[CMake] [Clang] Add CMake build option to specify long double format on PowerPC

[qiucf]

Summary

This method introduces new CMake variable PPC_LINUX_DEFAULT_IEEELONGDOUBLE (false by default) to enable fp128 as default long double format.

We need to make some tests explicitly set long double type, otherwise they fail in either config.

[nemanjai]

It is probably not worth the effort since there won't be that many test cases that test the front end's IR generation for long double, but there should be a way to set up lit to know the default through its configuration files.

[nemanjai]

clang/CMakeLists.txt

240

Do we need any error checking here? What happens if someone erroneously sets this on an AIX build (or big endian Linux, FreeBSD, etc.)?

[jsji]

In D118110#3269450, @nemanjai wrote:

It is probably not worth the effort since there won't be that many test cases that test the front end's IR generation for long double, but there should be a way to set up lit to know the default through its configuration files.

Yes, that is not a must for now, but we should try to do it .

There's one thing uncertain: whether this config only applies to ppc64le?

Let us apply it to ppc64le only for now.

clang/CMakeLists.txt

240

How about CLANG_DEFAULT_IEEELONGDOUBLE_ON_PPC_LINUX similar to CLANG_DEFAULT_PIE_ON_LINUX?

clang/include/clang/Driver/ToolChain.h

413

defaultToIEEELongDouble?

clang/lib/Driver/ToolChain.cpp

113

Check OS here ?

eg: CLANG_DEFAULT_IEEELONGDOUBLE_ON_PPC_LINUX && OS.Linux()

[qiucf]

It is probably not worth the effort since there won't be that many test cases that test the front end's IR generation for long double, but there should be a way to set up lit to know the default through its configuration files.

Yes, lit doesn't know about it. But lit can't do things in IR test like #ifdef in C nor change default semantics of clang, so it seems not able to help in such cases.

[jsji]

LGTM.