https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63189
Bug ID: 63189
Summary: Incorrect results from trivial loop when optimized
with O3 or O2+tree vectorization
Product: gcc
Version: 4.8.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: br...@mynah-software.com
The following code runs two identical loops with the exception that one
variable is marked as volatile:
#include <stdio.h>
short int srcdata[16] = { 0, 0, 0, 0, 0, 0, 0, 1,0,0,0,0,0,0,0,0 };
int main(int argc, char **argv)
{
int hix= 8;
int j;
volatile int s1ref = 0;
for(j = 0; j < hix; j++){ s1ref += srcdata[j]*j;}
int s1=0;
for(j = 0; j < hix; j++) { s1 += srcdata[j]*j;}
printf("s1ref=%d, s1=%d\n",s1ref,s1);
}
I would expect the results to be identical for s1ref and s1 under all
compilation options, however when compiling with O3 or O2 with
-ftree-vectorization I get incorrect results for S1:
bms20@arthas:/tmp$ gcc -O2 test.c && ./a.out
s1ref=7, s1=7
bms20@arthas:/tmp$ gcc -O3 test.c && ./a.out
s1ref=7, s1=1
bms20@arthas:/tmp$ gcc -O2 -ftree-vectorize test.c && ./a.out
s1ref=7, s1=1
bms20@arthas:/tmp$
Changing the type of srcdata to integers or unsigned shorts appears to fix the
problem; however, this is producing incorrect results with signed shorts.
It appears that the vector code is incorrect from examining the .s files
produced.
SETUP SPECIFICS:
Exact version of GCC:
bms20@arthas:/tmp$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1'
--with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap
--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-4.8-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-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 --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 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
System type:
Ubuntu Linux 14.04 running on x86_64.
