https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71539
Bug ID: 71539
Summary: incomplete execution of a nested loop for -O2 and -O3
Product: gcc
Version: 5.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: cubitect at gmail dot com
Target Milestone: ---
Created attachment 38700
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38700&action=edit
example source file
For the optimisation levels -O2 and -O3, the exact behaviour of the attached
program falsely depends on the values of BIGNUM and siz. For the given example,
the nested loop in init() terminates early.
Both -O0 and -O1 give the expected output:
$ gcc -Wall -Wextra -Wpedantic test.c -o test -O1 && ./test
1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2
1 1 1 1 1 1 1 1 1
4 4 4 4 4 4 4 4 4
1 1 1 1 1 1 1 1 1
6 6 6 6 6 6 6 6 6
1 1 1 1 1 1 1 1 1
8 8 8 8 8 8 8 8 8
1 1 1 1 1 1 1 1 1
However:
$ gcc -Wall -Wextra -Wpedantic test.c -o test -O2 && ./test
1 1 0 0 0 0 0 0 0
2 2 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0
4 4 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0
6 6 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0
8 8 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0
$ gcc -Wall -Wextra -Wpedantic test.c -o test -O3 && ./test
1 1 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
In my tests, the issue occurred for gcc versions 5.3.1 and 6.0, but not for
version 4.9.3.
$ uname -a
Linux cubuntu-pc 4.4.0-22-generic #40-Ubuntu SMP Thu May 12 22:03:46 UTC 2016
x86_64 x86_64 x86_64 GNU/Linux
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.3.1-14ubuntu2.1' --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-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.3.1 20160413 (Ubuntu 5.3.1-14ubuntu2.1)
