[MacPorts] #48637: python25 @2.5.6_5: clang: error: unknown argument: '-mno-fused-madd'

[MacPorts]

#48637: python25 @2.5.6_5: clang: error: unknown argument: '-mno-fused-madd'
------------------------+----------------------
  Reporter:  fhgwright  |      Owner:  larryv
      Type:  defect     |     Status:  accepted
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:  2.3.3
Resolution:             |   Keywords:
      Port:  python25   |
------------------------+----------------------
Changes (by ryandesign):

 * cc: ryandesign (added)


Comment:

 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 [http://www.network-
 theory.co.uk/docs/gccintro/gccintro_68.html 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.

--
Ticket URL: <https://trac.macports.org/ticket/48637#comment:8>
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