Give an escaped parameter a slot before reloading it

src/reg-alloc.c

@@ -986,6 +986,19 @@ int prepare_operand(basic_block_t *bb, var_t *var, int operand_0)
         return i;
     }
 
+    /* The reload below reads the variable's slot, so there has to be one
+     * holding its value. A parameter arrives in a register and is given a
+     * slot only when something spills it: reloading before that read whatever
+     * the frame happened to hold, and "int x = a - b; int *p = &b;" computed
+     * a - garbage. Writing the register back first is what makes the slot
+     * stand for the variable.
+     */
+    if (i > -1 && var->address_taken && !var->space_is_allocated) {
+        store_var(bb, var, i);
+        vreg_map_to_phys(var, i);
+        return i;
+    }
+
     for (i = 0; i < REG_CNT; i++) {
         if (reg_is_free(i)) {
             load_var(bb, var, i);

tests/driver.sh

@@ -477,6 +477,7 @@ begin_category "Standalone Programs" "Testing checked-in end-to-end programs"
 try_file 0 'F(10) = 55' "$TESTS_DIR/fib.c"
 try_file 0 $'1\nHello World' "$TESTS_DIR/hello.c"
 try_file 0 '' "$TESTS_DIR/strength-reduce.c"
+try_file 0 '' "$TESTS_DIR/escaped-param.c"
 
 # Category: Basic Literals and Constants
 begin_category "Literals and Constants" "Testing integer, character, and string literals"

tests/escaped-param.c

@@ -0,0 +1,42 @@
+/* prepare_operand() reloads an address-taken variable from its stack slot
+ * rather than reading the register it is already in, because a store through
+ * the pointer would not have gone to the register. That reload needs the slot
+ * to hold the value, and a parameter arrives in a register with no slot at
+ * all: one is reserved the first time something spills it.
+ *
+ * Taking the address after the expression is what orders the two so the
+ * reload comes first. Nothing has to be stored through the pointer -- the
+ * value is already wrong by the time the address is taken.
+ */
+
+int sub_then_escape(int a, int b)
+{
+    int x = a - b;
+    int *p = &b;
+    return x;
+}
+
+int add_then_escape(int a, int b)
+{
+    int x = a + b;
+    int *p = &b;
+    return x;
+}
+
+int escape_first_operand(int a, int b)
+{
+    int x = a - b;
+    int *p = &a;
+    return x;
+}
+
+int main()
+{
+    if (sub_then_escape(5, 2) != 3)
+        return 1;
+    if (add_then_escape(1, 2) != 3)
+        return 2;
+    if (escape_first_operand(5, 2) != 3)
+        return 3;
+    return 0;
+}