[llvm] [docs] Add a brief description of using -fveclib to enable some math library vectorizations (PR #119215)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon updated https://github.com/llvm/llvm-project/pull/119215

>From aec57fd0cead34b460abf2882598c5687b109184 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Mon, 9 Dec 2024 14:04:40 +0000
Subject: [PATCH] [docs] Add a brief description of using -fveclib to enable
 some math library vectorizations

Fixes #62283
---
 llvm/docs/Vectorizers.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/llvm/docs/Vectorizers.rst b/llvm/docs/Vectorizers.rst
index c2beb1669c4e3b..a4462e53edda09 100644
--- a/llvm/docs/Vectorizers.rst
+++ b/llvm/docs/Vectorizers.rst
@@ -345,6 +345,16 @@ instruction is available.
       f[i] = floorf(f[i]);
   }
 
+Many of these math functions are only vectorizable if the file has been built
+with a specified target vector library that provides a vector implemention
+of that math function. Using clang, this is handled by the "-fveclib" command
+line option with one of the following vector libraries:
+"accelerate,libmvec,massv,svml,sleef,darwin_libsystem_m,armpl,amdlibm"
+
+.. code-block:: console
+
+   $ clang ... -fno-math-errno -fveclib=libmvec file.c
+
 Partial unrolling during vectorization
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^