Document fastcomp setup

README.md

@@ -82,6 +82,10 @@ the website, run test cases, etc:
     $ cd duktape
     $ make
 
+    # If you want to build dukweb.js or run Emscripten targets, you need
+    # to setup Emscripten fastcomp manually, see doc/emscripten-status.rst
+    # for step-by-step instructions.
+
     # Run Ecmascript and API testcases, and some other tests
     $ make ecmatest
     $ make apitest

doc/emscripten-status.rst

@@ -31,3 +31,40 @@ following additional options:
 Since Duktape 1.3 there is support for Khronos/ES6 TypedArrays which allow
 Emscripten to run better than with Duktape 1.2, and also allows use of
 Emscripten fastcomp.
+
+Setting up fastcomp for Duktape
+===============================
+
+To build dukweb.js and to use Makefile targets like ``emscriptentest`` you
+need Emscripten "fastcomp".  Example steps to setup emscripten:
+
+* Compile fastcomp manually:
+
+  - https://kripken.github.io/emscripten-site/docs/building_from_source/building_fastcomp_manually_from_source.html
+
+* Checkout emscripten::
+
+      $ cd (duktape)
+      $ make emscripten  # duktape checkouts emscripten master
+
+* Create a ``~/.emscripten`` file which uses your manually compiled fastcomp.
+  You can create a default configuration as follows::
+
+      $ cd (duktape)/emscripten
+      $ ./emcc  # creates ~/.emscripten if it doesn't exist
+
+  Then change Emscripten to use the manually compiled fastcomp by changing
+  the LLVM_ROOT line in ``~/.emscripten`` to point to your fastcomp build::
+
+      LLVM_ROOT = '/home/user/myfastcomp/emscripten-fastcomp/build/Release/bin'
+
+You should now be able to build dukweb.js and run Emscripten Makefile
+targets::
+
+    $ make emscripteninceptiontest
+    [...]
+    emscripten/emcc -O2 -std=c99 -Wall --memory-init-file 0 -Idist/src dist/src/duktape.c dist/examples/hello/hello.c -o /tmp/duk-emcc-test.js
+    [...]
+    ./duk /tmp/duk-emcc-test-fixed.js
+    Hello world!
+    2+3=5