Get it working

tools/build.sh

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+#!/bin/bash
+set -e
+# Sections and commands of this script are copied from build-llvm.sh [1] in the
+# jprendes/emception original project.
+#
+# [1]: https://github.com/jprendes/emception/blob/366065547b1a59cb58011ed19aedce70c3bcbd2b/build-llvm.sh
+
+# Cross compiling llvm needs a native build of llvm-tblgen and clang-tblgen for
+# that platform. So first, we need to build them for a WebAssembly target so
+# that we can use them later when we build all the other binaries. It's like a
+# dependency that we need to make before making the parent project.
+cmake -G Ninja \
+  -S llvm \
+  -B build/llvm-native \
+  -DCMAKE_BUILD_TYPE=Release \
+  -DLLVM_TARGETS_TO_BUILD=WebAssembly \
+  -DLLVM_ENABLE_PROJECTS=clang
+cmake --build build/llvm-native -- llvm-tblgen clang-tblgen
+
+# Start a subshell so that we can incrementally export ... over multiple lines.
+# If we try to do all these env vars in one prefix, it gets to be a mess of
+# quotes and backslashes.
+#
+# This builds the clang, lld, and clang-tools-extra LLVM sub-projects using the
+# emcmake environment wrapper. It sets a bunch of magic stuff to make builds
+# like this work. I don't completely understand it.
+#
+# TODO: Add better comment about what the emcmake command does
+(
+  set -e
+  export CXXFLAGS=-Dwait4=__syscall_wait4
+  export LDFLAGS="-s LLD_REPORT_UNDEFINED=1 \
+    -s ALLOW_MEMORY_GROWTH=1 \
+    -s EXPORTED_FUNCTIONS=_main,_free,_malloc \
+    -s EXPORTED_RUNTIME_METHODS=FS,PROXYFS,ERRNO_CODES,allocateUTF8 \
+    -lproxyfs.js \
+    --js-library=$PWD/tools/fsroot.js"
+  emcmake cmake -G Ninja \
+    -S llvm \
+    -B build/llvm-wasm \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DLLVM_TARGETS_TO_BUILD=WebAssembly \
+    -DLLVM_ENABLE_PROJECTS='clang;lld;clang-tools-extra' \
+    -DLLVM_ENABLE_DUMP=OFF \
+    -DLLVM_ENABLE_ASSERTIONS=OFF \
+    -DLLVM_ENABLE_EXPENSIVE_CHECKS=OFF \
+    -DLLVM_ENABLE_BACKTRACES=OFF \
+    -DLLVM_BUILD_TOOLS=OFF \
+    -DLLVM_ENABLE_THREADS=OFF \
+    -DLLVM_BUILD_LLVM_DYLIB=OFF \
+    -DLLVM_INCLUDE_TESTS=OFF \
+    -DLLVM_TABLEGEN=build/llvm-native/bin/llvm-tblgen \
+    -DCLANG_TABLEGEN=build/llvm-native/bin/clang-tblgen
+)

tools/fsroot.js

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+/**
+ * @license
+ * Copyright 2013 The Emscripten Authors
+ * SPDX-License-Identifier: MIT
+ */
+
+mergeInto(LibraryManager.library, {
+  $FSROOT__deps: ["$FS"],
+  $FSROOT__postset: "FSROOT.staticInit();",
+  $FSROOT: {
+    staticInit: () => {
+      FS.root = null;
+
+      let opts = (Module.ROOT && Module.ROOT.opts) || {};
+      let type = (Module.ROOT && Module.ROOT.type) || "MEMFS";
+      if (typeof type === "string") {
+        type = FS.filesystems[type] || eval(type);
+      } else if (typeof type === "function") {
+        type = type(Module);
+      }
+      FS.mount(type, opts, '/');
+
+      FSROOT.createDefaultMountPoints();
+
+      // We need to ignore errors in mkdir
+      // since we are pre-creation mountpoints
+      // for directories otherwise created by the
+      // FS.create* functions
+      const restore_mkdir = FSROOT.safeMkdir();
+
+      FS.createDefaultDirectories();
+      FS.createDefaultDevices();
+      FS.createSpecialDirectories();
+
+      restore_mkdir();
+    },
+    createDefaultMountPoints: () => {
+      // Mount a new MEMFS for /dev
+      FS.mkdirTree("/dev");
+      FS.mount(MEMFS, {}, "/dev");
+
+      // Mount a new MEMFS for /proc/self
+      FS.mkdirTree('/proc/self');
+      FS.mount(MEMFS, {}, '/proc/self');
+    },
+    safeMkdir: () => {
+      const mkdir = FS.mkdir;
+      FS.mkdir = (path, mode) => {
+        try {
+          return mkdir(path, mode);
+        } catch {
+          // ignore errors
+          return FS.lookupPath(path, { follow: true }).node;
+        }
+      };
+      return () => {
+          FS.mkdir = mkdir;
+      };
+    },
+  },
+});
+
+DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.push('$FSROOT');