From: Al Viro <viro@zeniv.linux.org.uk>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Eric Dumazet <edumazet@google.com>
Subject: Re: [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user()
Date: Fri, 16 Apr 2021 19:44:06 +0000 [thread overview]
Message-ID: <YHnpBm36PcIINhWi@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <20210416192413.1514419-1-eric.dumazet@gmail.com>
On Fri, Apr 16, 2021 at 12:24:13PM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
>
> We have to loop only to copy u64 values.
> After this first loop, we copy at most one u32, one u16 and one byte.
Does it actually yield a better code?
FWIW, this
void bar(unsigned);
void foo(unsigned n)
{
while (n >= 8) {
bar(n);
n -= 8;
}
while (n >= 4) {
bar(n);
n -= 4;
}
while (n >= 2) {
bar(n);
n -= 2;
}
while (n >= 1) {
bar(n);
n -= 1;
}
}
will compile (with -O2) to
pushq %rbp
pushq %rbx
movl %edi, %ebx
subq $8, %rsp
cmpl $7, %edi
jbe .L2
movl %edi, %ebp
.L3:
movl %ebp, %edi
subl $8, %ebp
call bar@PLT
cmpl $7, %ebp
ja .L3
andl $7, %ebx
.L2:
cmpl $3, %ebx
jbe .L4
movl %ebx, %edi
andl $3, %ebx
call bar@PLT
.L4:
cmpl $1, %ebx
jbe .L5
movl %ebx, %edi
andl $1, %ebx
call bar@PLT
.L5:
testl %ebx, %ebx
je .L1
addq $8, %rsp
movl $1, %edi
popq %rbx
popq %rbp
jmp bar@PLT
.L1:
addq $8, %rsp
popq %rbx
popq %rbp
ret
i.e. loop + if + if + if...
next prev parent reply other threads:[~2021-04-16 19:44 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 19:24 [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user() Eric Dumazet
2021-04-16 19:44 ` Al Viro [this message]
2021-04-16 20:11 ` Eric Dumazet
2021-04-16 20:57 ` Eric Dumazet
2021-04-17 13:59 ` David Laight
2021-04-17 16:03 ` Linus Torvalds
2021-04-17 16:08 ` Linus Torvalds
2021-04-17 16:27 ` Linus Torvalds
2021-04-17 18:09 ` Al Viro
2021-04-17 20:30 ` Al Viro
2021-04-17 20:35 ` Al Viro
2021-04-17 22:11 ` Linus Torvalds
2021-04-18 0:50 ` Al Viro
2021-04-17 19:44 ` Eric Dumazet
2021-04-17 19:51 ` Linus Torvalds
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