Revert "Revert "Forbid 3-byte struct pass/return by value, document struct value pass/return""

doc/cc65.sgml

@@ -741,7 +741,7 @@ Here is a description of all the command line options:
   <tag><tt/return-type/</tag>
         Warn about no return statement in function returning non-void.
   <tag><tt/struct-param/</tag>
-        Warn when passing structs by value.
+        Warn when passing structs by value. (Disabled by default.)
   <tag><tt/unknown-pragma/</tag>
         Warn about #pragmas that aren't recognized by cc65.
   <tag><tt/unreachable-code/</tag>
@@ -806,9 +806,8 @@ and the one defined by the ISO standard:
 
 <item>  The datatypes "float" and "double" are not available.
         <p>
-<item>  C Functions may not return structs (or unions), and structs may not
-        be passed as parameters by value. However, struct assignment *is*
-        possible.
+<item>  C Functions may pass and return structs (or unions) by value, but only
+        of 1, 2 or 4 byte sizes.
         <p>
 <item>  Most of the C library is available with only the fastcall calling
         convention (<ref id="extension-fastcall" name="see below">).  It means

src/cc65/datatype.c

@@ -803,7 +803,6 @@ const Type* GetStructReplacementType (const Type* SType)
     switch (SizeOf (SType)) {
         case 1:     NewType = type_uchar;   break;
         case 2:     NewType = type_uint;    break;
-        case 3:     /* FALLTHROUGH */
         case 4:     NewType = type_ulong;   break;
         default:    NewType = SType;        break;
     }

test/val/struct-by-value.c

@@ -0,0 +1,144 @@
+/* Test of passing and returning structs by value.
+   Structs of 1, 2 and 4 bytes are supported.
+   Note that structs of 3 bytes are disabled, see:
+       https://github.com/cc65/cc65/issues/2022
+*/
+
+int fail = 0;
+
+struct s1 { char a; };
+struct s2 { char a, b; };
+struct s3 { char a, b, c; };
+struct s4 { char a, b, c, d; };
+
+const struct s1 c1 = { 1 };
+const struct s2 c2 = { 2, 3 };
+const struct s3 c3 = { 4, 5, 6 };
+const struct s4 c4 = { 7, 8, 9, 10 };
+
+struct s1 return1() { return c1; }
+struct s2 return2() { return c2; }
+/*struct s3 return3() { return c3; }*/
+struct s4 return4() { return c4; }
+
+int compare1(struct s1 a, struct s1 b)
+{
+    if (a.a != b.a) return 1;
+    return 0;
+}
+
+int compare2(struct s2 a, struct s2 b)
+{
+    if (a.a != b.a) return 1;
+    if (a.b != b.b) return 1;
+    return 0;
+}
+
+/*int compare3(struct s3 a, struct s3 b)
+{
+    if (a.a != b.a) return 1;
+    if (a.b != b.b) return 1;
+    if (a.c != b.c) return 1;
+    return 0;
+}*/
+
+int compare4(struct s4 a, struct s4 b)
+{
+    if (a.a != b.a) return 1;
+    if (a.b != b.b) return 1;
+    if (a.c != b.c) return 1;
+    if (a.d != b.d) return 1;
+    return 0;
+}
+
+int pass1(struct s1 p1)
+{
+    struct s1 a1;
+    a1 = p1;
+    if (a1.a != c1.a) return 1;
+    return 0;
+}
+
+int pass2(struct s2 p2)
+{
+    struct s2 a2;
+    a2 = p2;
+    if (a2.a != c2.a) return 1;
+    if (a2.b != c2.b) return 1;
+    return 0;
+}
+
+/*int pass3(struct s3 p3)
+{
+    struct s3 a3;
+    a3 = p3;
+    if (a3.a != c3.a) return 1;
+    if (a3.b != c3.b) return 1;
+    if (a3.c != c3.c) return 1;
+    return 0;
+}*/
+
+int pass4(struct s4 p4)
+{
+    struct s4 a4;
+    a4 = p4;
+    if (a4.a != c4.a) return 1;
+    if (a4.b != c4.b) return 1;
+    if (a4.c != c4.c) return 1;
+    if (a4.d != c4.d) return 1;
+    return 0;
+}
+
+void reset(char* gg)
+{
+    char i;
+    for (i=0;i<5;++i) gg[i] = 128+i;
+}
+
+int test(char* gg, char start)
+{
+    char i;
+    for (i=start;i<5;++i)
+        if (gg[i] != 128+i) return 1;
+    return 0;
+}
+
+int main()
+{
+    /* Used to check #2022 bug condition of extra bytes being overwritten. */
+    union
+    {
+        char gg[5];
+        struct s1 g1;
+        struct s2 g2;
+        struct s3 g3;
+        struct s4 g4;
+    } guard;
+
+    reset(guard.gg);
+    guard.g1 = return1();
+    fail += compare1(guard.g1,c1);
+    fail += test(guard.gg,1);
+
+    reset(guard.gg);
+    guard.g2 = return2();
+    fail += compare2(guard.g2,c2);
+    fail += test(guard.gg,2);
+
+    /*reset(guard.gg);
+    guard.g3 = return3();
+    fail += compare3(guard.g3,c3);
+    fail += test(guard.gg,3);*/
+
+    reset(guard.gg);
+    guard.g4 = return4();
+    fail += compare4(guard.g4,c4);
+    fail += test(guard.gg,4);
+
+    fail += pass1(c1);
+    fail += pass2(c2);
+    /*fail += pass3(c3);*/
+    fail += pass4(c4);
+
+    return fail;
+}