linux-kernel - Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed

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Message-ID: <7a3c0525-efac-debf-895c-cf984916daaf@rasmusvillemoes.dk>
Date:   Tue, 2 Mar 2021 16:16:17 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Petr Mladek <pmladek@...e.com>,
        Marco Elver <elver@...gle.com>, Timur Tabi <timur@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        roman.fietze@...na.com, Kees Cook <keescook@...omium.org>,
        John Ogness <john.ogness@...utronix.de>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all
 addresses as unhashed

On 02/03/2021 15.28, Geert Uytterhoeven wrote:

> Fortunately gcc is already smart enough to deduplicate identical strings,
> but only in the same source file.

Yeah, gcc can't do much more since it only handles one source file at a
time. However, the linker certainly deduplicates strings across
translation units :)

I don't think gcc bothers to do the tail merging since the linker does
it, but that may depend on optimization level and gcc version; it does
seem to merge identical strings within a TU, at least in very simple cases.

Rasmus

$ grep . *.c
a.c:const char *a1(void) { return "string"; }
a.c:const char *a2(void) { return "string"; }
a.c:const char *a3(void) { return "longer string"; }
b.c:const char *b1(void) { return "string"; }
b.c:const char *b2(void) { return "longer string"; }
b.c:const char *b3(void) { return "much longer string"; }
main.c:#include <stdio.h>
main.c:const char *a1(void);
main.c:const char *a2(void);
main.c:const char *a3(void);
main.c:const char *b1(void);
main.c:const char *b2(void);
main.c:const char *b3(void);
main.c:int main(int argc, char *argv[])
main.c:{
main.c:#define p(x) printf("%s(): %p - [%s]\n", #x, x(), x())
main.c: p(a1);
main.c: p(a2);
main.c: p(a3);
main.c: p(b1);
main.c: p(b2);
main.c: p(b3);
main.c:
main.c: return 0;
main.c:}

$ gcc -O2 -o a.o -c a.c ; gcc -O2 -o b.o -c b.c ; gcc -O2 -o main.o -c
main.c ; gcc -o main main.o a.o b.o ; ./main
a1(): 0x560b1bc3d033 - [string]
a2(): 0x560b1bc3d033 - [string]
a3(): 0x560b1bc3d02c - [longer string]
b1(): 0x560b1bc3d033 - [string]
b2(): 0x560b1bc3d02c - [longer string]
b3(): 0x560b1bc3d027 - [much longer string]