[llvm] [lit][docs] Document %if conditional substitution in lit CommandGuide (PR #225683)

[Ingo Müller via llvm-commits]

https://github.com/ingomueller-net updated https://github.com/llvm/llvm-project/pull/225683

>From 85986f64eb467901eaedced6589f0f24e96d0df3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ingo=20M=C3=BCller?= <ingomueller at google.com>
Date: Wed, 23 Sep 2026 13:02:41 +0200
Subject: [PATCH] [lit][docs] Document %if conditional substitution in lit
 CommandGuide.
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`%if ... %else` conditional substitutions (implemented in `TestRunner.py`
and mentioned in `TestingGuide.md`) were missing from the `SUBSTITUTIONS`
table and `available_features` description in `llvm/docs/CommandGuide/lit.md`.
Add `%if` to the base substitutions table and note that `available_features`
can be used in `%if` conditions.

Signed-off-by: Ingo Müller <ingomueller at google.com>
---
 llvm/docs/CommandGuide/lit.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/docs/CommandGuide/lit.md b/llvm/docs/CommandGuide/lit.md
index f652110bfc2cbb..43d664942d532b 100644
--- a/llvm/docs/CommandGuide/lit.md
+++ b/llvm/docs/CommandGuide/lit.md
@@ -606,7 +606,7 @@ executed, two important global variables are predefined:
 
   **available_features**
   : A set of features that can be used in `XFAIL`, `REQUIRES`, and `UNSUPPORTED`
-    directives.
+    directives and `%if` substitutions.
 
 ### TEST DISCOVERY
 
@@ -679,6 +679,7 @@ TestRunner.py:
 | %:p                     | On Windows, %/p but a `:` is removed if its the second character. Otherwise, %p but with a single leading `/` removed. |
 | %:t                     | On Windows, %/t but a `:` is removed if its the second character. Otherwise, %t but with a single leading `/` removed. |
 | %{readfile:\<filename>} | Reads the file specified.                                                                                              |
+| %if \<cond> %{...%}     | Conditional substitution (`%if <cond> %{<if-branch>%} [%else %{<else-branch>%}]`); `<cond>` uses `available_features`. |
 
 Other substitutions are provided that are variations on this base set and
 further substitution patterns can be defined by each test module. See the