[llvm] [LFI][AArch64] Add AArch64 LFI rewrites for system instructions (PR #186896)

[Vitaly Buka via llvm-commits]

================
@@ -270,50 +303,51 @@ System instructions
 
 System calls are rewritten into a sequence that loads the address of the first
 runtime call entrypoint and jumps to it. The runtime call entrypoint table is
-stored at the start of the sandbox, so it can be referenced by ``x27``. The
-rewrite also saves and restores the link register, since it is used for
-branching into the runtime.
-
-+-----------------+----------------------------+
-|    Original     |         Rewritten          |
-+-----------------+----------------------------+
-| .. code-block:: | .. code-block::            |
-|                 |                            |
-|    svc #0       |    mov w26, w30            |
-|                 |    ldr x30, [x27]          |
-|                 |    blr x30                 |
-|                 |    add x30, x27, w26, uxtw |
-|                 |                            |
-+-----------------+----------------------------+
+stored at a negative offset from the sandbox base, so it can be referenced by
+``x27``. The rewrite also saves and restores the link register, since it is
+used for branching into the runtime.
+
++-----------------+------------------------------+
+|    Original     |          Rewritten           |
++-----------------+------------------------------+
+| .. code-block:: | .. code-block::              |
+|                 |                              |
+|    svc #0       |    mov x26, x30              |
+|                 |    ldur x30, [x27, #-8]      |
+|                 |    blr x30                   |
+|                 |    add x30, x27, w26, uxtw   |
+|                 |                              |
++-----------------+------------------------------+
 
 Thread-local storage
 ~~~~~~~~~~~~~~~~~~~~
 
-TLS accesses are rewritten into accesses offset from ``x25``, which is a
-reserved register that points to a virtual register file, with a location for
-storing the sandbox's thread pointer. ``TP`` is the offset into that virtual
-register file where the thread pointer is stored.
-
-+----------------------+-----------------------+
-|       Original       |       Rewritten       |
-+----------------------+-----------------------+
-| .. code-block::      | .. code-block::       |
-|                      |                       |
-|    mrs xN, tpidr_el0 |    ldr xN, [x25, #TP] |
-|                      |                       |
-+----------------------+-----------------------+
-| .. code-block::      | .. code-block::       |
-|                      |                       |
-|    mrs tpidr_el0, xN |    str xN, [x25, #TP] |
-|                      |                       |
-+----------------------+-----------------------+
+TLS accesses are rewritten into loads/stores from the context register
+(``x25``), which holds the virtual thread pointer at offset 32 (see
----------------
vitalybuka wrote:

 offset 32 here and 16 on line 115?

https://github.com/llvm/llvm-project/pull/186896