linux-kernel - [PATCH] : Optimizes timespec

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Date:	Sat, 03 Mar 2007 00:16:56 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: [PATCH] : Optimizes timespec_trunc() 

The first thing done by timespec_trunc() is :

  if (gran <= jiffies_to_usecs(1) * 1000)

This should really be a test against a constant known at compile time.

Alas, it isnt. jiffies_to_usec() was unilined so C compiler emits a function 
call and a multiply to compute : a CONSTANT.

mov    $0x1,%edi
mov    %rbx,0xffffffffffffffe8(%rbp)
mov    %r12,0xfffffffffffffff0(%rbp)
mov    %edx,%ebx
mov    %rsi,0xffffffffffffffc8(%rbp)
mov    %rsi,%r12
callq  ffffffff80232010 <jiffies_to_usecs>
imul   $0x3e8,%eax,%eax
cmp    %ebx,%eax

This patch reorders kernel/time.c a bit so that jiffies_to_usecs() is defined 
before timespec_trunc() so that compiler now generates :

cmp    $0x3d0900,%edx  (HZ=250 on my machine)

This gives a better code (timespec_trunc() becoming a leaf function), and 
shorter kernel size as well.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

View attachment "time.patch" of type "text/plain" (2101 bytes)