Bug#823222: gcc-4.9: reordering of signed int operations triggers overflow

[g1 <g1pi@libero.it>]

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