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Message-ID: <84199633fd49db573c9ba71f1992936422e907d4.camel@perches.com> Date: Fri, 21 Dec 2018 14:20:25 -0800 From: Joe Perches <joe@...ches.com> To: Steven Rostedt <rostedt@...dmis.org>, Andreas Schwab <schwab@...ux-m68k.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Linux List Kernel Mailing <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Namhyung Kim <namhyung@...nel.org>, Masami Hiramatsu <mhiramat@...nel.org>, Tom Zanussi <zanussi@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org> Subject: Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro On Fri, 2018-12-21 at 16:08 -0500, Steven Rostedt wrote: > On Fri, 21 Dec 2018 21:58:32 +0100 > Andreas Schwab <schwab@...ux-m68k.org> 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
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