http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49031
Summary: powl() function gives wrong results in some situations
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: eliasrudb...@gmail.com
Created attachment 24270
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24270
The powlbug.i file generated by "gcc -v -save-temps -lm powlbug.c".
Hello!
There seems to be a bug in the powl() function in the gcc -lm library, as shown
by the following test program:
$ cat powlbug.c
#include <stdio.h>
#include <math.h>
#include <string.h>
int main() {
long double xx = 1e-20;
printf("xx = %12.6Lg\n", xx);
printf("powl(xx, 1.5) = %12.6Lg\n", powl(xx, 1.5));
if(1) {
long double a = 0.2;
long double b;
char chararr_b[16] = {1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, -16, 63};
memcpy(&b, chararr_b, 16);
printf("powl(a, b) = %9.4Lg\n", powl(a, b));
}
printf("powl(xx, 1.5) = %12.6Lg\n", powl(xx, 1.5));
return 0;
}
$ gcc -lm powlbug.c
$ ./a.out
xx = 1e-20
powl(xx, 1.5) = 1e-30
powl(a, b) = 1
powl(xx, 1.5) = 0.353553
Since xx is set to 1e-20, the result of powl(xx, 1.5) should always be 1e-30.
It seems like the powl(a, b) call messes up something inside the math library,
so that the following call powl(xx, 1.5) gives 0.353553 which is very wrong.
If I change if(1) to if(0) the second call to powl(xx, 1.5) gives 1e-30 as it
should.
$ gcc --version
gcc (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4)
$ gcc -v -save-temps -lm powlbug.c
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,lto --enable-plugin
--enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-ppl --with-cloog --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.5.1 20100924 (Red Hat 4.5.1-4) (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/cc1 -E -quiet -v powlbug.c
-mtune=generic -march=x86-64 -fpch-preprocess -o powlbug.i
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/4.5.1/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/include
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/cc1 -fpreprocessed powlbug.i -quiet
-dumpbase powlbug.c -mtune=generic -march=x86-64 -auxbase powlbug -version -o
powlbug.s
GNU C (GCC) version 4.5.1 20100924 (Red Hat 4.5.1-4) (x86_64-redhat-linux)
compiled by GNU C version 4.5.1 20100924 (Red Hat 4.5.1-4), GMP version
4.3.1, MPFR version 2.4.2, MPC version 0.8.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C (GCC) version 4.5.1 20100924 (Red Hat 4.5.1-4) (x86_64-redhat-linux)
compiled by GNU C version 4.5.1 20100924 (Red Hat 4.5.1-4), GMP version
4.3.1, MPFR version 2.4.2, MPC version 0.8.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: ea394b69293dd698607206e8e43d607e
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
as -V -Qy --64 -o powlbug.o powlbug.s
GNU assembler version 2.20.51.0.7 (x86_64-redhat-linux) using BFD version
version 2.20.51.0.7-6.fc14 20100318
COMPILER_PATH=/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/:/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/:/usr/libexec/gcc/x86_64-redhat-linux/:/usr/lib/gcc/x86_64-redhat-linux/4.5.1/:/usr/lib/gcc/x86_64-redhat-linux/
LIBRARY_PATH=/usr/lib/gcc/x86_64-redhat-linux/4.5.1/:/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/collect2 --build-id --no-add-needed
--eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker
/lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../lib64/crt1.o
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/crtbegin.o
-L/usr/lib/gcc/x86_64-redhat-linux/4.5.1
-L/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../lib64 -L/lib/../lib64
-L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../.. -lm
powlbug.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed
-lgcc_s --no-as-needed /usr/lib/gcc/x86_64-redhat-linux/4.5.1/crtend.o
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../lib64/crtn.o
The powlbug.i file generated by "gcc -v -save-temps -lm powlbug.c" is attached.
/ Elias
