From 7e0585fa35513df4dd2c8d3c8e7de3a659bcf879 Mon Sep 17 00:00:00 2001
From: Zoltan Varga <vargaz@gmail.com>
Date: Thu, 30 May 2019 21:01:52 -0400
Subject: [PATCH] [wasm] Add document with wasm feature requests.

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 sdks/wasm/docs/WASM-REQUESTS.md | 68 +++++++++++++++++++++++++++++++++++++++++
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 create mode 100644 sdks/wasm/docs/WASM-REQUESTS.md

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+# Introduction
+
+This document describes possible extensions to WebAssembly which would enable Mono/the
+WASM runtime to emit smaller and/or faster code or implement additional functionality.
+
+## Nullref checks
+
+Currently, reading from address 0 or close to it doesn't throw. This forces mono
+to emit explicit null checks which are very common and lead to increased code
+size.
+
+Division by 0 is a similar situation. For integer division, its possible
+to emit fairly simple explicit checks, but fp division is more complicated.
+
+Some possible solutions:
+* Add some kind of signal handler facility where hitting a nullref would transfer
+control to a specific wasm function which could throw an exception.
+* The 'macro operations' section below.
+
+## Branch hints
+
+Add support for llvm.expected ().
+
+This is useful for generating better native code for
+
+	if (unlikely(cond))
+	...
+
+which is fairly common in .net IL because of argument checks, etc.
+
+## Cold calling convention
+
+Add support for coldcc.
+
+This is useful for calls which are rarely executed, like the call in the GC safe
+point code:
+
+	if <need to do a gc> {
+		<call into runtime to do a gc>
+   	}
+
+## Macro operations
+
+Many higher level .net opcodes are translated to a series of llvm opcodes. To make
+the wasm executable smaller, it would be useful to have some kind of macro facility
+so these can be emitted in a more compact manner. Something similar to
+https://github.com/WebAssembly/decompressor-prototype/blob/master/CompressionLayer1.md.
+
+Alternatively, emitting small functions which are always inlined by the wasm JIT would also
+work.
+
+## Computed goto
+
+Currently, the llvm wasm backend doesn't support computed gotos. This is important for
+efficient implementation of interpreters.
+
+## Programmatic stack walking
+
+Languages like .net/java allow programs to obtain their own stacktraces as an array of stackframe objects.
+Would be useful to have this facility in wasm.
+
+This could be implemented by associating some data like an id
+with a subset of wasm methods, then having a wasm intrinsics which returns a list of such data,
+which the app could then use to construct the stack trace objects. Returning only
+data which the app writer explicitly added to the wasm module would help alleviate
+some of the security concerns with stack walks.
+
+
-- 
2.11.4.GIT