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Why is `abs()` implemented differently?
Why is “using namespace std” considered bad practice?abs vs std::abs, what does the reference say?What are the differences between a pointer variable and a reference variable in C++?Why can templates only be implemented in the header file?Why doesn't GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)?Why does changing 0.1f to 0 slow down performance by 10x?Why is it faster to process a sorted array than an unsorted array?opencv compile with clang ok, with gcc not ok os x 10.9Implement divided differences in C++ with variadic templatesBuilding valgrind on OS Xclang run error gcc okC++ regex_search breaks when compiled with -O1
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I've had a super frustrating bug in my code for the past few weeks. My code would work exactly as expected on my computer, but as soon as I farmed it out to the HPC server, it would produce weird results.
I've boiled it down to this: on my computer (iMac) the function abs() works with floating point numbers, but on the server abs() truncates it to an integer.
Example:
Server
abs(-1.1341234) = 1
My Mac
abs(-1.1341234) = 1.1341234
Now I know I can fix this by using fabs(), that's not the question. I looked at the versions of gcc on either machine and this is the output:
Server
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/apps/software/GCCcore/5.4.0/libexec/gcc/x86_64-unknown-linux-gnu/5.4.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-languages=c,c++,fortran --enable-lto --enable-checking=release --disable-multilib --enable-shared=yes --enable-static=yes --enable-threads=posix --enable-gold=default --enable-plugins --enable-ld --with-plugin-ld=ld.gold --prefix=/apps/software/GCCcore/5.4.0 --with-local-prefix=/apps/software/GCCcore/5.4.0 --enable-bootstrap --with-isl=/dev/shm/GCCcore/5.4.0/dummy-/gcc-5.4.0/stage2_stuff
Thread model: posix
gcc version 5.4.0 (GCC)
My Mac
g++ -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1
Apple LLVM version 10.0.1 (clang-1001.0.46.3)
Target: x86_64-apple-darwin18.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
So my question, why does abs() produce different results between gcc and clang? This issue literally cost me 3 weeks of progress, so as you can imagine I'm a little salty just now...
c++ gcc compilation clang
edited Apr 30 at 20:27
Bathsheba
184k27261388
asked Apr 30 at 8:10
PhillPhill
45549
|
show 1 more comment
I've had a super frustrating bug in my code for the past few weeks. My code would work exactly as expected on my computer, but as soon as I farmed it out to the HPC server, it would produce weird results.
I've boiled it down to this: on my computer (iMac) the function abs() works with floating point numbers, but on the server abs() truncates it to an integer.
Example:
Server
abs(-1.1341234) = 1
My Mac
abs(-1.1341234) = 1.1341234
Now I know I can fix this by using fabs(), that's not the question. I looked at the versions of gcc on either machine and this is the output:
Server
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/apps/software/GCCcore/5.4.0/libexec/gcc/x86_64-unknown-linux-gnu/5.4.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-languages=c,c++,fortran --enable-lto --enable-checking=release --disable-multilib --enable-shared=yes --enable-static=yes --enable-threads=posix --enable-gold=default --enable-plugins --enable-ld --with-plugin-ld=ld.gold --prefix=/apps/software/GCCcore/5.4.0 --with-local-prefix=/apps/software/GCCcore/5.4.0 --enable-bootstrap --with-isl=/dev/shm/GCCcore/5.4.0/dummy-/gcc-5.4.0/stage2_stuff
Thread model: posix
gcc version 5.4.0 (GCC)
My Mac
g++ -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1
Apple LLVM version 10.0.1 (clang-1001.0.46.3)
Target: x86_64-apple-darwin18.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
So my question, why does abs() produce different results between gcc and clang? This issue literally cost me 3 weeks of progress, so as you can imagine I'm a little salty just now...
c++ gcc compilation clang
edited Apr 30 at 20:27
Bathsheba
184k27261388
asked Apr 30 at 8:10
PhillPhill
45549
14
so as you can imagine I'm a little salty just now... Annoying but worth the lecture... This will make you very sensitive about these things. ;-) (I don't mean this offensive but rather as a "heads up". Every really annoying bug might at least produce a valuable experience. So, take it positively...)
– Scheff
Apr 30 at 8:31
3
@Scheff Oh man, I certainly won't ever forget this lesson...
– Phill
Apr 30 at 8:46
5
@Scheff No. A good programming language should not have such quirks. (There's still a lesson to be learned here, but I won't tell which one ;-) )
– Marco13
Apr 30 at 10:09
1
related: developers.redhat.com/blog/2016/02/29/…
– sudo rm -rf slash
Apr 30 at 14:37
7
-Wall, fix all warnings. Always.
