Add SVS binary size comparison demo and documentation (#4777)

Summary:
Pull Request resolved: #4777

Add a demo target `demo_sift1M_svs` to compare binary sizes between faiss (without SVS) and faiss_svs (with SVS).

Also add documentation explaining the methodology for binary size comparison:
- Without SVS: 33 MB
- With SVS: 41 MB
- Savings: 8.24 MB (20% reduction)

Reviewed By: mdouze

Differential Revision: D91717353

fbshipit-source-id: f9a8298e8ae6c77644e3c2b1b7d51533e5a0abbc

Author: junjieqi

faiss/docs/svs_binary_size_comparison.md

@@ -0,0 +1,114 @@
+# Faiss SVS Binary Size Comparison
+
+## Overview
+
+This document describes the methodology for comparing binary sizes between the default `faiss` target (without SVS) and the `faiss_svs` target (with SVS support).
+
+## Background
+
+SVS (Scalable Vector Search) is an optional feature in Faiss that adds additional index implementations. Including SVS increases binary size, so it was separated into an opt-in target to reduce binary bloat for users who don't need SVS functionality.
+
+## Targets
+
+| Target | Description |
+|--------|-------------|
+| `//faiss:faiss` | Default Faiss library without SVS |
+| `//faiss:faiss_svs` | Faiss library with SVS support |
+| `//faiss:pyfaiss` | Python bindings without SVS |
+| `//faiss:pyfaiss_svs` | Python bindings with SVS |
+
+## Methodology
+
+### Why `demo_sift1M`?
+
+The `demo_sift1M` binary is used for comparison because it uses `index_factory`, which creates indexes dynamically from string descriptions at runtime. This means:
+
+- The linker cannot determine at link time which index implementations will be used
+- All index implementations must be included in the binary
+- This provides a worst-case (maximum) binary size measurement
+
+### Build Commands
+
+```bash
+# Build without SVS
+buck2 build @mode/opt fbcode//faiss/demos:demo_sift1M --show-full-output
+
+# Build with SVS
+buck2 build @mode/opt fbcode//faiss/demos:demo_sift1M_svs --show-full-output
+```
+
+### Size Comparison
+
+```bash
+# Check sizes
+ls -lh <path_to_demo_sift1M>
+ls -lh <path_to_demo_sift1M_svs>
+
+# Verify no SVS symbols in default build
+nm <path_to_demo_sift1M> | grep -i svs
+```
+
+## Results
+
+| Configuration | Binary Size |
+|---------------|-------------|
+| Without SVS (`faiss`) | 33 MB |
+| With SVS (`faiss_svs`) | 41 MB |
+| **Difference** | **8.24 MB (20% reduction)** |
+
+## Files Changed
+
+The following files were modified to separate SVS from the default Faiss build:
+
+### `faiss/xplat.bzl`
+- Separated SVS source and header files into dedicated functions:
+  - `svs_header_files()` - SVS header files
+  - `svs_source_files()` - SVS source files
+
+### `faiss/BUCK`
+- Default `faiss` target no longer includes SVS
+- Added new `faiss_svs` target with SVS support
+- Updated `faiss_no_multithreading` and `faiss_omp_mock` to exclude SVS
+
+### `faiss/python/defs.bzl`
+- Added `with_svs` parameter to `pyfaiss_binary()` macro
+
+### `faiss/python/BUCK`
+- Added `pyfaiss_svs` target
+
+### `faiss/fbcode.bzl`
+- Updated `pyfaiss_libraries()` to include SVS variant
+
+### `faiss/tests/BUCK`
+- Updated SVS tests to use `faiss_svs` and `pyfaiss_svs`
+
+### `faiss/demos/BUCK`
+- Added `demo_sift1M_svs` for binary size comparison
+
+## Usage
+
+### For users who need SVS
+
+Replace dependencies on `//faiss:faiss` with `//faiss:faiss_svs`:
+
+```python
+# BUCK file
+cpp_binary(
+    name = "my_binary",
+    srcs = ["main.cpp"],
+    deps = ["//faiss:faiss_svs"],  # Use faiss_svs for SVS support
+)
+```
+
+For Python:
+```python
+python_binary(
+    name = "my_script",
+    srcs = ["main.py"],
+    deps = ["//faiss:pyfaiss_svs"],  # Use pyfaiss_svs for SVS support
+)
+```
+
+### For users who don't need SVS
+
+No changes needed - the default `//faiss:faiss` and `//faiss:pyfaiss` targets now exclude SVS automatically.