bpo-30104: Compile dtoa.c without strict aliasing

[vstinner]

dtoa.c uses union to cast double to unsigned long[2]. clang 4.0 with
-O2 or higher and strict aliasing miscompiles the ratio() function
causing rounding issues. Always disable strict aliasing on dtoa.c to
prevent any risk of bug caused by optimizations.

[vstinner]

I tested manually the patch on Linux using clang 4.0: dtoa.c is compiled with -fno-strict-aliasing, test_strtod test pass, and only dtoa.c is compiled with -fno-strict-aliasing so other C optimized are optimized with strict aliasing rules.

[vstinner]

The main question is if the -no-strict-aliasing option is supported by all UNIX and BSD compilers? What about Solaris and HP-UX compilers?

I propose to push the change and fix it later if someone complains. What do you think?

[vstinner]

cc @DimitryAndric

[vstinner]

I don't know well autotools, so I also tested to compile CPython from a different directory:

cd # $HOME
mkdir build_cpython
cd build_cpython
 ~/prog/python/master/configure CC=/opt/llvm-4.0.0/bin/clang
make
./python -m test -v test_strtod

dtoa.o is written into build_cpython/Python, not in ~/prog/python/master/Python: it works :-)

[vstinner]

The main question is if the -no-strict-aliasing option is supported by all UNIX and BSD compilers? What about Solaris and HP-UX compilers?

I chose to add the flag for any C compiler. If you think that a C compiler may not support that option, we can start by only adding the option on clang, and maybe add it on other C compilers if someone complains. configure is now able to detect correctly the clang compiler.

[DimitryAndric]

Originally, -fstrict-aliasing and -fno-strict-aliasing were gcc specific optimization flags (see https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fstrict-aliasing). But of course, other compilers that are meant as drop-in replacements for gcc, such as Intel's icc and LLVM's clang, support this flag to, with the same meaning.

However, there are definitely compilers out there which will not support this flag, for example the Microsoft C/C++ compiler. Does this commit affect the flags passed to Microsoft C/C++?

[vstinner]

Does this commit affect the flags passed to Microsoft C/C++?

No, we use the PCbuild directory for Microsoft Visual Studio and its C compiler. It's a completely different build system.

However, there are definitely compilers out there which will not support this flag,

Do you suggest to first only define the flag on clang?

[DimitryAndric]

Do you suggest to first only define the flag on clang?

Yes, that should the most minimal way of fixing the original issue, since gcc itself won't need it.

[vstinner]

Yes, that should the most minimal way of fixing the original issue, since gcc itself won't need it.

Ok, done. The new change now only impacts clang and only use -fno-strict-aliasing on dtoa.c to optimize correctly all other C files using strict aliasing, as decided in bpo-30124.

[vstinner]

I tested that clang uses -fno-strict-aliasing but only on dtoa.c and gcc never uses -fno-strict-aliasing.