[llvm] [RISCV][docs] GP Relaxation and Small Data Limit (PR #108592)

[Sam Elliott via llvm-commits]

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@@ -431,3 +431,29 @@ line.  This currently applies to the following extensions:
 * ``Zvksg``
 * ``Zvksh``
 * ``Zvkt``
+
+Global Pointer (GP) Relaxation and the Small Data Limit
+=======================================================
+
+Some of the RISC-V psABI variants reserve ``gp`` (``x3``) for use as a "Global Pointer", to make generating data addresses more efficient.
+
+To use this functionality, you need to be doing all of the following:
+* Use the ``medlow`` (aka ``small``) code model;
+* Not use the ``gp`` register for any other uses (some platforms use it for the shadow stack and others as a temporary -- as denoted by the ``Tag_RISCV_x3_reg_usage`` build attribute);
+* Compile your objects with Clang's ``-mrelax`` option, to enable relaxation annotations on relocatable objects;
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lenary wrote:

Yes, it is worth mentioning. I'll directly commit this addition.

https://github.com/llvm/llvm-project/pull/108592