linux-kernel - RE: [PATCH] capabilities: use logical OR

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Date:   Thu,  1 Jun 2023 03:32:31 +0800
From:   Min-Hua Chen <minhuadotchen@...il.com>
To:     david.laight@...lab.com
Cc:     jmorris@...ei.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, minhuadotchen@...il.com,
        paul@...l-moore.com, serge@...lyn.com
Subject: RE: [PATCH] capabilities: use logical OR

>From: Min-Hua Chen <minhuadotchen@...il.com>
>> Sent: 29 May 2023 23:55
>> 
>> Use logical OR to fix the following sparse warnings:
>> 
>> security/commoncap.c:1358:41: sparse: warning: dubious: !x | y
>> 
>> No functional changes intended.
>
>Except it will run just that teeny, weeny bit slower.
>
>	David
>

Thanks for the comment.

I wrote a test case on my x86 machine to see the difference
between '|' and '||' versions.

The '|' version does 2 OR operations and 1 test. (all cases)
The '||' (logical or) version does 3 cmpl (worst case)
and 1 cmpl and 1 jump if the first argument is TRUE (best case).

For the readers, I think the logical OR version is clearer.


thanks,
Min-Hua

my test case:

void foo(void)
{
}

void bar(void)
{
}

int main(void)
{
	int a, b, c;
	if (a || b || c) 
		foo();
	else
		bar();
}

if (a || b || c) 
logical OR
Dump of assembler code for function main:
0x000000000000113f <+0>:     endbr64
0x0000000000001143 <+4>:     push   %rbp
0x0000000000001144 <+5>:     mov    %rsp,%rbp
0x0000000000001147 <+8>:     sub    $0x10,%rsp
0x000000000000114b <+12>:    cmpl   $0x0,-0xc(%rbp)
0x000000000000114f <+16>:    jne    0x115d <main+30>
0x0000000000001151 <+18>:    cmpl   $0x0,-0x8(%rbp)
0x0000000000001155 <+22>:    jne    0x115d <main+30>
0x0000000000001157 <+24>:    cmpl   $0x0,-0x4(%rbp)
0x000000000000115b <+28>:    je     0x1164 <main+37>
0x000000000000115d <+30>:    call   0x1129 <foo>
0x0000000000001162 <+35>:    jmp    0x1169 <main+42>
0x0000000000001164 <+37>:    call   0x1134 <bar>
0x0000000000001169 <+42>:    mov    $0x0,%eax
0x000000000000116e <+47>:    leave
0x000000000000116f <+48>:    ret
End of assembler dump.

if (a | b | c)
Dump of assembler code for function main:
0x000000000000113f <+0>:     endbr64
0x0000000000001143 <+4>:     push   %rbp
0x0000000000001144 <+5>:     mov    %rsp,%rbp
0x0000000000001147 <+8>:     sub    $0x10,%rsp
0x000000000000114b <+12>:    mov    -0xc(%rbp),%eax
0x000000000000114e <+15>:    or     -0x8(%rbp),%eax
0x0000000000001151 <+18>:    or     -0x4(%rbp),%eax
0x0000000000001154 <+21>:    test   %eax,%eax
0x0000000000001156 <+23>:    je     0x115f <main+32>
0x0000000000001158 <+25>:    call   0x1129 <foo>
0x000000000000115d <+30>:    jmp    0x1164 <main+37>
0x000000000000115f <+32>:    call   0x1134 <bar>
0x0000000000001164 <+37>:    mov    $0x0,%eax
0x0000000000001169 <+42>:    leave
0x000000000000116a <+43>:    ret
End of assembler dump.