[Bug c/44081] New: Incorrect nonnull assumed in code generation

[Bug c/44081] New: Incorrect nonnull assumed in code generation

[hv at crypt dot org]

zen% /opt/gcc-4.5.0/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/opt/gcc-4.5.0/bin/gcc
COLLECT_LTO_WRAPPER=/opt/gcc-4.5.0/libexec/gcc/i686-pc-linux-gnu/4.5.0/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: /src/package/lang/other/gcc-4.5.0/configure
--prefix=/opt/gcc-4.5.0 --with-gmp=/opt/gmp-4.2.2 --with-mpfr=/opt/mpfr-2.4.1
--with-mpc=/opt/mpc-0.8.1
Thread model: posix
gcc version 4.5.0 (GCC) 
zen% 

test.S is generated from 'gcc -S -O2 -fno-inline -Wall gcctest2.c -o
gcctest2.S' using gcc-4.5.0. The assembler shows that the function test() does
not check if its third parameter o2 is NULL, but one path in the calling
function peep() does not guarantee that it is nonnull.

I was unable to reproduce this with gcc-4.4.3; I was also unable to reproduce
it without the '__attribute__((nonnull(2)))' declaration - it seems possible
that in some circumstances the nonnull may be getting applied to the wrong
parameter, but that may be a red herring.

This is cut down from a problem found by Alex Hunsaker compiling perl-5.12.1,
where the erroneous generated code caused a segfault
<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2010-05/msg00316.html>.


-- 
           Summary: Incorrect nonnull assumed in code generation
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hv at crypt dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug c/44081] Incorrect nonnull assumed in code generation

[hv at crypt dot org]

------- Comment #1 from hv at crypt dot org  2010-05-11 14:41 -------
Created an attachment (id=20630)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=20630&action=view)
C source code


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug c/44081] Incorrect nonnull assumed in code generation

[hv at crypt dot org]

------- Comment #2 from hv at crypt dot org  2010-05-11 14:41 -------
Created an attachment (id=20631)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=20631&action=view)
Generated assembly code, with annotation


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug c/44081] Incorrect nonnull assumed in code generation

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2010-05-11 14:51 -------
That's a user bug.  You shouldn't pass NULL to arguments declared nonnull.
To quote gcc documentation:
"The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null."


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug c/44081] Incorrect nonnull assumed in code generation

[hv at crypt dot org]

------- Comment #4 from hv at crypt dot org  2010-05-11 15:38 -------
(In reply to comment #3)
> That's a user bug.  You shouldn't pass NULL to arguments declared nonnull.
> To quote gcc documentation:
> "The compiler may also choose to make optimizations based
> on the knowledge that certain function arguments will not be null."
> 

Parameter 2 has been declared nonnull. It isn't being passed as NULL.

The generated code elides a test for parameter 3 being NULL. This parameter has
not been declared nonnull.


-- 

hv at crypt dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2010-05-11 22:59 -------
Works for me with:
        .ident  "GCC: (GNU) 4.6.0 20100427 (experimental) [trunk revision
158795]"
on x86_64 with -m32 -O2 -fno-inline.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[pmoulder at mail dot csse dot monash dot edu dot au]

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------- Comment #6 from pmoulder at mail dot csse dot monash dot edu dot au  2010-05-12 05:29 -------
Part of the problem is a documentation bug: the nonnull attribute's parameter
is named "arg-index", suggesting that the first parameter is 0, while the
example strongly suggests that the first parameter is 1.

If parameters are numbered from 1, then please change the name to "arg-number";
if they are numbered from 0, then please change the example to match.

On the subject of nonnull's documentation (and only weakly related to the
current bug report): To judge from the fact that this attribute is being
incorrectly used in glibc on functions like memcmp,memcpy,memmove,qsort etc.
where NULL is a valid value (so long as n==0), and also from comments such as
at
http://chtekk.longitekk.com/index.php?/archives/43-GCCs-__attribute__-nonnull-...-not-helpful-at-all.html,
I suggest that the "The compiler may also choose to make optimizations based on
the knowledge that certain function arguments will not be null" sentence be
given more emphasis: I suggest putting it in a paragraph by itself, and adding
a warning that a consequence would be that any checks for NULL in the function
will be “optimized away”, and also warning that the attribute is
inappropriate for functions such as memcmp,memcpy,memmove,qsort where NULL is
only unusual (but valid when n==0) rather than unconditionally invalid.


