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Bug#823222: marked as done (gcc-4.9: reordering of signed int operations triggers overflow)

Your message dated Tue, 20 Sep 2016 22:15:11 +0000
with message-id <[🔎] E1bmTJn-0005QY-0D@franck.debian.org>
and subject line Bug#838316: Removed package(s) from unstable
has caused the Debian Bug report #823222,
regarding gcc-4.9: reordering of signed int operations triggers overflow
to be marked as done.

This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

(NB: If you are a system administrator and have no idea what this
message is talking about, this may indicate a serious mail system
misconfiguration somewhere. Please contact owner@bugs.debian.org
immediately.)


-- 
823222: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=823222
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems
--- Begin Message --- 
Package: gcc-4.9
Version: 4.9.2-10
Severity: normal

I compiled the program

    #include <stdio.h>
    #include <limits.h>
    int main() {
	int s = 1 << 30;
	s += (s - 1);
	printf("%d\n%d\n%d\n", sizeof s, s, INT_MAX);
	return 0;
    }

with "gcc -W -Wall -ansi -pedantic -O0 -fsanitize=undefined".
The executable emits warnings for undefined behaviour at runtime:

a.c:6:7: runtime error: signed integer overflow: 1073741824 + 1073741824 cannot be represented in type 'int'
a.c:6:7: runtime error: signed integer overflow: -2147483648 + -1 cannot be represented in type 'int'
4
2147483647
2147483647

As far as I can tell, the program is correct, and does not involve undefined
behaviour because no signed-integer overflow occurs.  It turns out, however,
that gcc transforms 

    s += (s - 1);

into 

    s += s;
    s -= 1;

even at -O0, as is apparent in the generated assembly code (gcc -O0 -S)
    ...
    movl    $1073741824, -12(%ebp)
    movl    -12(%ebp), %eax
    addl    %eax, %eax
    subl    $1, %eax
    movl    %eax, -12(%ebp)
    ...

Here, it seems that the compiler and libubsan disagree, and I'm not sure which
one is right.

Best regards,
    g.

-- System Information:
Debian Release: 8.4
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: i386 (i686)

Kernel: Linux 3.16.0-4-686-pae (SMP w/2 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages gcc-4.9 depends on:
ii  binutils        2.25-5
ii  cpp-4.9         4.9.2-10
ii  gcc-4.9-base    4.9.2-10
ii  libc6           2.19-18+deb8u4
ii  libcloog-isl4   0.18.2-1+b2
ii  libgcc-4.9-dev  4.9.2-10
ii  libgmp10        2:6.0.0+dfsg-6
ii  libisl10        0.12.2-2
ii  libmpc3         1.0.2-1
ii  libmpfr4        3.1.2-2
ii  zlib1g          1:1.2.8.dfsg-2+b1

Versions of packages gcc-4.9 recommends:
ii  libc6-dev  2.19-18+deb8u4

Versions of packages gcc-4.9 suggests:
ii  gcc-4.9-doc       4.9.1-3
pn  gcc-4.9-locales   <none>
pn  gcc-4.9-multilib  <none>
pn  libasan1-dbg      <none>
pn  libatomic1-dbg    <none>
pn  libcilkrts5-dbg   <none>
pn  libgcc1-dbg       <none>
pn  libgomp1-dbg      <none>
pn  libitm1-dbg       <none>
pn  liblsan0-dbg      <none>
pn  libquadmath0-dbg  <none>
pn  libtsan0-dbg      <none>
pn  libubsan0-dbg     <none>

-- no debconf information

--- End Message ---
--- Begin Message --- 
Version: 4.9.4-2+rm

Dear submitter,

as the package gcc-4.9 has just been removed from the Debian archive
unstable we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/838316

The version of this package that was in Debian prior to this removal
can still be found using http://snapshot.debian.org/.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

Debian distribution maintenance software
pp.
Chris Lamb (the ftpmaster behind the curtain)

--- End Message ---
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