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paper 1.1
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‎docs/paper.pdf‎

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‎docs/paper.tex‎

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@@ -499,9 +499,24 @@ \subsubsection{Emulating Code Semantics}
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exact same behavior, in particular, we must handle overflows properly, which would not be the case if we just implement
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this as $\%1 + \%2$ in JavaScript. For example, with inputs of $255$ and $1$, the
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correct output is 0, but simple addition in JavaScript will give us 256. We
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can emulate the proper behavior by adding additional code, one way
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(not necessarily the most optimal) would be to check for overflows after
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each addition, and correct them as necessary. This however significantly degrades performance.
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can of course emulate the proper behavior by adding additional code.
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This however significantly degrades performance,
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because modern JavaScript engines can often translate something like $z = x + y$ into
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native code containing a single instruction (or very close to that), but if instead we had
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something like $z = (x + y)\&255$ (in order to correct overflows), the JavaScript engine
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would need to generate additional code to perform the AND operation.\footnote{
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In theory, the JavaScript engine could determine that we are implicitly working
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on 8-bit values here, and generate machine code that no longer needs the AND operation.
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However, most or all modern JavaScript engines have just two internal numeric types, doubles and
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32-bit integers. This is so because they are tuned for `normal' JavaScript code
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on the web, which in most cases is served well by just those two types.
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In addition, even if JavaScript engines did analyze code containing $\&255$, etc.,
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in order to deduce that a variable can be implemented
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as an 8-bit integer, there is a cost to including all the necessary $\&255$ text
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in the script, because code size is a significant factor on the web. Adding even
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a few characters for every single mathematic operation, in a large JavaScript file,
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could add up to a significant increase in download size.}
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Emscripten's approach to this problem is to allow the generation of both accurate code,
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that is identical in behavior to LLVM assembly, and inaccurate code which is

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