrev8(x >> 8);
-}
-
 /**
  * bitrev32 - reverse the order of bits in a u32 value
  * @x: value to be bit-reversed
  */
 u32 bitrev32(u32 x)
 {
-	return (bitrev16(x & 0xffff) << 16) | bitrev16(x >> 16);
+	return  (bitrev8(x) << 24) |
+		(bitrev8(x >> 8) << 16) |
+		(bitrev8(x >> 16) << 8) |
+		(bitrev8(x >> 24));
 }
 EXPORT_SYMBOL(bitrev32);
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/