[Fwd: Re: pure-parser warning: ‘yylval’ is used uninitialized in this function]

[Kaiwang Chen]

Hello,

Maybe there is a gcc bug, according to Joel's comment in the "Forwarded
Message" section at the end of this mail.

$ bison -o parse.c parse.y
$ gcc -v -save-temps -Os -Wall -Werror  -c parse.c -o parse.o
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-languages=c,c++,objc,obj-c
++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
/usr/libexec/gcc/x86_64-redhat-linux/4.1.2/cc1 -E -quiet -v parse.c
-mtune=generic -Wall -Werror -Os -fpch-preprocess
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/include
/usr/include
End of search list.
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-languages=c,c++,objc,obj-c
++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
/usr/libexec/gcc/x86_64-redhat-linux/4.1.2/cc1
-fpreprocessed /home/kwchen/.ccache/parse.tmp.test64.16477.i -quiet
-dumpbase parse.tmp.test64.16477.i -mtune=generic
-auxbase-strip /home/kwchen/.ccache/tmp.hash.test64.16477.o -Os -Wall
-Werror -version -o parse.tmp.test64.16477.s
GNU C version 4.1.2 20070925 (Red Hat 4.1.2-33) (x86_64-redhat-linux)
compiled by GNU C version 4.1.2 20070925 (Red Hat 4.1.2-33).
GGC heuristics: --param ggc-min-expand=100 --param
ggc-min-heapsize=131072
Compiler executable checksum: 9f833c3d322ccff4b0d52ae42234eccc
cc1: warnings being treated as errors
parse.c: In function ‘yyparse’:
parse.c:1247: warning: ‘yylval’ is used uninitialized in this function


$ cat parse.tmp.test64.16477.s 
.file "parse.c"
.ident "GCC: (GNU) 4.1.2 20070925 (Red Hat 4.1.2-33)"
.section .note.GNU-stack,"",@progbits


$ uname -a
Linux test64 2.6.21-2950.fc8xen #1 SMP Tue Oct 23 12:23:33 EDT 2007
x86_64 x86_64 x86_64 GNU/Linux
$ gcc --version
gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-33)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.
$ bison --version
bison (GNU Bison) 2.3
Written by Robert Corbett and Richard Stallman.

Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.


$ cat parse.y
%{
#define YYSTYPE int

static int yylex(YYSTYPE *lvalp);
static int yyerror(char const *s);
%}
%pure-parser
%start t
%%

t: 'a'

%%

static int yylex(YYSTYPE *lvalp)
{
    return YYEOF;
}

static int yyerror(char const *s)
{
       return 0;
}

int main()
{
    return yyparse();
}


Thanks,
Kaiwang

-------- Forwarded Message --------
From: Joel E. Denny <address@hidden>
To: Kaiwang Chen <address@hidden>
Cc: address@hidden
Subject: Re: pure-parser warning: ‘yylval’ is used uninitialized in this
function
Date: Sat, 10 May 2008 19:43:29 -0400 (EDT)

On Thu, 8 May 2008, Kaiwang Chen wrote:

> GCC with "-Os -Wall -Werror" complains in the case of pure-parser, while
> without -Os it works well. Any help? Thanks in advance.
> 
> $ bison -o parse.c parse.y
> $ gcc -Os -Wall -Werror  -c parse.c -o parse.o
> cc1: warnings being treated as errors
> parse.c: In function ‘yyparse’:
> parse.c:1247: warning: ‘yylval’ is used uninitialized in this function

According to the gcc documentation, -Wuninitialized (implied by -Wall) is 
affected by the optimization level.

> static int yylex(YYSTYPE *lvalp)
> {
>     return YYEOF;
> }

The warning seems legitimate to me because you're not setting *lvalp in 
yylex.

If yylex is not declared static, the warning goes away.  I'm thinking that 
gcc believes yylex is capable of initializing yylval, but gcc gives up 
when it sees that yylex could have external linking.

When I keep yylex static and initialize *lvalp within it, the warning line 
number moves to 1035, which is the declaration of yylval.  I don't 
understand what that means.  I would've guessed the warning should always 
have the line number of a usage of yylval.  Maybe gcc has a bug.

I don't have time to pursue this further right now.  Maybe someone can 
take this to the gcc maintainers for advice.  If you find an answer, 
please let us know.

As a workaround, you can add this to your definitions section:

  %initial-action { $$ = 0; }