[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
----------------
omjavaid wrote:

Here are you trying to tell the user that "program being debugged can also change vector length during execution"? so 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.

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