[FFmpeg-devel] [PATCH] x86: cabac: Disable the inline asm on clang on windows on i386

[Martin Storsjö]

On Sun, 24 May 2020, Carl Eugen Hoyos wrote:

> Am So., 24. Mai 2020 um 21:53 Uhr schrieb Martin Storsjö <martin at martin.st>:
>
>> configure --enable-gpl --arch=i686 --cc=clang-cl --ld=lld-link
>> --target-os=win32 --toolchain=msvc --enable-cross-compile --ar=llvm-ar
>> --nm=llvm-nm --disable-stripping --extra-cflags=-m32
>
> Why are you cross-compiling?
> On which system are you testing this?

Because I normally only ever cross compile for windows.

There seems to be a subtle difference in this case, between using clang-cl 
and "clang -target i686-win32-msvc"; clang-cl passes "-mdisable-fp-elim" 
to the compiler internals, while "clang -target i686-win32-msvc" doesn't.

And cross compiling does seem to affect this particular case; there's a 
check for whether ebp is available, which requires running the built 
executable. When cross compiling, it's assumed the built executable is ok 
and it isn't test run.

So with clang-cl running on windows, you'll end up with "#define 
HAVE_EBP_AVAILABLE 0" in config.h, and the x86 cabac code won't end up 
used at all - i.e. this patch wouldn't make any difference.

With clang-cl in cross compilation, and other invocations of "clang" 
instead of "clang-cl", targeting i686 windows, you'll end up with 
ebp_available enabled.

For a case where it does make a difference, in the same setup, try 
configuring this way instead:

--cc='clang -target i686-win32-msvc' --ld=lld-link 
--extra-ldflags='msvcrt.lib oldnames.lib' --toolchain=msvc

In this case, the ebp_available test will succeed, it will proceed to 
trying to build the x86 inline cabac code, which fails.

// Martin