– Ben
Apr 30 at 19:01
|
show 1 more comment
I've had a super frustrating bug in my code for the past few weeks. My code would work exactly as expected on my computer, but as soon as I farmed it out to the HPC server, it would produce weird results.
I've boiled it down to this: on my computer (iMac) the function abs() works with floating point numbers, but on the server abs() truncates it to an integer.
Example:
Server
abs(-1.1341234) = 1
My Mac
abs(-1.1341234) = 1.1341234
Now I know I can fix this by using fabs(), that's not the question. I looked at the versions of gcc on either machine and this is the output:
Server
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/apps/software/GCCcore/5.4.0/libexec/gcc/x86_64-unknown-linux-gnu/5.4.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-languages=c,c++,fortran --enable-lto --enable-checking=release --disable-multilib --enable-shared=yes --enable-static=yes --enable-threads=posix --enable-gold=default --enable-plugins --enable-ld --with-plugin-ld=ld.gold --prefix=/apps/software/GCCcore/5.4.0 --with-local-prefix=/apps/software/GCCcore/5.4.0 --enable-bootstrap --with-isl=/dev/shm/GCCcore/5.4.0/dummy-/gcc-5.4.0/stage2_stuff
Thread model: posix
gcc version 5.4.0 (GCC)
My Mac
g++ -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1
Apple LLVM version 10.0.1 (clang-1001.0.46.3)
Target: x86_64-apple-darwin18.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
So my question, why does abs() produce different results between gcc and clang? This issue literally cost me 3 weeks of progress, so as you can imagine I'm a little salty just now...
c++ gcc compilation clang
edited Apr 30 at 20:27
Bathsheba
184k27261388
asked Apr 30 at 8:10
PhillPhill
45549
I've had a super frustrating bug in my code for the past few weeks. My code would work exactly as expected on my computer, but as soon as I farmed it out to the HPC server, it would produce weird results.
I've boiled it down to this: on my computer (iMac) the function abs() works with floating point numbers, but on the server abs() truncates it to an integer.
Example:
Server
abs(-1.1341234) = 1
My Mac
abs(-1.1341234) = 1.1341234
Now I know I can fix this by using fabs(), that's not the question. I looked at the versions of gcc on either machine and this is the output:
Server
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/apps/software/GCCcore/5.4.0/libexec/gcc/x86_64-unknown-linux-gnu/5.4.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-languages=c,c++,fortran --enable-lto --enable-checking=release --disable-multilib --enable-shared=yes --enable-static=yes --enable-threads=posix --enable-gold=default --enable-plugins --enable-ld --with-plugin-ld=ld.gold --prefix=/apps/software/GCCcore/5.4.0 --with-local-prefix=/apps/software/GCCcore/5.4.0 --enable-bootstrap --with-isl=/dev/shm/GCCcore/5.4.0/dummy-/gcc-5.4.0/stage2_stuff
Thread model: posix
gcc version 5.4.0 (GCC)
My Mac
g++ -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1
Apple LLVM version 10.0.1 (clang-1001.0.46.3)
Target: x86_64-apple-darwin18.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
So my question, why does abs() produce different results between gcc and clang? This issue literally cost me 3 weeks of progress, so as you can imagine I'm a little salty just now...
c++ gcc compilation clang
c++ gcc compilation clang
edited Apr 30 at 20:27
Bathsheba
184k27261388
asked Apr 30 at 8:10
PhillPhill
45549
edited Apr 30 at 20:27
Bathsheba
184k27261388
asked Apr 30 at 8:10
PhillPhill
45549
edited Apr 30 at 20:27
Bathsheba
184k27261388
edited Apr 30 at 20:27
Bathsheba
184k27261388
edited Apr 30 at 20:27
Bathsheba
184k27261388
184k27261388
asked Apr 30 at 8:10
PhillPhill
45549
asked Apr 30 at 8:10
PhillPhill
45549
asked Apr 30 at 8:10
PhillPhill
45549
45549
14
so as you can imagine I'm a little salty just now... Annoying but worth the lecture... This will make you very sensitive about these things. ;-) (I don't mean this offensive but rather as a "heads up". Every really annoying bug might at least produce a valuable experience. So, take it positively...)
– Scheff
Apr 30 at 8:31
3
@Scheff Oh man, I certainly won't ever forget this lesson...
– Phill
Apr 30 at 8:46
5
@Scheff No. A good programming language should not have such quirks. (There's still a lesson to be learned here, but I won't tell which one ;-) )
– Marco13
Apr 30 at 10:09
1
related: developers.redhat.com/blog/2016/02/29/…
– sudo rm -rf slash
Apr 30 at 14:37
7
-Wall, fix all warnings. Always.
