[clang] [AVX10][Doc] Add documentation about AVX10 options and their attentions (PR #77925)

[Evgenii Kudriashov via cfe-commits]

================
@@ -3963,6 +3963,60 @@ implicitly included in later levels.
 - ``-march=x86-64-v3``: (close to Haswell) AVX, AVX2, BMI1, BMI2, F16C, FMA, LZCNT, MOVBE, XSAVE
 - ``-march=x86-64-v4``: AVX512F, AVX512BW, AVX512CD, AVX512DQ, AVX512VL
 
+`Intel AVX10 ISA <https://cdrdv2.intel.com/v1/dl/getContent/784267>`_ is
+a major new vector ISA incorporating the modern vectorization aspects of
+Intel AVX-512. This ISA will be supported on all future Intel processor.
+Users are supposed to use the new options ``-mavx10.N`` and ``-mavx10.N-512``
+on these processors and should not use traditional AVX512 options anymore.
+
+The ``N`` in ``-mavx10.N`` represents a continuous integer number starting
+from ``1``. ``-mavx10.N`` is an alias of ``-mavx10.N-256``, which means to
+enable all instructions within AVX10 version N at a maximum vector length of
+256 bits. ``-mavx10.N-512`` enables all instructions at a maximum vector
+length of 512 bits, which is a superset of instructions ``-mavx10.N`` enabled.
+
+Current binaries built with AVX512 features can run on Intel AVX10/512 capable
+processor without re-compile, but cannot run on AVX10/256 capable processor.
+Users need to re-compile their code with ``-mavx10.N``, and maybe update some
+code that calling to 512-bit X86 specific intrinsics and passing or returning
+512-bit vector types in function call, if they want to run on AVX10/256 capable
+processor. Binaries built with ``-mavx10.N`` can run on both AVX10/256 and
+AVX10/512 capable processor.
+
+Users can add a ``-mno-evex512`` in the command line with AVX512 options if
+they want to run the binary on both legacy AVX512 and new AVX10/256 capable
+processors. The option has the same constraints as ``-mavx10.N``, i.e.,
+cannot call to 512-bit X86 specific intrinsics and pass or return 512-bit vector
+types in function call.
+
+Users should avoid using AVX512 features in function target attributes when
+developing code for AVX10. If they have to do so, they need to add an explicit
+``evex512`` or ``no-evex512`` together with AVX512 features for 512-bit or
+non-512-bit functions respectively to avoid unexpected code generation. Both
+command line option and target attribute of EVEX512 feature can only be used
+with AVX512. They don't affect vector size of AVX10.
+
+User should not mix the use AVX10 and AVX512 options together at any time,
+because the option combinations are conflicting sometimes. For example, a
+combination of ``-mavx512f -mavx10.1-256`` doesn't show a clear intention to
+compiler, since instructions in AVX512F and AVX10.1/256 intersect but do not
+overlap. In this case, compiler will emit warning for it, but the behavior
+is determined. It will generate the same code as option ``-mavx10.1-512``.
+A similar case is ``-mavx512f -mavx10.2-256``, which equals to
+``-mavx10.1-512 -mavx10.2-256``, because ``avx10.2-256`` implies ``avx10.1-256``
+and ``-mavx512f -mavx10.1-256`` equals to ``-mavx10.1-512``.
+
+There are some new macros introduced with AVX10 support. ``-mavx10.1-256`` will
+enable ``__AVX10_1__`` and ``__EVEX256__``, while ``-mavx10.1-512`` enables
+``__AVX10_1__``, ``__EVEX256__``, ``__EVEX512__``  and ``__AVX10_1_512__``.
+Besides, both ``-mavx10.1-256`` and ``-mavx10.1-512`` will enable all AVX512
+feature specific macros. A AVX512 feature will enable both ``__EVEX256__``,
+``__EVEX512__`` and its own macro. So ``__EVEX512__`` can be used to guard code
+that can run on both legacy AVX512 and AVX10/512 capable processors but cannot
+run on AVX10/256, while a AVX512 macro like ``__AVX512F__`` cannot tell the
+difference among the three options. Users need to check additional macros
+``__AVX10_1__`` and ``__EVEX512__`` if they want to make distinction.
----------------
e-kud wrote:

My idea was about scenario when you've migrated your sources to avx10 (e.g. using EVEX512 instead of AVX512F) but still have CI with different compilers that does not support it. I thought that it may be useful to provide some guidance. But I'm OK to leave it up to users. Whether they decide to specify versions or avx10 support availability.

https://github.com/llvm/llvm-project/pull/77925