[Docs] Add documentation for llvm-remark-size-diff

Add documentation describing how to

- Use `llvm-remark-size-diff`
- Interpret the output from the tool

Differential Revision: https://reviews.llvm.org/D122744

diff --git a/llvm/docs/CommandGuide/llvm-remark-size-diff.rst b/llvm/docs/CommandGuide/llvm-remark-size-diff.rst
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+llvm-remark-size-diff - diff size remarks
+=========================================
+
+.. program:: llvm-remark-size-diff
+
+SYNOPSIS
+--------
+
+:program:`llvm-remark-size-diff` [*options*] *file_a* *file_b* **--parser** *parser*
+
+DESCRIPTION
+-----------
+
+:program:`llvm-remark-size-diff` diffs size
+`remarks <https://llvm.org/docs/Remarks.html>`_ in two remark files: ``file_a``
+and ``file_b``.
+
+:program:`llvm-remark-size-diff` can be used to gain insight into which
+functions were impacted the most by code generation changes.
+
+In most common use-cases ``file_a`` and ``file_b`` will be remarks output by
+compiling a **fixed source** with **differing compilers** or
+**differing optimization settings**.
+
+:program:`llvm-remark-size-diff` handles both
+`YAML <https://llvm.org/docs/Remarks.html#yaml-remarks>`_ and
+`bitstream <https://llvm.org/docs/Remarks.html#llvm-bitstream-remarks>`_
+remarks.
+
+OPTIONS
+-------
+
+.. option:: --parser=<yaml|bitstream>
+
+  Select the type of input remark parser. Required.
+  * ``yaml``: The tool will parse YAML remarks.
+  * ``bitstream``: The tool will parse bitstream remarks.
+
+.. option:: --report-style=<human|json>
+
+  Output style.
+  * ``human``: Human-readable textual report. Default option.
+  * ``json``: JSON report.
+
+.. option:: --pretty
+
+  Pretty-print JSON output. Optional.
+
+  If output is not set to JSON, this does nothing.
+
+.. option:: -o=<file>
+
+  Output file for the report. Outputs to stdout by default.
+
+HUMAN-READABLE OUTPUT
+---------------------
+
+The human-readable format for :program:`llvm-remark-size-diff` is composed of
+two sections:
+
+* Per-function changes.
+* A high-level summary of all changes.
+
+Changed Function Section
+########################
+
+Suppose you are comparing two remark files OLD and NEW.
+
+For each function with a **changed instruction count** in OLD and NEW,
+:program:`llvm-remark-size-diff` will emit a line like below:
+
+::
+
+  (++|--|==) (>|<) function_name, N instrs, M stack B
+
+A breakdown of the format is below:
+
+``(++|--|==)``
+  Which of OLD and NEW the ``function_name`` is present in.
+
+  * ``++``: Only in NEW. ("Added")
+  * ``--``: Only in OLD. ("Removed")
+  * ``==``: In both.
+
+``(>|<)``
+  Denotes if ``function_name`` has more instructions or fewer instructions in
+  the second file.
+
+  *  ``>``: More instructions in second file than first file.
+  *  ``<``: Fewer instructions in second file than in first file.
+
+``function_name``
+  The name of the changed function.
+
+``N instrs``
+  Second file instruction count - first file instruction count.
+
+``M stack B``
+  Second file stack byte count - first file stack byte count.
+
+Summary Section
+###############
+
+:program:`llvm-remark-size-diff` will output a high-level summary after
+printing all changed functions.
+
+::
+
+  instruction count: N (inst_pct_change%)
+  stack byte usage: M (sb_pct_change%)
+
+``N``
+  Sum of all instruction count changes between the second and first file.
+
+``inst_pct_change%``
+  Percent increase or decrease in instruction count between the second and first
+  file.
+
+``M``
+  Sum of all stack byte count changes between the second and first file.
+
+``sb_pct_change%``
+  Percent increase or decrease in stack byte usage between the second and first
+  file.
+
+JSON OUTPUT
+-----------
+
+High-Level view
+###############
+
+Suppose we are comparing two files, OLD and NEW.
+
+:program:`llvm-remark-size-diff` will output JSON as follows.
+
+::
+
+  "Files": [
+    "A": "path/to/OLD",
+    "B": "path/to/NEW"
+  ]
+
+  "InBoth": [
+    ...
+  ],
+
+  "OnlyInA": [
+    ...
+  ],
+
+  "OnlyInB": [
+    ...
+  ]
+
+
+``Files``
+  Original paths to remark files.
+
+  * ``A``: Path to the first file.
+  * ``B``: Path to the second file.
+
+``InBoth``
+  Functions present in both files.
+
+``OnlyInA``
+  Functions only present in the first file.
+
+``OnlyInB``
+  Functions only present in the second file.
+
+Function JSON
+#############
+
+The ``InBoth``, ``OnlyInA``, and ``OnlyInB`` sections contain size information
+for each function in the input remark files.
+
+::
+
+  {
+    "FunctionName" : "function_name"
+    "InstCount": [
+        INST_COUNT_A,
+        INST_COUNT_B
+      ],
+    "StackSize": [
+        STACK_BYTES_A,
+        STACK_BYTES_B
+      ],
+  }
+
+``FunctionName``
+  Name of the function.
+
+``InstCount``
+  Instruction counts for the function.
+
+  * ``INST_COUNT_A``: Instruction count in OLD.
+  * ``INST_COUNT_B``: Instruction count in NEW.
+
+``StackSize``
+  Stack byte counts for the function.
+
+  * ``STACK_BYTES_A``: Stack bytes in OLD.
+  *  ``STACK_BYTES_B``: Stack bytes in NEW.
+
+Computing Diffs From Function JSON
+**********************************
+
+Function JSON does not contain the diffs. Tools consuming JSON output from
+:program:`llvm-remark-size-diff` are responsible for computing the diffs
+separately.
+
+**To compute the diffs:**
+
+* Instruction count diff: ``INST_COUNT_B - INST_COUNT_A``
+* Stack byte count diff: ``STACK_BYTES_B - STACK_BYTES_A``
+
+EXIT STATUS
+-----------
+
+:program:`llvm-remark-size-diff` returns 0 on success, and a non-zero value
+otherwise.