[cfe-dev] Intel intrinsics on clang-cl

[Markus Böck via cfe-dev]

Hello everyone!

Recently I was in the process of compiling Qt 6.1 with a wide variety
of toolchains on Windows. While doing so I was also attempting to
build the library using clang-cl, which sadly led to a build failure
due to _rdrand64_step not being declared.
Someone happened to have already reported the bug to Qt here
https://bugreports.qt.io/browse/QTBUG-88434 but they won't fix it and
rightfully so. With further research I discovered that there are many
intrinsics in immitrin.h and associate headers that are simply
disabled when using clang-cl unless explicitly enabled by various
feature macros (one's that aren't documented for clang-cl).

An example of a few of these:
_rdrand16_step, _rdrand32_step and on x86_64 _rdrand64_step are behind
the guard:

#if !(defined(_MSC_VER) || defined(__SCE__)) || __has_feature(modules) ||      \
    defined(__RDRND__)

So therefore if one wanted to use these intrinsics one would have to
define __RDRND__ on the commandline. This is contrary to how MSVC
handles it, as those are always available during compilation.

Similarly many BMI intrinsics such as _blsi_u32, _blsmsk_u32 or
_blsr_u32 are guarded behind such a header guard (except checking for
the __BMI__ macro instead).

A git log revealed that these header guards were introduced in this
commit here https://github.com/llvm/llvm-project/commit/379a1952b37247975d2df8d23498675c9c8cc730
for the sake of compile time. I think this makes a lot of sense for
the various intrinsics for AVX and similar, as these need to be
accompanied by a /arch: option on the command line anyways, but the
above intrinsics do not have such an option.

I am now wondering if it would make sense to enable intrinsics such as
the above (ones that aren't dependent on /arch: ) by default when
compiling for an MSVC target? This seems to be what MSVC users expect.
And besides, without looking at clangs headers it is currently
impossible to find out how to enable these intrinsics as it's
undocumented which defines one would have to set on the command line
to enable these.

Kind regards,

Markus