-- 

pmoulder at mail dot csse dot monash dot edu dot au changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pmoulder at mail dot csse
                   |                            |dot monash dot edu dot au


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[jakub at gcc dot gnu dot org]

------- Comment #7 from jakub at gcc dot gnu dot org  2010-05-12 06:25 -------
I can't reproduce the problem either, neither in 4.6 nor in 4.4.  4.5.0 (rather
than current 4.5 snapshots) had a bug with nonnull attribute if a function is
IPA optimized and some arguments are optimized out, but I don't think that's
the case here.

The nonnull arguments are 1 based, sorry for not reading the testcase carefully
when replying in #c3.

memcpy etc. with NULL argument is invalid, the standard say that those
functions
copy from object pointed by the argument (or to object pointed by the
argument).
NULL doesn't point to any object (the standard say that it compares unequal to
pointer to any object).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[pmoulder at mail dot csse dot monash dot edu dot au]

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------- Comment #8 from pmoulder at mail dot csse dot monash dot edu dot au  2010-05-12 09:25 -------
OK, on careful reading, I agree re memcpy etc. (see below).

Consequently, I amend my suggested change to the documentation to:

  - Add a sentence about implicit `this' argument (copied & pasted from the
format_arg entry).

  - Although largely taken care of by the above, I'd still be inclined to
rename the attribute parameter from ‘arg-index’ to ‘arg-number’; and
similarly to rename ‘string-index’ to ‘string-arg-number’ elsewhere in
the texinfo node (in format and format_arg items).

  - Reword ‘argument index list’ to ‘argument number list’ (in the
nonnull item).

  - Despite retracting the “wrongly used in glibc” grounds, I think the
concerns raised on the page I linked to are sufficient to merit giving more
emphasis to the "The compiler may also choose to make optimizations based on
the knowledge that certain function arguments will not be null" sentence: that
it be put it in a paragraph by itself, and adding a warning that a consequence
would be that any checks for NULL in the function may be optimized away.


Coming back to memcpy etc.: As this is such an edge case, I'll give more detail
for my (revised) reasoning (partly for the benefit of certain others that I
intend to refer to this discussion):

  - C99 §7.21.2.1 [#2]: “The memcpy function copies n [i.e. 0] characters
from the object pointed to by s2 into the object pointed to by s1.”

    Text is similar for memcmp, memmove, memset, and their w- (wmemcmp etc.)
counterparts, in each case referring to “the object pointed to by s/s1/s2”;
while text for qsort says “sorts an array of nmemb objects, the initial
element of which is pointed to by base”.

  - C99 §6.3.2.3 [#3]: “Such a pointer [as NULL], called a null pointer, is
guaranteed to compare unequal to a pointer to any object or function.”

Thus, if s/s1/s2 is a null pointer, then “the object pointed to by s/s1/s2”
isn't defined, so the sentence doesn't apply (despite copying/comparing only 0
bytes/characters from or to this undefined thing); and in each case there's no
other text giving the behaviour when s/s1/s2 is a null pointer; so I conclude
that the behaviour of these functions is indeed undefined when s/s1/s2 is a
null pointer, so the “undefined behaviour” provisions of §3.18 apply, and
optimizing out the test for NULL is conformant behaviour as an instance of
“ignoring the situation [of s/s1/s2 == NULL] completely with unpredictable
results” (§3.18 [#2]).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[hv at crypt dot org]

------- Comment #9 from hv at crypt dot org  2010-05-12 10:54 -------
The direction of discussion has centred so far on the documentation, but as far
as I can tell the only point at which the documentation confused someone was
the triage at #3. Should there not be a separate bug opened for problems with
the documentation?

Can somone with access to a stock 4.5.0 confirm that the originally reported
testcase does indeed show a bug?

In #7, Jakub says:
> 4.5.0 (rather than current 4.5 snapshots) had a bug with nonnull attribute
> if a function is IPA optimized and some arguments are optimized out, but I
> don't think that's the case here.

Do you have a reference for this bug? What makes you think this isn't the same?

Note that in the process of cutting down the original code to a testcase, one
constant that I found had to be preserved was that the test() function be
static, so that gcc had freedom to fiddle with the calling conventions; another
was that the first parameter be preserved even though unused; another was that
the second parameter needed its nonnull attribute. It sounds very similar to
me.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[joseph at codesourcery dot com]

------- Comment #10 from joseph at codesourcery dot com  2010-05-12 11:52 -------
Subject: Re:  Incorrect nonnull assumed in code generation

On Wed, 12 May 2010, pmoulder at mail dot csse dot monash dot edu dot au wrote:

> Coming back to memcpy etc.: As this is such an edge case, I'll give more detail
> for my (revised) reasoning (partly for the benefit of certain others that I
> intend to refer to this discussion):

It's a case about which the standard is completely explicit.  7.21.1#2.  
"Where an argument declared as size_t n specifies the length of the array 
for a function, n can have the value zero on a call to that function. 
Unless explicitly stated otherwise in the description of a particular 
function in this subclause, pointer arguments on such a call shall still 
have valid values, as described in 7.1.4."


