/* Redefine flan.cell.twice from outside the program, without a compiler.
 *
 * A dev build exports one cell per function -- a mutable global holding the
 * address of the body that is current -- and it is -rdynamic, so the cell is
 * in .dynsym and dlsym can find it by name. Storing a different function
 * pointer there is the whole of what a redefinition does to an existing call
 * site; the rest of the dev loop is about producing the new body, and none of
 * that is needed to answer "does a call actually read the cell".
 *
 * A Flan function's signature is its parameters followed by the transfer
 * channel, so this takes (i64, void *) where the Flan body takes (n i64). It
 * never transfers, so it never writes through the channel.
 *
 * A release build has no cells, dlsym answers NULL, and this does nothing --
 * which is the control: it shows the change below comes from the indirection
 * and not from ordinary symbol interposition. See spike/x86/cells.sh.
 */
#define _GNU_SOURCE
#include <dlfcn.h>
#include <stddef.h>
#include <stdint.h>

static int64_t instead(int64_t n, void *xfer) {
  (void)xfer;
  return n + 1;
}

__attribute__((constructor)) static void install(void) {
  void **cell = (void **)dlsym(RTLD_DEFAULT, "flan.cell.twice");
  if (cell != NULL) *cell = (void *)instead;
}