Bug 12818 - [3.4/4.0 Regression] -fwritable strings triggers bad code generation
Summary: [3.4/4.0 Regression] -fwritable strings triggers bad code generation
Status: RESOLVED FIXED
Alias: None
Product: gcc
Classification: Unclassified
Component: c (show other bugs)
Version: 3.4.0
: P3 normal
Target Milestone: 3.4.0
Assignee: Eric Botcazou
URL:
Keywords: patch, wrong-code
Depends on:
Blocks:
 
Reported: 2003-10-28 22:10 UTC by fnf
Modified: 2004-09-13 14:15 UTC (History)
1 user (show)

See Also:
Host: i686-pc-linux-gnu
Target: i686-pc-linux-gnu
Build: i686-pc-linux-gnu
Known to work: 3.3
Known to fail:
Last reconfirmed: 2004-01-19 19:37:26


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Description fnf 2003-10-28 22:10:41 UTC
When using -fwritable-strings, the compiler emits two copies of the
strings.  In the supplied example, one of the copies is emitted into a
data area that is supposed to contain pointers to the strings.

In checking some older versions of gcc, it would appear that this
bug first appeared in late May 2003 or early June 2003.

Environment:
System: Linux fred.ninemoons.com 2.4.20-20.9 #1 Mon Aug 18 11:27:43 EDT 2003 i686 athlon i386 GNU/Linux
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: /src/sourceware/gcc/gcc/configure -v --prefix=/usr/local/sourceware --enable-languages=c,c++

How-To-Repeat:

Here is the source code:

	char *names[] = {"alice", "bob", "john"};

	main ()
	{
	  printf ("names[0] = %s\n", names[0]);
	}

This works without -fwritable-strings:

	$ gcc -o j j.c
	$ ./j
	names[0] = alice
	$

It coredumps with -fwritable-strings:

	$ gcc -fwritable-strings -o j j.c
	$ ./j
	Segmentation fault (core dumped)
	$

Here is the generated code:

		 .file   "j.c"
		 .data
	 .LC0:
		 .string "alice"
	 .LC1:
		 .string "bob"
	 .LC2:
		 .string "john"
	 .globl names
		 .align 4
		 .type   names, @object
		 .size   names, 12
	 names:
	 .LC3:
		 .string "alice"
		 .long   .LC3
	 .LC4:
		 .string "bob"
		 .long   .LC4
	 .LC5:
		 .string "john"
		 .long   .LC5
	 .LC6:
		 .string "names[0] = %s\n"
		 .text
	 .globl main
		 .type   main, @function
	 main:
		 pushl   %ebp
		 movl    %esp, %ebp
		 subl    $8, %esp
		 andl    $-16, %esp
		 movl    $0, %eax
		 addl    $15, %eax
		 addl    $15, %eax
		 shrl    $4, %eax
		 sall    $4, %eax
		 subl    %eax, %esp
		 movl    names, %eax
		 movl    %eax, 4(%esp)
		 movl    $.LC6, (%esp)
		 call    printf
		 leave
		 ret
		 .size   main, .-main
		 .section        .note.GNU-stack,"",@progbits
		 .ident  "GCC: (GNU) 3.4 20031028 (experimental)"

 >Fix:
	Workaround is to not use -fwritable-strings
Comment 1 Andrew Pinski 2003-10-28 22:27:53 UTC
I can confirm this on the mainline (20031028), I want to say it is related to deferring of writing 
globals.
Comment 2 Andrew Pinski 2003-12-29 17:51:28 UTC
We might remove or oboslete -fwritable-strings for 3.4.
Comment 3 Andrew Pinski 2004-01-11 00:05:53 UTC
Can we remove this option, please or do something with this bug.
Comment 4 Eric Botcazou 2004-01-29 13:14:29 UTC
Investigating.
Comment 5 Andrew Pinski 2004-01-29 18:40:28 UTC
Path here: <http://gcc.gnu.org/ml/gcc-patches/2004-01/msg03268.html>.
Comment 6 GCC Commits 2004-01-30 14:16:53 UTC
Subject: Bug 12818

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	ebotcazou@gcc.gnu.org	2004-01-30 14:16:44

Modified files:
	gcc            : ChangeLog varasm.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/gcc.dg: fwritable-strings-1.c 

Log message:
	PR c/12818
	* varasm.c (const_hash_1) <STRING_CST>: Use the
	address to compute the hash value if flag_writable_strings.
	(compare_constant) <STRING_CST>: Compare the addresses
	if flag_writable_strings.
	(build_constant_desc): Do not copy the expression for a
	STRING_CST if flag_writable_strings.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.2561&r2=2.2562
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/varasm.c.diff?cvsroot=gcc&r1=1.405&r2=1.406
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.3442&r2=1.3443
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/fwritable-strings-1.c.diff?cvsroot=gcc&r1=NONE&r2=1.1

Comment 7 GCC Commits 2004-01-30 14:23:30 UTC
Subject: Bug 12818

CVSROOT:	/cvs/gcc
Module name:	gcc
Branch: 	gcc-3_4-branch
Changes by:	ebotcazou@gcc.gnu.org	2004-01-30 14:23:24

Modified files:
	gcc            : ChangeLog varasm.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/gcc.dg: fwritable-strings-1.c 

Log message:
	PR c/12818
	* varasm.c (const_hash_1) <STRING_CST>: Use the
	address to compute the hash value if flag_writable_strings.
	(compare_constant) <STRING_CST>: Compare the addresses
	if flag_writable_strings.
	(build_constant_desc): Do not copy the expression for a
	STRING_CST if flag_writable_strings.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=2.2326.2.111&r2=2.2326.2.112
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/varasm.c.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.405&r2=1.405.2.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.3389.2.32&r2=1.3389.2.33
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/fwritable-strings-1.c.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=NONE&r2=1.1.2.1

Comment 8 Eric Botcazou 2004-01-30 14:25:27 UTC
See the aforementioned patch.