Fix win abi bug causing stack dealignment/crash

The accounting for push_struct was wrong (double counted by both |stack|
and |by_ref_copies_size|) and the push of r11 wasn't being counted.
Sometimes they cancelled each other out, but only accidentally (because
the stack is always either 8 or 16 aligned, it's not too hard to get
accidentally right.)

src/codegen.in.c

@@ -835,7 +835,9 @@ static int push_args_win(Node* node, int* by_ref_copies_size) {
   assert((*by_ref_copies_size == 0 && !has_by_ref_args) ||
          (*by_ref_copies_size && has_by_ref_args));
 
-  if ((C(depth) + stack + (*by_ref_copies_size / 8)) % 2 == 1) {
+  // Realign the stack to 16 bytes if rsp fiddling mucked it up.
+  size_t r11_push_size = has_by_ref_args ? 1 : 0;
+  if ((C(depth) + stack + r11_push_size) % 2 == 1) {
     ///| sub rsp, 8
     C(depth)++;
     stack++;

test/substructs.c

@@ -0,0 +1,44 @@
+#include <stdio.h>
+
+struct B {
+  int x;
+  int y;
+  int z;
+};
+
+struct C {
+  int x;
+  int y;
+  int z;
+  int w;
+};
+
+struct D {
+  int x;
+  int y;
+  int z;
+  int w;
+  int v;
+};
+
+void f(struct B b) {
+  printf("%d %d %d\n", b.x, b.y, b.z);
+}
+
+void g(struct C c) {
+  printf("%d %d %d %d\n", c.x, c.y, c.z, c.w);
+}
+
+void h(struct D d) {
+  printf("%d %d %d %d %d\n", d.x, d.y, d.z, d.w, d.v);
+}
+
+int main(void) {
+  struct B b = {10, 20, 30};
+  f(b);
+  struct C c = {10, 20, 30, 40};
+  g(c);
+  struct D d = {10, 20, 30, 40, 50};
+  h(d);
+  return 0;
+}

test/test_helpers_for_update.py

@@ -45,6 +45,7 @@
       .get_function_address = get_host_helper_func,
       .output_function = NULL,
       .use_ansi_codes = false,
+      .generate_debug_symbols = false,
   };
 
   DyibiccContext* ctx = dyibicc_set_environment(&env_data);

test/update_structabi2.py

@@ -0,0 +1,52 @@
+from test_helpers_for_update import *
+
+HOST = '''\
+#include <stdio.h>
+
+struct B {
+    int x;
+    int y;
+    int z;
+};
+
+void test_f(struct B b) {
+  printf("%d %d %d\\n", b.x, b.y, b.z);
+}
+'''
+
+SRC = '''\
+#include <stdio.h>
+
+struct B {
+    int x;
+    int y;
+    int z;
+};
+
+extern void test_f(struct B b);
+
+// This was causing stack un-alignment due to how struct copying was
+// implemented in codegen.
+int main(void) {
+  struct B b = {10, 20, 30};
+  test_f(b);
+  printf("OK\\n");
+  return 0;
+}
+'''
+
+# This is not really an "update" test, but because the update tests happen to
+# build a separate host binary rather than using the standard dyibicc.exe, it
+# gives us a way to have C code to call that's test-specific and compiled by
+# the host compiler, rather than by ours.
+
+add_to_host(HOST);
+add_host_helper_func("test_f")
+include_path("../../test")
+
+initial({'main.c': SRC})
+update_ok()
+expect(0)
+
+done()
+