add FAQ regarding int64s in JS (emscripten-core#7224) [ci skip]

WizardOfArc · kripken · commit 6edf480a4e45 · 2018-10-05T17:44:01.000-07:00
diff --git a/site/source/docs/getting_started/FAQ.rst b/site/source/docs/getting_started/FAQ.rst
@@ -473,6 +473,13 @@ To check if this is the issue you are seeing, build with ``-s PRECISE_F32=1``. T
 
 (This is not an issue for wasm, which has native float types.)
 
+
+How do I pass int64_t and uint64_t values from js into wasm functions?
+======================================================================
+
+JS can't represent int64s, so what happens is that in exported functions (that you can call from JS) we "legalize" the types, by turning an i64 argument into two i32s (low and high bits), and an i64 return value becomes an i32, and you can access the high bits by calling a helper function called getTempRet0.
+
+
 Can I use multiple Emscripten-compiled programs on one Web page?
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