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[nordq at ya dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

Valeriy <nordq at ya dot ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nordq at ya dot ru

--- Comment #11 from Valeriy <nordq at ya dot ru> 2010-12-26 22:57:48 UTC ---
/*
nonnull_2.c

gcc -O0 nonnull_2.c
gcc -O1 nonnull_2.c
gcc -O3 nonnull_2.c

00000000  if (0 or 8)  <- Ok
00000004  else
00000008  if (0 or 8)

gcc -O2 nonnull_2.c
gcc -Os nonnull_2.c

00000000  else         <- Error
00000004  else
00000008  if (0 or 8)
*/

#include <stdio.h>

static void attribute_nonnull_2(int i1, int *i2, int *i3)
__attribute__((__nonnull__(2)));
static void attribute_nonnull_2(int i1, int *i2, int *i3) {
    *i2 = i1;

    if (i3 == 0 || i3 == (int *) 8)
        printf("%p  if (0 or 8)\n", i3);
    else
        printf("%p  else       \n", i3);
}

int i2a;

int main(int argc, char *argv[]) {
    attribute_nonnull_2(1, &i2a, (int *) 0);
    attribute_nonnull_2(2, &i2a, (int *) 4);
    attribute_nonnull_2(3, &i2a, (int *) 8);
    return 0;
}

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[nordq at ya dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

--- Comment #12 from Valeriy <nordq at ya dot ru> 2011-01-20 18:23:29 UTC ---
/*    attr_nn.c    */

#include <stdio.h>

static void attr_nn(int i1, int *i2, int *i3) __attribute__((__nonnull__(2)));
static void attr_nn(int i1, int *i2, int *i3) {
    *i2 = i1;

    if (i3 == 0 || i3 == (int *) 8)  printf("%p  if (0 or 8)\n", i3);
    else                             printf("%p  else       \n", i3);
}

int i2a;

int main(int argc, char *argv[]) {
    attr_nn(1, &i2a, (int *) 0);
    attr_nn(2, &i2a, (int *) 4);
    attr_nn(3, &i2a, (int *) 8);
    return 0;
}




/*
-----------------------------------------------------------------------------

Ubuntu GCC-4.4.5  -  Error


$ gcc -O0 attr_nn.c && ./a.out
$ gcc -O1 attr_nn.c && ./a.out
$ gcc -O3 attr_nn.c && ./a.out

(nil)  if (0 or 8)       <- Ok
0x4  else
0x8  if (0 or 8)

$ gcc -O2 attr_nn.c && ./a.out
$ gcc -Os attr_nn.c && ./a.out

(nil)  else           <- Error
0x4  else
0x8  if (0 or 8)


$ gcc -v
Using built-in specs.
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.4.4-14ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc
--enable-targets=all --disable-werror --with-arch-32=i686 --with-tune=generic
--enable-checking=release --build=i686-linux-gnu --host=i686-linux-gnu
--target=i686-linux-gnu
Thread model: posix
gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5) 

-----------------------------------------------------------------------------

Cygwin GCC-4.3.4  - Ok


$ gcc -O0 attr_nn.c && ./a.exe
$ gcc -O1 attr_nn.c && ./a.exe
$ gcc -O2 attr_nn.c && ./a.exe
$ gcc -O3 attr_nn.c && ./a.exe
$ gcc -Os attr_nn.c && ./a.exe

0x0  if (0 or 8)         <- Ok
0x4  else
0x8  if (0 or 8)

-----------------------------------------------------------------------------

Cygwin GCC-4.5.0  - Error


$ gcc -O0 attr_nn.c && ./a.exe
$ gcc -O1 attr_nn.c && ./a.exe
$ gcc -O3 attr_nn.c && ./a.exe

0x0  if (0 or 8)         <- Ok
0x4  else
0x8  if (0 or 8)

$ gcc -O2 attr_nn.c && ./a.exe
$ gcc -Os attr_nn.c && ./a.exe

0x0  else             <- Error
0x4  else
0x8  if (0 or 8)


