Bug#780000: gcc-4.9: LTO makes the build unreproducible

[Thorsten Glaser <tg@mirbsd.de>]

Package: gcc-4.9
Version: 4.9.2-10
Severity: minor

With the attached file, or really any program (e.g. src:pax):

1$ gcc -O2 -g -fPIE -fstack-protector-strong -flto=jobserver -c bottles.c 
2$ gcc -O2 -g -fPIE -fstack-protector-strong -flto=jobserver -o x1 bottles.o
3$ gcc -O2 -g -fPIE -fstack-protector-strong -flto=jobserver -o x2 bottles.o
4$ md5sum x1 x2
9d58f432cfe6437fd83a807578400efe  x1
a57d1253be60b9a520c140e9c55e5b47  x2

Removing “-flto=jobserver” from line #1 (!) makes it succeed.
If LTO information is included in the object file, even not
using LTO during link (line #2 and #3) creates diverging output.

While I like the reproducible builds effort in Debian, I kinda
don’t want to eliminate LTO for it… though src:mksh hasn’t been
using LTO for a while, as it regularily breaks in newer versions
of GCC…

-- System Information:
Debian Release: 8.0
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: i386 (i686)

Kernel: Linux 3.16.0-4-686-pae (SMP w/2 CPU cores)
Locale: LANG=C, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/lksh
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           2.19-15
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         1.0.3-1
ii  libmpfr4        3.1.2-3
ii  zlib1g          1:1.2.8.dfsg-2+b1

Versions of packages gcc-4.9 recommends:
ii  libc6-dev  2.19-15

Versions of packages gcc-4.9 suggests:
pn  gcc-4.9-doc       <none>
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

/* $MirOS: contrib/hosted/tg/hellow/bottles.c,v 1.1 2011/04/02 19:49:44 tg Exp $ */

#include <stdio.h>

int
main(void)
{
	int b;

	for (b = 99; b >= 0; b--) {
		switch (b) {
		case 0:
			printf("No more bottles of beer on the wall, no more bottles of beer.\n");
			printf("Go to the store and buy some more, 99 bottles of beer on the wall.\n");
			break;
		case 1:
			printf("1 bottle of beer on the wall, 1 bottle of beer.\n");
			printf("Take one down and pass it around, no more bottles of beer on the wall.\n\n");
			break;
		default:
			printf("%d bottles of beer on the wall, %d bottles of beer.\n", b, b);
			printf("Take one down and pass it around, %d %s of beer on the wall.\n\n",
			    b - 1, ((b - 1) > 1) ? "bottles" : "bottle");
			break;
		}
	}

	return (0);
}