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Why is `abs()` implemented differently?

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53

4

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

share|improve this question

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

53

4

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

share|improve this question

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

share|improve this question

edited Apr 30 at 20:27

Bathsheba

184k27261388

asked Apr 30 at 8:10

PhillPhill

45549

abs(-1.1341234) = 1

abs(-1.1341234) = 1.1341234

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) 

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

share|improve this question

edited Apr 30 at 20:27

Bathsheba

184k27261388

asked Apr 30 at 8:10

PhillPhill

45549

share|improve this question

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

asked Apr 30 at 8:10

PhillPhill

45549

asked Apr 30 at 8:10

PhillPhill

45549

1 Answer
1

active

oldest

votes

83

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.

share|improve this answer

edited Apr 30 at 9:07

answered Apr 30 at 8:14

BathshebaBathsheba

184k27261388

• 
  
  
  

  
  34
  

  
  
  
  
  
  

  
  
  @BenjaminBihler: That kicks the can down the road - fabs breaks for long double for 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: Use fabs instead of abs OR std::abs which 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 bit int, and IEEE754 double.
  
  – Bathsheba
  May 1 at 12:44
  

  
  
  
  

 | 
show 5 more comments

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83

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.

share|improve this answer

edited Apr 30 at 9:07

answered Apr 30 at 8:14

BathshebaBathsheba

184k27261388

• 
  
  
  

  
  34
  

  
  
  
  
  
  

  
  
  @BenjaminBihler: That kicks the can down the road - fabs breaks for long double for 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: Use fabs instead of abs OR std::abs which 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 bit int, and IEEE754 double.
  
  – Bathsheba
  May 1 at 12:44
  

  
  
  
  

 | 
show 5 more comments

83

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.

share|improve this answer

edited Apr 30 at 9:07

answered Apr 30 at 8:14

BathshebaBathsheba

184k27261388

• 
  
  
  

  
  34
  

  
  
  
  
  
  

  
  
  @BenjaminBihler: That kicks the can down the road - fabs breaks for long double for 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: Use fabs instead of abs OR std::abs which 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 bit int, and IEEE754 double.
  
  – 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.

share|improve this answer

edited Apr 30 at 9:07

answered Apr 30 at 8:14

BathshebaBathsheba

184k27261388

share|improve this answer

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