Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro

[Joe Perches]

On Fri, 2018-12-21 at 16:08 -0500, Steven Rostedt wrote:
> On Fri, 21 Dec 2018 21:58:32 +0100
> Andreas Schwab <schwab@xxxxxxxxxxxxxx> wrote:
> 
> 
> > > Well, perhaps I can just remove the ending ones. I get paranoid with
> > > macro variables, and tend to over do it so that there's no question.  
> > 
> > Why not make it an inline function?
> 
> Matters if that removes the strlen(const) optimization. I could try it
> and see what happens.

Using

static inline bool str_has_prefix(const char *str, const char prefix[])
{
	return !strncmp(str, prefix, strlen(prefix));
}

eliminates the strlen with gcc 4.8 (oldest I still have)

$ cat lib/test_module.c
 * This module emits "Hello, world" on printk when loaded.
 *
 * It is designed to be used for basic evaluation of the module loading
 * subsystem (for example when validating module signing/verification). It
 * lacks any extra dependencies, and will not normally be loaded by the
 * system unless explicitly requested by name.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/init.h>
#include <linux/module.h>
#include <linux/printk.h>

static inline bool str_has_prefix(const char *str, const char prefix[])
{
	return !strncmp(str, prefix, strlen(prefix));
}

bool test_str_has_prefix(const char *foo)
{
	return str_has_prefix("TomJonesPrefix", foo);
}

bool test_str_has_prefix_TomJones(void)
{
	return str_has_prefix("TomJonesPrefix", "TomJones");
}

$ make CC=/usr/bin/gcc-4.8 lib/test_module.o
$ objdump -d -s lib/test_module.o

lib/test_module.o:     file format elf64-x86-64

Contents of section .text:
 0000 534889fb e8000000 00b90f00 00004883  SH............H.
 0010 f80f4889 df480f4e c848c7c6 00000000  ..H..H.N.H......
 0020 4839c9f3 a65b0f94 c0c3660f 1f440000  H9...[....f..D..
 0030 b8010000 00c3                        ......          
Contents of section .rodata.str1.1:
 0000 546f6d4a 6f6e6573 50726566 697800    TomJonesPrefix. 
Contents of section .comment:
 0000 00474343 3a202855 62756e74 7520342e  .GCC: (Ubuntu 4.
 0010 382e352d 34756275 6e747539 2920342e  8.5-4ubuntu9) 4.
 0020 382e3500                             8.5.            
Contents of section .orc_unwind_ip:
 0000 00000000 00000000 00000000 00000000  ................
 0010 00000000 00000000                    ........        
Contents of section .orc_unwind:
 0000 08000000 05001000 00000500 08000000  ................
 0010 05000000 00000000 08000000 05000000  ................
 0020 00000000                             ....            

Disassembly of section .text:

0000000000000000 <test_str_has_prefix>:
   0:	53                   	push   %rbx
   1:	48 89 fb             	mov    %rdi,%rbx
   4:	e8 00 00 00 00       	callq  9 <test_str_has_prefix+0x9>
   9:	b9 0f 00 00 00       	mov    $0xf,%ecx
   e:	48 83 f8 0f          	cmp    $0xf,%rax
  12:	48 89 df             	mov    %rbx,%rdi
  15:	48 0f 4e c8          	cmovle %rax,%rcx
  19:	48 c7 c6 00 00 00 00 	mov    $0x0,%rsi
  20:	48 39 c9             	cmp    %rcx,%rcx
  23:	f3 a6                	repz cmpsb %es:(%rdi),%ds:(%rsi)
  25:	5b                   	pop    %rbx
  26:	0f 94 c0             	sete   %al
  29:	c3                   	retq   
  2a:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)

0000000000000030 <test_str_has_prefix_TomJones>:
  30:	b8 01 00 00 00       	mov    $0x1,%eax
  35:	c3                   	retq