Document some emsdk patches (#863)

pyodide · Dec 15, 2020 · 9cd79f4 · 9cd79f4
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diff --git a/emsdk/patches/emulate_pointer.patch b/emsdk/patches/emulate_pointer.patch
@@ -1,3 +1,9 @@
+This fixes an upstream bug introduced in emscripten 1.38.34 that causes dlsym
+to always return 0 for function pointers when EMULATE_FUNCTION_POINTER_CASTS is
+enabled. This is a simple typo, since `mangled == "_" + symbol`. This piece of
+code is no longer present in upstream master, hence there is no effort to
+upstream the fix.
+
 diff --git a/emsdk/fastcomp/emscripten/src/library.js b/emsdk/fastcomp/emscripten/src/library.js
 index 97cca10..5c002d7 100644
 --- a/emsdk/fastcomp/emscripten/src/library.js

diff --git a/emsdk/patches/num_params.patch b/emsdk/patches/num_params.patch
@@ -1,3 +1,25 @@
+Quoting emscripten documentation:
+
+> Function pointers must be called with the correct type: it is undefined
+> behavior in C and C++ to cast a function pointer to another type and call it
+> that way. This does work in most native platforms, however, despite it being
+> UB, but in asm.js and in wasm it can fail.
+
+Unfortunately, Python code exploits this behaviour quite a lot.
+
+FuncCastEmulation is a wasm-opt pass that helps bypass this issue by
+redefining every function to take a fixed number of i64s, and then manually
+converting the arguments to feed into the original function.
+
+To do so, the fixed number of i64s must be at least the number of actual
+arguments our functions take, and binaryen decided that 16 is enough for
+everyone. Turns out scipy has functions that take in a huge number of
+arguments, so we bump it up to 61.
+
+An upstream patch has been submitted and accepted that makes this number
+configurable at runtime, and should be present in binaryen version_99:
+https://github.com/WebAssembly/binaryen/pull/3411
+
 --- a/emsdk/binaryen/src/passes/FuncCastEmulation.cpp	2019-04-03 11:30:13.556074729 -0400
 +++ b/emsdk/binaryen/src/passes/FuncCastEmulation.cpp	2019-04-03 11:30:32.817143862 -0400
 @@ -39,7 +39,7 @@