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docs/GypVsCMake.md

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+# GYP vs. CMake #
+
+GYP was originally created to generate native IDE project files (Visual Studio, Xcode) for building [Chromium](http://www.chromim.org).
+
+The functionality of GYP is very similar to the [CMake](http://www.cmake.org)
+build tool.  Bradley Nelson wrote up the following description of why the team
+created GYP instead of using CMake.  The text below is copied from
+http://www.mail-archive.com/webkit-dev@lists.webkit.org/msg11029.html
+
+```
+
+Re: [webkit-dev] CMake as a build system?
+Bradley Nelson
+Mon, 19 Apr 2010 22:38:30 -0700
+
+Here's the innards of an email with a laundry list of stuff I came up with a
+while back on the gyp-developers list in response to Mike Craddick regarding
+what motivated gyp's development, since we were aware of cmake at the time
+(we'd even started a speculative port):
+
+
+I did an exploratory port of portions of Chromium to cmake (I think I got as
+far as net, base, sandbox, and part of webkit).
+There were a number of motivations, not all of which would apply to other
+projects. Also, some of the design of gyp was informed by experience at
+Google with large projects built wholly from source, leading to features
+absent from cmake, but not strictly required for Chromium.
+
+1. Ability to incrementally transition on Windows. It took us about 6 months
+to switch fully to gyp. Previous attempts to move to scons had taken a long
+time and failed, due to the requirement to transition while in flight. For a
+substantial period of time, we had a hybrid of checked in vcproj and
+gyp generated
+vcproj. To this day we still have a good number of GUIDs pinned in the
+gyp files,
+because different parts of our release pipeline have leftover assumptions
+regarding manipulating the raw sln/vcprojs. This transition occurred from
+the bottom up, largely because modules like base were easier to convert, and
+had a lower churn rate. During early stages of the transition, the majority
+of the team wasn't even aware they were using gyp, as it integrated into
+their existing workflow, and only affected modules that had been converted.
+
+2. Generation of a more 'normal' vcproj file. Gyp attempts, particularly on
+Windows, to generate vcprojs which resemble hand generated projects. It
+doesn't generate any Makefile type projects, but instead produces msvs
+Custom Build Steps and Custom Build Rules. This makes the resulting projects
+easier to understand from the IDE and avoids parts of the IDE that simply
+don't function correctly if you use Makefile projects. Our early hope with
+gyp was to support the least common denominator of features present in each
+of the platform specific project file formats, rather than falling back on
+generated Makefiles/shell scripts to emulate some common abstraction. CMake by
+comparison makes a good faith attempt to use native project features, but
+falls back on generated scripts in order to preserve the same semantics on
+each platforms.
+
+3. Abstraction on the level of project settings, rather than command line
+flags. In gyp's syntax you can add nearly any option present in a hand
+generated xcode/vcproj file. This allows you to use abstractions built into
+the IDEs rather than reverse engineering them possibly incorrectly for
+things like: manifest generation, precompiled headers, bundle generation.
+When somebody wants to use a particular menu option from msvs, I'm able to
+do a web search on the name of the setting from the IDE and provide them
+with a gyp stanza that does the equivalent. In many cases, not all project
+file constructs correspond to command line flags.
+
+4. Strong notion of module public/private interface. Gyp allows targets to
+publish a set of direct_dependent_settings, specifying things like
+include_dirs, defines, platforms specific settings, etc. This means that
+when module A depends on module B, it automatically acquires the right build
+settings without module A being filled with assumptions/knowledge of exactly
+how module B is built. Additionally, all of the transitive dependencies of
+module B are pulled in. This avoids their being a single top level view of
+the project, rather each gyp file expresses knowledge about its immediate
+neighbors. This keep local knowledge local. CMake effectively has a large
+shared global namespace.
+
+5. Cross platform generation. CMake is not able to generate all project
+files on all platforms. For example xcode projects cannot be generated from
+windows (cmake uses mac specific libraries to do project generation). This
+means that for instance generating a tarball containing pregenerated
+projects for all platforms is hard with Cmake (requires distribution to
+several machine types).
+
+6. Gyp has rudimentary cross compile support. Currently we've added enough
+functionality to gyp to support x86 -> arm cross compiles. Last I checked
+this functionality wasn't present in cmake. (This occurred later).
+
+
+That being said there are a number of drawbacks currently to gyp:
+
+1. Because platform specific settings are expressed at the project file
+level (rather than the command line level). Settings which might otherwise
+be shared in common between platforms (flags to gcc on mac/linux), end up
+being repeated twice. Though in fairness there is actually less sharing here
+than you'd think. include_dirs and defines actually represent 90% of what
+can be typically shared.
+
+2. CMake may be more mature, having been applied to a broader range of
+projects. There a number of 'tool modules' for cmake, which are shared in a
+common community.
+
+3. gyp currently makes some nasty assumptions about the availability of
+chromium's hermetic copy of cygwin on windows. This causes you to either
+have to special case a number of rules, or swallow this copy of cygwin as a
+build time dependency.
+
+4. CMake includes a fairly readable imperative language. Currently Gyp has a
+somewhat poorly specified declarative language (variable expansion happens
+in sometimes weird and counter-intuitive ways). In fairness though, gyp assumes
+that external python scripts can be used as an escape hatch. Also gyp avoids
+a lot of the things you'd need imperative code for, by having a nice target
+settings publication mechanism.
+
+5. (Feature/drawback depending on personal preference). Gyp's syntax is
+DEEPLY nested. It suffers from all of Lisp's advantages and drawbacks.
+
+-BradN
+```