$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-cygwin/4.5.0/lto-wrapper.exe
Target: i686-pc-cygwin
Configured with:
/gnu/gcc/releases/respins/4.5.0-1/gcc4-4.5.0-1/src/gcc-4.5.0/configure
--srcdir=/gnu/gcc/releases/respins/4.5.0-1/gcc4-4.5.0-1/src/gcc-4.5.0
--prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin
--libexecdir=/usr/lib --datadir=/usr/share --localstatedir=/var
--sysconfdir=/etc --datarootdir=/usr/share --docdir=/usr/share/doc/gcc4
--datadir=/usr/share --infodir=/usr/share/info --mandir=/usr/share/man -v
--with-gmp=/usr --with-mpfr=/usr --enable-bootstrap
--enable-version-specific-runtime-libs --libexecdir=/usr/lib --enable-static
--enable-shared --enable-shared-libgcc --disable-__cxa_atexit --with-gnu-ld
--with-gnu-as --with-dwarf2 --disable-sjlj-exceptions
--enable-languages=ada,c,c++,fortran,java,lto,objc,obj-c++ --enable-graphite
--enable-lto --enable-java-awt=gtk --disable-symvers --enable-libjava
--program-suffix=-4 --enable-libgomp --enable-libssp --enable-libada
--enable-threads=posix --with-arch=i686 --with-tune=generic
--enable-libgcj-sublibs CC=gcc-4 CXX=g++-4 CC_FOR_TARGET=gcc-4
CXX_FOR_TARGET=g++-4 GNATMAKE_FOR_TARGET=gnatmake GNATBIND_FOR_TARGET=gnatbind
--with-ecj-jar=/usr/share/java/ecj.jar
Thread model: posix
gcc version 4.5.0 (GCC)

-----------------------------------------------------------------------------

MinGW GCC-4.4.3 (Strawberry Perl)  -  Error


>gcc -O0 attr_nn.c && a.exe
>gcc -O1 attr_nn.c && a.exe
>gcc -O3 attr_nn.c && a.exe

00000000  if (0 or 8)    <- Ok
00000004  else
00000008  if (0 or 8)

>gcc -O2 attr_nn.c && a.exe
>gcc -Os attr_nn.c && a.exe

00000000  else           <- Error
00000004  else
00000008  if (0 or 8)


>gcc -v
Using built-in specs.
Target: i686-w64-mingw32
Configured with: ../gcc44-svn/configure --target=i686-w64-mingw32
--host=i686-w64-mingw32 --disable-multilib --disable-nls
--disable-win32-registry --prefix=/mingw32 --with-gmp=/mingw32
-with-mpfr=/mingw32 --enable-languages=c,c++
Thread model: win32
gcc version 4.4.3 (GCC)

-----------------------------------------------------------------------------

MinGW GCC-4.5.0 (20101030)  -  Error


>gcc -O0 attr_nn.c && a.exe
>gcc -O1 attr_nn.c && a.exe
>gcc -O3 attr_nn.c && a.exe

00000000  if (0 or 8)    <- Ok
00000004  else
00000008  if (0 or 8)

>gcc -O2 attr_nn.c && a.exe
>gcc -Os attr_nn.c && a.exe

00000000  else           <- Error
00000004  else
00000008  if (0 or 8)


gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=g:/mingw/bin/../libexec/gcc/mingw32/4.5.0/lto-wrapper.exe
Target: mingw32
Configured with: ../gcc-4.5.0/configure
--enable-languages=c,c++,ada,fortran,objc,obj-c++ --disable-sjlj-exceptions
--with-dwarf2 --enable-shared --enable-libgomp --disable-win32-registry
--enable-libstdcxx-debug --enable-version-specific-runtime-libs
--disable-werror --build=mingw32 --prefix=/mingw

Thread model: win32
gcc version 4.5.0 (GCC)

-----------------------------------------------------------------------------
*/

[Bug middle-end/44081] Incorrect nonnull assumed in code generation

[hv at crypt dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081

--- Comment #14 from Hugo van der Sanden <hv at crypt dot org> ---
(In reply to Manuel López-Ibáñez from comment #13)
> Should we close this?

With what status?

I think it should at least be updated to CONFIRMED, based on the comments from
valeriy.

I should note also that I was discouraged by the quality of response, and in
particular that I never got a reply to #9.
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From: "anders.granlund.0 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/67016] New: Redeclaration of enum
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idg016

            Bug ID: 67016
           Summary: Redeclaration of enum
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: anders.granlund.0 at gmail dot com
  Target Milestone: ---

Consider the following program:

  enum E {};
  enum E;
  int main() {}

Compile it with the following command line:

  g++ prog.cc -std=c++98 -pedantic-errors

No error messages are given. I expect to get at least one error message, since
the program is ill-formed according to the c++ standard, see [dcl.type.elab]/1:

  http://eel.is/c++draft/dcl.dcl#dcl.type.elab-1

I have tried compiling with gcc HEAD 6.0.0 20150725 here:

  http://melpon.org/wandbox/permlink/KtDN7vjZN3bc1gtc