Add initial overview (#1)

README.md

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-[![Build Status](https://travis-ci.org/WebAssembly/spec.svg?branch=master)](https://travis-ci.org/WebAssembly/spec)
+[![Build Status](https://travis-ci.org/WebAssembly/extended-const.svg?branch=master)](https://travis-ci.org/WebAssembly/extended-const)
+
+# Extented Const Expressesions Proposal for WebAssembly
+
+This repository is a clone of [github.com/WebAssembly/spec/](https://github.com/WebAssembly/spec/).
+It is meant for discussion, prototype specification and implementation of a proposal to add tail call support to WebAssembly.
+
+* See the [overview](proposals/extended-const/Overview.md) for a summary of the proposal.
+
+* See the [modified spec](https://webassembly.github.io/extended-const/core/) for details.
+
+Original `README` from upstream repository follows...
 
 # spec
 

proposals/extended-const/Overview.md

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+# Extended Constant Expressions
+
+## Introduction
+
+This page describes a proposal for extending constant expressions in
+WebAssembly.  The current [spec][] for constant expressions is fairly limited,
+and was always intended to be extended.
+
+This proposal adds new instructions to the list of *constant instructions* that
+can be used in *constant expressions*.
+
+An overview of this proposal was presented at the 01-19-20 CG meeting along with
+a brief [presentation][].  This issue was originally discusses in
+https://github.com/WebAssembly/design/issues/1392.
+
+### Motivation
+
+The primay/initial motivation comes from LLVM where we could benefit from using
+integer addition in both global initializers and in segment initializers.  Both
+of these use cases stem from dynamic linking, which is currently
+[experimental][abi].
+
+1. With dynamic linking the data segments are relative to a global import called
+   `__memory_base` which is supplied by the dynamic linker.   We currently have
+   to have [combine][] all our memory segments into one because there is no way
+   to do `__memory_base + CONST_OFFSET` in a segment initilizer.
+
+2. The linker currently has to generate [dynamic relocations][reloc] for certain
+   WebAssembly globals because its currently not possible to initialize a global
+   with a value of `__memory_base + CONST_OFFSET`.  Specifically, this happens
+   when the static linker decides that a given symbol is local to the currently
+   module.  In this case, rather than importing a global it creates a new
+   global which points insides the a data segment (i.e. it's value is an offset
+   from `__memory_base` or `__table_base` which are themselves imported).
+
+## New Instructions
+
+This proposal adds the following new instructions to the list of valid constant
+instruction:
+
+ - `i32.add`
+ - `i32.sub`
+ - `i64.add`
+ - `i64.sub`
+ - `i32.mul`
+ - `i64.mul`
+
+[spec]: https://webassembly.github.io/spec/core/valid/instructions.html#constant-expressions
+[presentation]: https://docs.google.com/presentation/d/1sM9mJJ6iM7D8324ipYxot91hSKnWCtB8jX4Kh3bde5E
+[abi]: https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md
+[combine]: https://github.com/llvm/llvm-project/blob/5f9be2c3e37c0428ba56876dd84af04b8d9d8915/lld/wasm/Writer.cpp#L868
+[reloc]: https://github.com/llvm/llvm-project/blob/5f9be2c3e37c0428ba56876dd84af04b8d9d8915/lld/wasm/SyntheticSections.cpp#L311