Bug#799953: gcc-4.9: incorrect double to integer conversion on i386
Package: gcc
Version: 4:5.2.1-4
Severity: normal
Tags: upstream
Hello,
I believe I've found a bug in GCC that affects plymouth and maybe
other packages on i386. The following minimal code produces an
incorrect result.
----
$ cat test.c
#include <stdio.h>
#include <stdlib.h>
void main(int argc, char **argv) {
float f = 9999999;
double d = f*atoi(argv[1]);
float g = d;
int i = d;
printf("f = %f\n", f);
printf("d = %f\n", d);
printf("g = %f\n", g);
/* printf("i = %d\n", i); */
printf("fractional part of d is %f\n", d-i);
}
$ gcc-4.9 -O1 -o test test.c && ./test 100
f = 9999999.000000
d = 999999900.000000
g = 999999900.000000
fractional part of d is 28.000000
----
The variable 'i' should contain the integral part of 'd', and 'd-i'
should give 0.000000. This is a standard method to calculate the
fractional part of a number. The bug disappears with optimization
turned off (-O0) or with uncommented printf statement. Assembly output
shows that an intermediate float is used for conversion instead of a
double.
The bug is present in gcc-snapshot package from unstable, but not in
gcc-4.6 from wheezy.
----
$ gcc-5 -O1 -o test test.c && ./test 100
f = 9999999.000000
d = 999999900.000000
g = 999999900.000000
fractional part of d is 28.000000
$ gcc-4.6 -O1 -o test test.c && ./test 100
f = 9999999.000000
d = 999999872.000000
g = 999999872.000000
fractional part of d is 0.000000
----
Cheers,
MU
----
$ gcc-4.9 -v
Using built-in specs.
COLLECT_GCC=gcc-4.9
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i586-linux-gnu/4.9/lto-wrapper
Target: i586-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.9.2-10' --with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.9 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-i386/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-i386 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-i386 --with-arch-directory=i386 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-targets=all --enable-multiarch --with-arch-32=i5
86 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=i586-linux-gnu --host=i586-linux-gnu --target=i586-linux-gnu
Thread model: posix
gcc version 4.9.2 (Debian 4.9.2-10)
$ gcc-5 -v
Using built-in specs.
COLLECT_GCC=gcc-5
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i586-linux-gnu/5/lto-wrapper
Target: i586-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 5.2.1-17' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-i386/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-i386 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-i386 --with-arch-directory=i386 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-targets=all --enable-multiarch --with-arch-32=i58
6 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=i586-linux-gnu --host=i586-linux-gnu --target=i586-linux-gnu
Thread model: posix
gcc version 5.2.1 20150911 (Debian 5.2.1-17)
gcc-4.6 -v
Using built-in specs.
COLLECT_GCC=gcc-4.6
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.6/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.6.3-14' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --enable-targets=all --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.6.3 (Debian 4.6.3-14)
----
-- System Information:
Debian Release: 8.2
APT prefers stable-updates
APT policy: (500, 'stable-updates'), (500, 'unstable'), (500, 'stable'), (500, 'oldstable')
Architecture: i386 (i686)
Kernel: Linux 3.16.0-4-686-pae (SMP w/1 CPU core)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
Versions of packages gcc depends on:
ii cpp 4:5.2.1-4
ii gcc-5 5.2.1-17
Versions of packages gcc recommends:
ii libc6-dev [libc-dev] 2.19-22
Versions of packages gcc suggests:
ii autoconf 2.69-8
ii automake 1:1.14.1-4
pn bison <none>
pn flex <none>
pn gcc-doc <none>
ii gcc-multilib 4:5.2.1-4
ii gdb 7.7.1+dfsg-5
ii libtool 2.4.2-1.11
ii make 4.0-8.1
ii manpages-dev 3.74-1
-- no debconf information
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