aosp12/external/harfbuzz_ng/CONFIG.md

Configuring HarfBuzz

Most of the time you will not need any custom configuration. The configuration options provided by configure or cmake should be enough. In particular, if you just want HarfBuzz library plus hb-shape / hb-view utilities, make sure FreeType and Cairo are available and found during configuration.

If you are building for distribution, you should more carefully consider whether you need Glib, ICU, Graphite2, as well as CoreText / Uniscribe / DWrite. Make sure the relevant ones are enabled.

If you are building for custom environment (embedded, downloadable app, etc) where you mostly just want to call hb_shape() and the binary size of the resulting library is very important to you, the rest of this file guides you through your options to disable features you may not need, in exchange for binary size savings.

Compiler Options

Make sure you build with your compiler's "optimize for size" option. On gcc this is -Os, and can be enabled by passing CXXFLAGS=-Os either to configure (sticky) or to make (non-sticky). On clang there is an even more extreme flag, -Oz.

HarfBuzz heavily uses inline functions and the optimize-size flag can make the library smaller by 20% or more. Moreover, sometimes, based on the target CPU, the optimize-size builds perform faster as well, thanks to lower code footprint and caching effects. So, definitely try that even if size is not extremely tight but you have a huge application. For example, Chrome does that. Note that this configuration also automatically enables certain internal optimizations. Search for HB_OPTIMIZE_SIZE for details, if you are using other compilers, or continue reading.

Another compiler option to consider is "link-time optimization", also known as 'lto'. To enable that, with gcc or clang, add -flto to both CXXFLAGS and LDFLAGS, either on configure invocation (sticky) or on make (non-sticky). This, also, can have a huge impact on the final size, 20% or more.

Finally, if you are making a static library build or otherwise linking the library into your app, make sure your linker removes unused functions. This can be tricky and differ from environment to environment, but you definitely want to make sure this happens. Otherwise, every unused public function will be adding unneeded bytes to your binary. The following pointers might come handy:

• https://lwn.net/Articles/741494/ (all of the four-part series)
• https://elinux.org/images/2/2d/ELC2010-gc-sections_Denys_Vlasenko.pdf

Combining the above three build options should already shrink your library a lot. The rest of this file shows you ways to shrink the library even further at the expense of removing functionality (that may not be needed). The remaining options are all enabled by defining pre-processor macros, which can be done via CXXFLAGS or CPPFLAGS similarly.

Unicode-functions

Access to Unicode data can be configured at compile time as well as run-time. By default, HarfBuzz ships with its own compact subset of properties from Unicode Character Database that it needs. This is a highly-optimized implementation that depending on compile settings (optimize-size or not) takes around ~40kb or ~60kb. Using this implementation (default) is highly recommended, as HarfBuzz always ships with data from latest version of Unicode. This implementation can be disabled by defining HB_NO_UCD.

For example, if you are enabling ICU as a built-in option, or GLib, those can provide Unicode data as well, so defining HB_NO_UCD might save you space without reducing functionality (to the extent that the Unicode version of those implementations is recent.)

If, however, you provide your own Unicode data to HarfBuzz at run-time by calling hb_buffer_set_unicode_funcs on every buffer you create, and you do not rely on hb_unicode_funcs_get_default() results, you can disable the internal implementation by defining both HB_NO_UCD and HB_NO_UNICODE_FUNCS. The latter is needed to guard against accidentally building a library without any default Unicode implementations.

Font-functions

Access to certain font functionalities can also be configured at run-time. By default, HarfBuzz uses an efficient internal implementation of OpenType functionality for this. This internal implementation is called hb-ot-font. All newly-created hb_font_t objects by default use hb-ot-font. Using this is highly recommended, and is what fonts use by default when they are created.

Most embedded uses will probably use HarfBuzz with FreeType using hb-ft.h. In that case, or if you otherwise provide those functions by calling hb_font_set_funcs() on every font you create, you can disable hb-ot-font without loss of functionality by defining HB_NO_OT_FONT.

Shapers

Most HarfBuzz clients use it for the main shaper, called "ot". However, it is legitimate to want to compile HarfBuzz with only another backend, eg. CoreText, for example for an iOS app. For that, you want HB_NO_OT_SHAPE. If you are going down that route, check if you want HB_NO_OT.

This is very rarely what you need. Make sure you understand exactly what you are doing.

Defining HB_NO_FALLBACK_SHAPE however is pretty harmless. That removes the (unused) "fallback" shaper.

Thread-safety

By default HarfBuzz builds as a thread-safe library. The exception is that the HB_TINY predefined configuring (more below) disables thread-safety.

If you do /not/ need thread-safety in the library (eg. you always call into HarfBuzz from the same thread), you can disable thread-safety by defining HB_NO_MT. As noted already, this is enabled by HB_TINY.

Pre-defined configurations

The hb-config.hh internal header supports three pre-defined configurations as well grouping of various configuration options. The pre-defined configurations are:

• HB_MINI: Disables shaping of AAT as well as legacy fonts. Ie. it produces a capable OpenType shaper only.

• HB_LEAN: Disables various non-shaping functionality in the library, as well as esoteric or rarely-used shaping features. See the definition for details.

• HB_TINY: Enables both HB_MINI and HB_LEAN configurations, as well as disabling thread-safety and debugging, and use even more size-optimized data tables.

Tailoring configuration

Most of the time, one of the pre-defined configuration is exactly what one needs. Sometimes, however, the pre-defined configuration cuts out features that might be desired in the library. Unfortunately there is no quick way to undo those configurations from the command-line. But one can add a header file called config-override.h to undefine certain HB_NO_* symbols as desired. Then define HAVE_CONFIG_OVERRIDE_H to make hb-config.hh include your configuration overrides at the end.

Notes

Note that the config option HB_NO_CFF, which is enabled by HB_LEAN and HB_TINY does /not/ mean that the resulting library won't work with CFF fonts. The library can shape valid CFF fonts just fine, with or without this option. This option disables (among other things) the code to calculate glyph exntents for CFF fonts.