https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90248
Bug ID: 90248
Summary: larger than 0 compare fails with -ffinite-math-only
-funsafe-math-optimizations
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kau at zurich dot ibm.com
Target Milestone: ---
Created attachment 46242
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46242&action=edit
preprocessed minimal example file
Hello,
the following code produces unexpected/different results using gcc 8.2.1 when
compared to gcc 7.3.1
test.cc:
#include <iostream>
int main() {
float a = 0;
std::cout << a << std::endl;
a = (a > 0) ? 1.0 : -1.0;
std::cout << a << std::endl;
return 0;
}
I would expect this code to print
0
-1
with "gcc version 7.3.1 20180130 (Red Hat 7.3.1-2) (GCC)" it does.
with "gcc version 8.2.1 20181215 (Red Hat 8.2.1-6) (GCC)" it prints
0
1
This behavior depends on the combination of two compiler flags. If either one
is missing, the code behaves as expected.
# g++ -Wall -Wextra -ffinite-math-only -funsafe-math-optimizations test.cc -o
test; ./test
0
1
# g++ -Wall -Wextra -funsafe-math-optimizations test.cc -o test; ./test
0
-1
# g++ -Wall -Wextra -ffinite-math-only test.cc -o test; ./test
0
-1
# g++ -Wall -Wextra test.cc -o test; ./test
0
-1
Note that if I replace
a = (a > 0) ? 1.0 : -1.0;
with
if (a > 0) a = 1.0; else a = -1.0;
it also works as expected regardless of the compiler version or flags.
Compiler output
# g++ -v -save-temps -Wall -Wextra -ffinite-math-only
-funsafe-math-optimizations test.cc -o test
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --with-isl --enable-libmpx
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-gnu-indirect-function --enable-cet --with-tune=generic
--with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 8.2.1 20181215 (Red Hat 8.2.1-6) (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-ffinite-math-only'
'-funsafe-math-optimizations' '-o' 'test' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/8/cc1plus -E -quiet -v -D_GNU_SOURCE
test.cc -mtune=generic -march=x86-64 -Wall -Wextra -ffinite-math-only
-funsafe-math-optimizations -fpch-preprocess -o test.ii
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/8/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/8/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8/x86_64-redhat-linux
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8/backward
/usr/lib/gcc/x86_64-redhat-linux/8/include
/usr/local/include
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-ffinite-math-only'
'-funsafe-math-optimizations' '-o' 'test' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/8/cc1plus -fpreprocessed test.ii -quiet
-dumpbase test.cc -mtune=generic -march=x86-64 -auxbase test -Wall -Wextra
-version -ffinite-math-only -funsafe-math-optimizations -o test.s
GNU C++14 (GCC) version 8.2.1 20181215 (Red Hat 8.2.1-6) (x86_64-redhat-linux)
compiled by GNU C version 8.2.1 20181215 (Red Hat 8.2.1-6), GMP version
6.1.2, MPFR version 3.1.6-p2, MPC version 1.0.2, isl version isl-0.16.1-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++14 (GCC) version 8.2.1 20181215 (Red Hat 8.2.1-6) (x86_64-redhat-linux)
compiled by GNU C version 8.2.1 20181215 (Red Hat 8.2.1-6), GMP version
6.1.2, MPFR version 3.1.6-p2, MPC version 1.0.2, isl version isl-0.16.1-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 2c6c0283460e534bb8025645f70a7cfb
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-ffinite-math-only'
'-funsafe-math-optimizations' '-o' 'test' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'
as -v --64 -o test.o test.s
GNU assembler version 2.29.1 (x86_64-redhat-linux) using BFD version version
2.29.1-23.fc28
COMPILER_PATH=/usr/libexec/gcc/x86_64-redhat-linux/8/:/usr/libexec/gcc/x86_64-redhat-linux/8/:/usr/libexec/gcc/x86_64-redhat-linux/:/usr/lib/gcc/x86_64-redhat-linux/8/:/usr/lib/gcc/x86_64-redhat-linux/
LIBRARY_PATH=/usr/lib/gcc/x86_64-redhat-linux/8/:/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-redhat-linux/8/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-ffinite-math-only'
'-funsafe-math-optimizations' '-o' 'test' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/8/collect2 -plugin
/usr/libexec/gcc/x86_64-redhat-linux/8/liblto_plugin.so
-plugin-opt=/usr/libexec/gcc/x86_64-redhat-linux/8/lto-wrapper
-plugin-opt=-fresolution=test.res -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --build-id
--no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -o test
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/8/crtbegin.o
-L/usr/lib/gcc/x86_64-redhat-linux/8
-L/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64 -L/lib/../lib64
-L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/8/../../.. test.o
-lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/lib/gcc/x86_64-redhat-linux/8/crtfastmath.o
/usr/lib/gcc/x86_64-redhat-linux/8/crtend.o
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-ffinite-math-only'
'-funsafe-math-optimizations' '-o' 'test' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'
Compiler version with unexpected behavior:
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --with-isl --enable-libmpx
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-gnu-indirect-function --enable-cet --with-tune=generic
--with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 8.3.1 20190223 (Red Hat 8.3.1-2) (GCC)
Compiler version with expected behavior regardless of compiler flags or code
changes:
$ gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --with-isl --enable-libmpx
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 7.3.1 20180130 (Red Hat 7.3.1-2) (GCC)
I'm aware that using -ffinite-math-only -funsafe-math-optimizations allows gcc
to divert from the IEEE standard. I am surprised though, that this simple
comparison produces different results depending on the speicifc gcc version and
whe way I code it.
Best regards,
Michael
