[llvm] [lldb][AArch64] Add release notes and documentation for SME (PR #66767)

[Omair Javaid via llvm-commits]

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+Using LLDB On AArch64 Linux
+===========================
+
+This page explains the details of debugging certain AArch64 extensions using
+LLDB. If something is not mentioned here, it likely works as you would expect.
+
+This is not a replacement for ptrace and Linux Kernel documentation. This covers
+how LLDB has chosen to use those things and how that effects your experience as
+a user.
+
+Scalable Vector Extension (SVE)
+-------------------------------
+
+See `here <https://developer.arm.com/Architectures/Scalable%20Vector%20Extensions>`__
+to learn about the extension and `here <https://kernel.org/doc/html/latest/arch/arm64/sve.html>`__
+for the Linux Kernel's handling of it.
+
+In LLDB you will be able to see the following new registers:
+
+* ``z0-z31`` vector registers, each one has size equal to the vector length.
+* ``p0-p15`` predicate registers, each one containing 1 bit per byte in the vector
+  length. Making each one vector length / 8 sized.
+* ``ffr`` the first fault register, same size as a predicate register.
+* ``vg``, the vector length in "granules". Each granule is 8 bytes.
+
+.. code-block::
+
+       Scalable Vector Extension Registers:
+             vg = 0x0000000000000002
+             z0 = {0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01 <...> }
+           <...>
+             p0 = {0xff 0xff}
+           <...>
+            ffr = {0xff 0xff}
+
+The example above has a vector length of 16 bytes. Within LLDB you will always
+see "vg" as in the ``vg`` register, which is 2 in this case (8*2 = 16).
+Elsewhere you may see "vq" which is the vector length in quadwords (16 bytes)
+elsewhere. Where you see "vl", it is in bytes.
+
+Changing the Vector Length
+..........................
+
+While you can count the size of a P or Z register, it is intended that ``vg`` be
+used to find the current vector length.
+
+vg can be written. Writing the current vector length changes nothing. If you
+increase the vector length, the registers will likely be reset to 0. If you
+decrease it, LLDB will truncate the Z registers but everything else will be reset
+to 0.
+
+Generally you should not assume that SVE state after changing the vector length
+is in any way the same as it was previously. If you need to do it, do it before
+a function's first use of SVE.
+
+Z Register Presentation
+.......................
+
+LLDB makes no attempt to predict how an SVE Z register will be used. Even if the
----------------
omjavaid wrote:

LLDB makes no attempt to predict how an SVE Z register should be visualized. 

https://github.com/llvm/llvm-project/pull/66767