Allow char bit-fields

These are not required to be supported (only int, signed int, and
unsigned int are required), but most compilers support it.

https://port70.net/~nsz/c/c89/c89-draft.html#3.5.2.1
https://port70.net/~nsz/c/c89/c89-draft.html#A.6.5.8

For consistency with other integral types, plain `char` bit-fields
are unsigned, regardless of the `--signed-chars` option.

Fixes #1047

doc/cc65.sgml

@@ -807,6 +807,11 @@ This cc65 version has some extensions to the ISO C standard.
         <tt/_Static_assert/ is also available as the macro <tt/static_assert/ in
         <tt/assert.h/.
 
+<item>  cc65 supports bit-fields of any integral type that is int-sized or
+        smaller, and enumerated types with those types as their underlying
+        type.  (Only <tt/int/, <tt/signed int/, and <tt/unsigned int/ are
+        required.)
+
 <item>  Computed gotos, a GCC extension, has limited support. With it you can
         use fast jump tables from C. You can take the address of a label with
         a double ampersand, putting them in a static const array of type void *.

src/cc65/declare.c

@@ -764,9 +764,9 @@ static int ParseFieldWidth (Declaration* Decl)
     /* TODO: This can be relaxed to be any integral type, but
     ** ParseStructInit currently only supports up to int.
     */
-    if (SizeOf (Decl->Type) != SizeOf (type_uint)) {
-        /* Only int sized types may be used for bit-fields for now */
-        Error ("cc65 currently only supports unsigned int bit-fields");
+    if (SizeOf (Decl->Type) > SizeOf (type_uint)) {
+        /* Only int-sized or smaller types may be used for bit-fields for now */
+        Error ("cc65 currently only supports char-sized and int-sized bit-fields");
         return -1;
     }
 

src/cc65/symtab.c

@@ -870,9 +870,13 @@ SymEntry* AddBitField (const char* Name, const Type* T, unsigned Offs,
         if (!SignednessSpecified) {
             /* int is treated as signed int everywhere except bit-fields; switch it to unsigned,
             ** since this is allowed for bit-fields and avoids sign-extension, so is much faster.
-            ** enums set SignednessSpecified to 1 to avoid this adjustment.
+            ** enums set SignednessSpecified to 1 to avoid this adjustment.  Character types
+            ** actually distinguish 3 types of char; char may either be signed or unsigned, which
+            ** is controlled by `--signed-chars`.  In bit-fields, however, we perform the same
+            ** `char -> unsigned char` adjustment that is performed with other integral types.
             */
-            CHECK ((Entry->Type->C & T_MASK_SIGN) == T_SIGN_SIGNED);
+            CHECK ((Entry->Type->C & T_MASK_SIGN) == T_SIGN_SIGNED ||
+                   IsTypeChar (Entry->Type));
             Entry->Type->C &= ~T_MASK_SIGN;
             Entry->Type->C |= T_SIGN_UNSIGNED;
         }

