[llvm] [llvm][docs] Add example use of `llvm-reduce`. (PR #73237)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-binary-utilities

Author: Francesco Petrogalli (fpetrogalli)

<details>
<summary>Changes</summary>

The script is based on an exmaple provided by Tim
Northover (https://github.com/TNorthover).

---
Full diff: https://github.com/llvm/llvm-project/pull/73237.diff


1 Files Affected:

- (modified) llvm/docs/CommandGuide/llvm-reduce.rst (+29) 


``````````diff
diff --git a/llvm/docs/CommandGuide/llvm-reduce.rst b/llvm/docs/CommandGuide/llvm-reduce.rst
index cd49ceba2d6b6e8..b03784696d55786 100644
--- a/llvm/docs/CommandGuide/llvm-reduce.rst
+++ b/llvm/docs/CommandGuide/llvm-reduce.rst
@@ -105,4 +105,33 @@ EXIT STATUS
 :program:`llvm-reduce` returns 0 under normal operation. It returns a non-zero
 exit code if there were any errors.
 
+EXAMPLE
+-------
 
+`llvm-reduce` can be used to simplify a test that causes a compiler
+crash.
+
+For example, let's assume that `opt` is crashing on the IR file
+`test.ll` with error message `Assertion failed at line 1234 of
+WhateverFile.cpp`, when running at `-O2`.
+
+The test case of `test.ll` can be reduced by invoking the following
+command:
+
+.. code-block:: bash
+
+   $(LLVM_BUILD_FOLDER)/bin/llvm-reduce --test=script.sh <path to>/test.ll
+
+The shell script passed to the option `test` consists of the
+following:
+
+.. code-block:: bash
+
+   $(LLVM_BUILD_FOLDER)/bin/opt -O2 -S $1 -o /dev/null \
+     |& grep "Assertion failed at line 1234 of WhateverFile.cpp"
+
+(In this script, `grep` exits with 0 if it finds the string and that
+becomes the whole script's status.)
+
+This example can be generalized to other tools that process IR files,
+for example `llc`.

``````````

</details>


https://github.com/llvm/llvm-project/pull/73237