[llvm] [Docs] Update coding standard for TD files (PR #200848)

[Renato Golin via llvm-commits]

https://github.com/rengolin created https://github.com/llvm/llvm-project/pull/200848

This PR proposes an update to the coding standards document to make explicit that we do not want unnecessary formatting changes to TD files.

This is in response to this merged PR (#199346), which lead to this RFC (https://discourse.llvm.org/t/80-column-limit-for-td-files/90950/).

>From d5e9cd9aaa2068b25eedd56ceb803ab1af450cba Mon Sep 17 00:00:00 2001
From: Renato Golin <rengolin at systemcall.eu>
Date: Mon, 1 Jun 2026 16:45:40 +0100
Subject: [PATCH] [Docs] Update coding standard for TD files

This PR proposes an update to the coding standards document to make explicit that we do not want unnecessary formatting changes to TD files.

This is in response to this merged PR (#199346), which lead to this RFC (https://discourse.llvm.org/t/80-column-limit-for-td-files/90950/).
---
 llvm/docs/CodingStandards.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/llvm/docs/CodingStandards.rst b/llvm/docs/CodingStandards.rst
index 12704ce606e08..43d1035eea9cd 100644
--- a/llvm/docs/CodingStandards.rst
+++ b/llvm/docs/CodingStandards.rst
@@ -463,6 +463,15 @@ for it (vs something else, like 90 columns).
 However, documentation files are not source code files, and instead of fitting into 80 columns, they must be formatted to one sentence per line.
 This way a change in the middle of a paragraph doesn't cause unnecessary changes in subsequent lines, making it easier for reviewers to see what has changed when documentation is updated.
 
+Another exception are table gen files.
+There are no tools that can parse, verify and edit them to match our coding style, so they are excluded from this limit.
+It is still good practice to try to fit them into 80 columns, but it is not a requirement.
+As with other changes, if you patch a TD file, do not reformat the entire file, but only the lines you are changing.
+
+If your reformatting is causing unnecessary changes in subsequent lines, please just follow the existing formatting.
+Unecessary changes in TD files cause unnecessary churn in the file history and force recompilation of many unnecessary files.
+It also causes unnecessary changes in forks working on the same TD files, which makes it harder to rebase and merge later.
+
 Whitespace
 ^^^^^^^^^^