– Ben
Apr 30 at 19:01
|
show 1 more comment
14
so as you can imagine I'm a little salty just now... Annoying but worth the lecture... This will make you very sensitive about these things. ;-) (I don't mean this offensive but rather as a "heads up". Every really annoying bug might at least produce a valuable experience. So, take it positively...)
– Scheff
Apr 30 at 8:31
3
@Scheff Oh man, I certainly won't ever forget this lesson...
– Phill
Apr 30 at 8:46
5
@Scheff No. A good programming language should not have such quirks. (There's still a lesson to be learned here, but I won't tell which one ;-) )
– Marco13
Apr 30 at 10:09
1
related: developers.redhat.com/blog/2016/02/29/…
– sudo rm -rf slash
Apr 30 at 14:37
7
-Wall, fix all warnings. Always.
– Ben
Apr 30 at 19:01
14
14
so as you can imagine I'm a little salty just now... Annoying but worth the lecture... This will make you very sensitive about these things. ;-) (I don't mean this offensive but rather as a "heads up". Every really annoying bug might at least produce a valuable experience. So, take it positively...)
– Scheff
Apr 30 at 8:31
so as you can imagine I'm a little salty just now... Annoying but worth the lecture... This will make you very sensitive about these things. ;-) (I don't mean this offensive but rather as a "heads up". Every really annoying bug might at least produce a valuable experience. So, take it positively...)
– Scheff
Apr 30 at 8:31
3
3
@Scheff Oh man, I certainly won't ever forget this lesson...
– Phill
Apr 30 at 8:46
@Scheff Oh man, I certainly won't ever forget this lesson...
– Phill
Apr 30 at 8:46
5
5
@Scheff No. A good programming language should not have such quirks. (There's still a lesson to be learned here, but I won't tell which one ;-) )
– Marco13
Apr 30 at 10:09
@Scheff No. A good programming language should not have such quirks. (There's still a lesson to be learned here, but I won't tell which one ;-) )
– Marco13
Apr 30 at 10:09
1
1
related: developers.redhat.com/blog/2016/02/29/…
– sudo rm -rf slash
Apr 30 at 14:37
related: developers.redhat.com/blog/2016/02/29/…
– sudo rm -rf slash
Apr 30 at 14:37
7
7
-Wall, fix all warnings. Always.– Ben
Apr 30 at 19:01
-Wall, fix all warnings. Always.– Ben
Apr 30 at 19:01
|
show 1 more comment
1 Answer
1
active
oldest
votes
You must use std::abs, which has overloads for the primitive types.
You are falling through to the C++ version in one case, and the old C version (which converts its parameters to integral types) in the other.
Things to avoid: (1) using namespace std; (here is why) and (2) not having the appropriate #includes to bring in the functionality you need. Don't rely on C++ standard library implementations to include files for you implicitly.
Some compilers will warn you of a "lossy" conversion if you set the warning level appropriately.
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
34
@BenjaminBihler: That kicks the can down the road - fabs breaks forlong doublefor example.
– Bathsheba
Apr 30 at 9:24
6
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
1
Correction: Usefabsinstead ofabsORstd::abswhich is a template function.
– mid
Apr 30 at 17:30
8
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
2
@Rakete1111: It's lossy in full generality. E.g. 64 bitint, and IEEE754double.
– Bathsheba
May 1 at 12:44
|
show 5 more comments
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1 Answer
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1 Answer
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votes
You must use std::abs, which has overloads for the primitive types.
You are falling through to the C++ version in one case, and the old C version (which converts its parameters to integral types) in the other.
Things to avoid: (1) using namespace std; (here is why) and (2) not having the appropriate #includes to bring in the functionality you need. Don't rely on C++ standard library implementations to include files for you implicitly.
Some compilers will warn you of a "lossy" conversion if you set the warning level appropriately.
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
34
@BenjaminBihler: That kicks the can down the road - fabs breaks forlong doublefor example.
– Bathsheba
Apr 30 at 9:24
6
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
1
Correction: Usefabsinstead ofabsORstd::abswhich is a template function.
– mid
Apr 30 at 17:30
8
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
2
@Rakete1111: It's lossy in full generality. E.g. 64 bitint, and IEEE754double.
– Bathsheba
May 1 at 12:44
|
show 5 more comments
You must use std::abs, which has overloads for the primitive types.
You are falling through to the C++ version in one case, and the old C version (which converts its parameters to integral types) in the other.
