python25 @2.5.6_5: clang: error: unknown argument: '-mno-fused-madd'

[fhgwright (Fred Wright)]

An "upgrade outdated" attempted to rebuild python25, but the build failed with a bunch of errors from clang like:

:info:build clang: error: unknown argument: '-mno-fused-madd'

Given the way Python changes from one version to the next, it's useful to keep older versions around for testing.

Attaching full log.

[fhgwright (Fred Wright)]

Logfile from failed build

[ryandesign (Ryan Carsten Schmidt)]

What version of clang? (run /usr/bin/clang -v)

[fhgwright (Fred Wright)]

Replying to ryandesign@…:

What version of clang? (run /usr/bin/clang -v)

MacPro:~ fw$ which -a clang
/Library/Developer/CommandLineTools/usr/bin/clang
/usr/bin/clang
MacPro:~ fw$ for c in $(which -a clang); do $c -v; done
Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
Thread model: posix
Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
Thread model: posix

[snarkhunter (Steve Langer)]

Cc Me!

[mmpagani (Marco Pagani)]

Cc Me!

[petrrr]

Cc Me!

[larryv (Lawrence Velázquez)]

Relevant upstream bug report: ​http://bugs.python.org/issue1779871

[ryandesign (Ryan Carsten Schmidt)]

The problem is that the -mno-fused-madd flag is unknown to clang, and as of clang 3.4 from Xcode 5.1, clang considers unknown flags to be an error.

Here's the commit that fixed that upstream bug:

​https://github.com/python/cpython/commit/1b4e45bab940d385386e9442de5f5fbc7983dd50

It just removed -Wno-long-double -no-cpp-precomp -mno-fused-madd from $BASECFLAGS for python 2.6 and later.

MacPorts already removed -Wno-long-double -no-cpp-precomp from $BASECFLAGS in [2675b181e576879eb901c6e5c125a2e8494a92b4/macports-ports] for python24 and [2c612b374253451055485ad1ad2b640c8aab17be/macports-ports] for python25, apparently for the benefit of gcc 4.2 on Snow Leopard.

It looks like we discovered way back in #2029 that -mno-fused-madd is a PowerPC-only option, and removed it only for non-PowerPC systems in [1f2207ac14f5a209fcb73354eb0f6594b5f7817f/macports-ports] for python23, and we have the same patch in python24. That patch was never added to python25 however.

What does the option do? Here's a description from ​An Introduction to GCC:

The POWER/PowerPC processors include a combined "multiply and add" instruction a * x + b, which performs the two operations simultaneously for speed--this is referred to as a fused multiply and add, and is used by GCC by default. Due to differences in the way intermediate values are rounded, the result of a fused instruction may not be exactly the same as performing the two operations separately. In cases where strict IEEE arithmetic is required, the use of the combined instructions can be disabled with the option -mno-fused-madd.

So on the theory that this option may do something important for PowerPC systems, I'll fix the issue by doing for python25 what we already do for python24 and did for python23 before it was deleted and keep the option for PowerPC systems.

[ryandesign (Ryan Carsten Schmidt)]

In 3d4f53db83d3c702ac48e8247ae42883a19a1e4b/macports-ports:

python25: Don't use -mno-fused-madd on non-PowerPC systems

Closes: #48637