Issue 13061: Decimal module yields incorrect results when Python compiled with clang

When Python is compiled on OS X with llvm, the decimal module behaves erratically (simple calculations are wrong, most doctests fail). This was originally reported here:

http://stackoverflow.com/questions/7590137/dividing-decimals-yields-invalid-results-in-python-2-5-to-2-7

and the compiler dependency was tracked down here:

https://trac.macports.org/ticket/31444

Although it looks like MacPorts is going to force use of gcc, it'd still be better for Python to behave correctly when compiled with llvm.

Possibly related to http://bugs.python.org/issue11149?

> Possibly related to http://bugs.python.org/issue11149?

Maybe I missed it in the links you gave, but that is easily
settled by compiling with and without -fwrapv.

>> Possibly related to http://bugs.python.org/issue11149?

> Maybe I missed it in the links you gave, but that is easily
> settled by compiling with and without -fwrapv.

Apologies -- I didn't do enough homework on this one. Yes, I can confirm that compiling with -fwrapv solves this issue. Not sure whether that makes this bug a straight duplicate of http://bugs.python.org/issue11149...

It's more a straight duplicate of #12973, but the underlying cause
(signed integer overflow) is the same. For people who are finding
this via a search engine: A lot of bugs have been fixed in #12973,
but even if the test suite passes without -fwrapv it is *still* recommended to use -fwrapv.


Could anyone test if the attached patch works for llvm-gcc?

There is some confusion here, I think. As best as I can tell, the original problem reported in the Stackoverflow question and to MacPorts involved using the clang compiler, not llvm-gcc and the Decimal problem reported here was only ever a problem when using clang.  It's not an issue when using Apple's llvm-gcc compiler because the pre-#11149 configure test works properly with llvm-gcc.  That test looks for "-fwrapv" in the compiler help string:

$ llvm-gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ llvm-gcc -v --help 2>/dev/null | grep -- -fwrapv
  -fwrapv                     Assume signed arithmetic overflow wraps around
$ gcc-4.2 --version
i686-apple-darwin11-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc-4.2 -v --help 2>/dev/null | grep -- -fwrapv
  -fwrapv                     Assume signed arithmetic overflow wraps around
$ clang --version
Apple clang version 2.1 (tags/Apple/clang-163.7.1) (based on LLVM 3.0svn)
Target: x86_64-apple-darwin11.1.0
Thread model: posix
$ clang -v --help 2>/dev/null | grep -- -fwrapv

The above was using the 3 Apple-supplied compilers with the OS X 10.7 (Lion) version of Xcode 4.1 (Build version 4B110).  The 10.6 compiler versions should work the same way.

So this issue should be closed as a duplicate of Issue11149.  I'm setting it to pending and will close it if there are no objections.