Double conversion to memory and from memory array is broken with O3
optimization enabled. Both functions conv1 ( conversion from double to
uint16_t[4] array ) and conv2 ( conversion from uint16_t array to double ) are
broken.
To my knowlege the problem also exists in previous versions of gcc/g++ ( I've
used gcc 4.2.4 and 4.1.x on my previous machine (32 bit environment)- and I've
had problems with HPA ( high precision arithmetic ) library with O3 enabled ).
Correct test program output ( with -O0 ) is
r=(0,0,0,16420) <- double converted to array
d=10.000000 <- double reconverted from array
Enabling optimization -O3 gives
r=(1600,64,0,0)
d=0.000000
Using g++ with -O3 gives
r=(1840,64,0,0)
d=0.000000
The following code shows the base of the problem I found in the HPA library.
------------- test5.c -------------
#include <stdio.h>
#include <stdint.h>
// move double to array
void conv1( double y, uint16_t* pr )
{
uint16_t* py;
int i;
py = (uint16_t*)&y;
for(i=0;i<4;i++) pr[i] = py[i];
}
// get double from array
double conv2( uint16_t* pr )
{
uint16_t pa[4];
int i;
for(i=0;i<4;i++) pa[i] = pr[i];
return *((double*)pa);
}
int main( int argc, char* argv[] )
{
uint16_t r[4];
double d;
int i;
conv1(10.,r);
printf("r=(%i,%i,%i,%i)\n", (int)r[0], (int)r[1], (int)r[2], (int)r[3]);
d = conv2(r);
printf("d=%lf\n", d);
}
------------- test5.c -------------
Result of gcc -v -save-temps -O3 test5.c
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.2-1ubuntu10'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib
--without-included-gettext--enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu10)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mtune=generic'
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/cc1 -E -quiet -v test5.c
-D_FORTIFY_SOURCE=2 -mtune=generic -O3 -fpch-preprocess -o test5.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/usr/include/x86_64-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/include
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mtune=generic'
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/cc1 -fpreprocessed test5.i -quiet
-dumpbase test5.c -mtune=generic -auxbase test5 -O3 -version -fstack-protector
-o test5.s
GNU C (Ubuntu 4.3.2-1ubuntu10) version 4.3.2 (x86_64-linux-gnu)
compiled by GNU C version 4.3.2, GMP version 4.2.2, MPFR version 2.3.1.
warning: MPFR header version 2.3.1 differs from library version 2.3.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 9fb216f9df26c4d658fb4fb0c8a8291a
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mtune=generic'
as -V -Qy -o test5.o test5.s
GNU assembler version 2.18.93 (x86_64-linux-gnu) using BFD version (GNU
Binutils for Ubuntu) 2.18.93.20081009
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mtune=generic'
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/collect2 --eh-frame-hdr -m elf_x86_64
--hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2 -zrelro
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/crtbegin.o
-L/usr/lib/gcc/x86_64-linux-gnu/4.3.2 -L/usr/lib/gcc/x86_64-linux-gnu/4.3.2
-L/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../.. test5.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/crtend.o
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/crtn.o
--
Summary: double to array conversion
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: hazeman at aster dot pl
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37849
