Generate thunks for address-taken imports (#554)

Under emscripten, C code can take the address of a function implemented
in Javascript (which is exposed via an import in wasm). Because imports
do not have linear memory address in wasm, we need to generate a thunk
to be the target of the indirect call; it call the import directly.

This is facilited by a new .s directive (.functype) which declares the
types of functions which are declared but not defined.

Fixes https://github.com/WebAssembly/binaryen/issues/392

1 files changed, 27 insertions, 0 deletions

diff --git a/test/dot_s/indirect-import.c b/test/dot_s/indirect-import.c
new file mode 100644
index 000000000..006424d84
--- /dev/null
+++ b/test/dot_s/indirect-import.c
@@ -0,0 +1,27 @@
+#include <stdint.h>
+
+struct big {
+	float a;
+	double b;
+	int32_t c;
+};
+
+float extern_fd(double);
+void extern_vj(uint64_t);
+void extern_v(void);
+int32_t extern_ijidf(int64_t, int32_t, double, float);
+void extern_struct(struct big);
+struct big extern_sret(void);
+
+intptr_t bar() {
+  float (*fd)(double) = &extern_fd;
+  void (*vj)(uint64_t) = &extern_vj;
+  vj(1ULL);
+  void (*v)(void) = &extern_v;
+  v();
+  int32_t (*ijidf)(int64_t, int32_t, double, float) = &extern_ijidf;
+  ijidf(1LL, 2, 3.0, 4.0f);
+  void (*vs)(struct big) = &extern_struct;
+  struct big (*s)(void) = &extern_sret;
+  return (intptr_t)fd;
+}