Things to avoid: (1) using namespace std; (here is why) and (2) not having the appropriate #includes to bring in the functionality you need. Don't rely on C++ standard library implementations to include files for you implicitly.
Some compilers will warn you of a "lossy" conversion if you set the warning level appropriately.
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
34
@BenjaminBihler: That kicks the can down the road - fabs breaks forlong doublefor example.
– Bathsheba
Apr 30 at 9:24
6
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
1
Correction: Usefabsinstead ofabsORstd::abswhich is a template function.
– mid
Apr 30 at 17:30
8
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
2
@Rakete1111: It's lossy in full generality. E.g. 64 bitint, and IEEE754double.
– Bathsheba
May 1 at 12:44
|
show 5 more comments
You must use std::abs, which has overloads for the primitive types.
You are falling through to the C++ version in one case, and the old C version (which converts its parameters to integral types) in the other.
Things to avoid: (1) using namespace std; (here is why) and (2) not having the appropriate #includes to bring in the functionality you need. Don't rely on C++ standard library implementations to include files for you implicitly.
Some compilers will warn you of a "lossy" conversion if you set the warning level appropriately.
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
You must use std::abs, which has overloads for the primitive types.
You are falling through to the C++ version in one case, and the old C version (which converts its parameters to integral types) in the other.
Things to avoid: (1) using namespace std; (here is why) and (2) not having the appropriate #includes to bring in the functionality you need. Don't rely on C++ standard library implementations to include files for you implicitly.
Some compilers will warn you of a "lossy" conversion if you set the warning level appropriately.
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
edited Apr 30 at 9:07
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
answered Apr 30 at 8:14
BathshebaBathsheba
184k27261388
184k27261388
34
@BenjaminBihler: That kicks the can down the road - fabs breaks forlong doublefor example.
– Bathsheba
Apr 30 at 9:24
6
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
1
Correction: Usefabsinstead ofabsORstd::abswhich is a template function.
– mid
Apr 30 at 17:30
8
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
2
@Rakete1111: It's lossy in full generality. E.g. 64 bitint, and IEEE754double.
– Bathsheba
May 1 at 12:44
|
show 5 more comments
34
@BenjaminBihler: That kicks the can down the road - fabs breaks forlong doublefor example.
– Bathsheba
Apr 30 at 9:24
6
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
1
Correction: Usefabsinstead ofabsORstd::abswhich is a template function.
– mid
Apr 30 at 17:30
8
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
2
@Rakete1111: It's lossy in full generality. E.g. 64 bitint, and IEEE754double.
– Bathsheba
May 1 at 12:44
34
34
@BenjaminBihler: That kicks the can down the road - fabs breaks for
long double for example.– Bathsheba
Apr 30 at 9:24
@BenjaminBihler: That kicks the can down the road - fabs breaks for
long double for example.– Bathsheba
Apr 30 at 9:24
6
6
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
@BenjaminBihler if you are using c++ there is normally no good reason not to use the c++ version of the functions. Keep C to C and C++ to C++.
– Kami Kaze
Apr 30 at 12:54
1
1
Correction: Use
fabs instead of abs OR std::abs which is a template function.– mid
Apr 30 at 17:30
Correction: Use
fabs instead of abs OR std::abs which is a template function.– mid
Apr 30 at 17:30
8
8
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
All compilers should warn you of a floating-point to integer, or integer to floating-point, conversion. If your compiler doesn't, and you can't adjust its warning levels accordingly, discard it immediately and get yourself a better compiler.
– Cody Gray♦
May 1 at 2:29
2
2
@Rakete1111: It's lossy in full generality. E.g. 64 bit
int, and IEEE754 double.– Bathsheba
May 1 at 12:44
@Rakete1111: It's lossy in full generality. E.g. 64 bit
int, and IEEE754 double.– Bathsheba
May 1 at 12:44
|
show 5 more comments
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14
so as you can imagine I'm a little salty just now... Annoying but worth the lecture... This will make you very sensitive about these things. ;-) (I don't mean this offensive but rather as a "heads up". Every really annoying bug might at least produce a valuable experience. So, take it positively...)
– Scheff
Apr 30 at 8:31
3
@Scheff Oh man, I certainly won't ever forget this lesson...
– Phill
Apr 30 at 8:46
5
@Scheff No. A good programming language should not have such quirks. (There's still a lesson to be learned here, but I won't tell which one ;-) )
– Marco13
Apr 30 at 10:09
1
related: developers.redhat.com/blog/2016/02/29/…
– sudo rm -rf slash
Apr 30 at 14:37
7
-Wall, fix all warnings. Always.– Ben
Apr 30 at 19:01