test/val/char-bitfield.c

@@ -0,0 +1,280 @@
+/*
+  Copyright 2020 The cc65 Authors
+
+  This software is provided 'as-is', without any express or implied
+  warranty. In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/*
+  Tests of char bit-fields; see https://github.com/cc65/cc65/issues/1047
+*/
+
+#include <stdio.h>
+
+static unsigned char failures = 0;
+
+static struct four_bits {
+    unsigned char x : 4;
+} fb = {1};
+
+static void test_four_bits (void)
+{
+    if (sizeof (struct four_bits) != 1) {
+        printf ("Got sizeof (struct four_bits) = %zu, expected 1.\n",
+                sizeof (struct four_bits));
+        failures++;
+    }
+
+    if (fb.x != 1) {
+        printf ("Got fb.x = %u, expected 1.\n", fb.x);
+        failures++;
+    }
+
+    fb.x = 3;
+
+    if (fb.x != 3) {
+        printf ("Got fb.x = %u, expected 3.\n", fb.x);
+        failures++;
+    }
+}
+
+static struct four_bits_signed {
+    signed char x : 4;
+} fbs = {1};
+
+static void test_four_bits_signed (void)
+{
+    if (sizeof (struct four_bits_signed) != 1) {
+        printf ("Got sizeof (struct four_bits_signed) = %zu, expected 1.\n",
+                sizeof (struct four_bits));
+        failures++;
+    }
+
+    if (fbs.x != 1) {
+        printf ("Got fbs.x = %d, expected 1.\n", fbs.x);
+        failures++;
+    }
+
+    fbs.x = 3;
+
+    if (fbs.x != 3) {
+        printf ("Got fbs.x = %d, expected 3.\n", fbs.x);
+        failures++;
+    }
+}
+
+static struct four_bits_plain {
+    char x : 4;
+} fbp = {1};
+
+static void test_four_bits_plain (void)
+{
+    if (sizeof (struct four_bits_plain) != 1) {
+        printf ("Got sizeof (struct four_bits_plain) = %zu, expected 1.\n",
+                sizeof (struct four_bits));
+        failures++;
+    }
+
+    if (fbp.x != 1) {
+        printf ("Got fbp.x = %d, expected 1.\n", fbp.x);
+        failures++;
+    }
+
+    fbp.x = 3;
+
+    if (fbp.x != 3) {
+        printf ("Got fbp.x = %d, expected 3.\n", fbp.x);
+        failures++;
+    }
+}
+
+/*
+  Logic is somewhat diferent for bit-fields that end a struct vs
+  having additional fields.
+*/
+
+static struct four_bits_with_char {
+    unsigned char x : 4;
+    unsigned char y;
+} fbi = {1, 2};
+
+static void test_four_bits_with_char (void)
+{
+    if (sizeof (struct four_bits_with_char) != 2) {
+        printf ("Got sizeof (struct four_bits_with_char) = %zu, expected 2.\n",
+                sizeof (struct four_bits_with_char));
+        failures++;
+    }
+
+    if (fbi.x != 1) {
+        printf ("Got fbi.x = %u, expected 1.\n", fbi.x);
+        failures++;
+    }
+
+    if (fbi.y != 2) {
+        printf ("Got fbi.y = %u, expected 2.\n", fbi.y);
+        failures++;
+    }
+
+    fbi.x = 3;
+    fbi.y = 17;
+
+    if (fbi.x != 3) {
+        printf ("Got fbi.x = %u, expected 3.\n", fbi.x);
+        failures++;
+    }
+
+    if (fbi.y != 17) {
+        printf ("Got fbi.y = %u, expected 17.\n", fbi.y);
+        failures++;
+    }
+}
+
+static struct two_chars {
+    unsigned char x : 4;
+    unsigned char y : 4;
+} o = {11, 7};
+
+/* Tests that bit-fields can share allocation units. */
+static void test_two_chars (void)
+{
+    if (sizeof (struct two_chars) != 1) {
+        printf ("Got sizeof (struct two_chars) = %zu, expected 1.\n",
+                sizeof (struct two_chars));
+        failures++;
+    }
+
+    if (o.x != 11) {
+        printf ("Got o.x = %u, expected 11.\n", o.x);
+        failures++;
+    }
+
+    if (o.y != 7) {
+        printf ("Got o.y = %u, expected 7.\n", o.y);
+        failures++;
+    }
+
+    o.x = 3;
+    o.y = 4;
+
+    if (o.x != 3) {
+        printf ("Got o.x = %u, expected 3.\n", o.x);
+        failures++;
+    }
+
+    if (o.y != 4) {
+        printf ("Got o.y = %u, expected 4.\n", o.y);
+        failures++;
+    }
+}
+
+static struct full_width {
+    unsigned char x : 8;
+} fw = {255};
+
+static void test_full_width (void)
+{
+    if (sizeof (struct full_width) != 1) {
+        printf ("Got sizeof (struct full_width) = %zu, expected 1.\n",
+                sizeof (struct full_width));
+        failures++;
+    }
+
+    if (fw.x != 255) {
+        printf ("Got fw.x = %u, expected 255.\n", fw.x);
+        failures++;
+    }
+
+    fw.x = 42;
+
+    if (fw.x != 42) {
+        printf ("Got fw.x = %u, expected 42.\n", fw.x);
+        failures++;
+    }
+}
+
+static struct aligned_end {
+    unsigned char : 2;
+    unsigned char x : 6;
+    unsigned char : 3;
+    unsigned char y : 5;
+} ae = {63, 17};
+
+static void test_aligned_end (void)
+{
+    if (sizeof (struct aligned_end) != 2) {
+        printf ("Got sizeof (struct aligned_end) = %zu, expected 2.\n",
+                sizeof (struct aligned_end));
+        failures++;
+    }
+
+    if (ae.x != 63) {
+        printf ("Got ae.x = %u, expected 63.\n", ae.x);
+        failures++;
+    }
+
+    if (ae.y != 17) {
+        printf ("Got ae.y = %u, expected 17.\n", ae.y);
+        failures++;
+    }
+
+    ae.x = 42;
+    ae.y = 15;
+
+    if (ae.x != 42) {
+        printf ("Got ae.x = %u, expected 42.\n", ae.x);
+        failures++;
+    }
+
+    if (ae.y != 15) {
+        printf ("Got ae.y = %u, expected 15.\n", ae.y);
+        failures++;
+    }
+}
+
+struct { signed char x : 1; } sc = {-1};
+struct { unsigned char x : 1; } uc = {1};
+struct { char x : 1; } pc = {1};
+
+static void test_signedness (void)
+{
+    if (sc.x != -1) {
+        printf ("Got sc.x = %d, expected -1.\n", sc.x);
+        failures++;
+    }
+
+    if (uc.x != 1) {
+        printf ("Got uc.x = %u, expected 1.\n", uc.x);
+        failures++;
+    }
+
+    if (pc.x != 1) {
+        printf ("Got pc.x = %u, expected 1.\n", pc.x);
+        failures++;
+    }
+}
+
+int main (void)
+{
+    test_four_bits ();
+    test_four_bits_with_char ();
+    test_two_chars ();
+    test_full_width ();
+    test_aligned_end ();
+    test_signedness ();
+    printf ("failures: %u\n", failures);
+    return failures;
+}

test/val/enum-bitfield.c

@@ -149,10 +149,117 @@ static void test_enum_bitfield_int(void)
     }
 }
 
+/* Enum with underlying type unsigned char. */
+enum e7uc {
+    E7UC_100 = 100,
+};
+
+static struct enum_bitfield_uchar {
+    enum e7uc x : 1;
+    enum e7uc y : 4;
+    enum e7uc z : 8;
+} e7ucbf = {0, 10, E7UC_100};
+
+static void test_enum_bitfield_uchar(void)
+{
+    if (sizeof (struct enum_bitfield_uchar) != 2) {
+        printf ("Got sizeof(struct enum_bitfield_uchar) = %zu, expected 2.\n",
+                sizeof(struct enum_bitfield_uchar));
+        failures++;
+    }
+
+    if (e7ucbf.x != 0) {
+        printf ("Got e7ucbf.x = %u, expected 0.\n", e7ucbf.x);
+        failures++;
+    }
+    if (e7ucbf.y != 10) {
+        printf ("Got e7ucbf.y = %u, expected 10.\n", e7ucbf.y);
+        failures++;
+    }
+    if (e7ucbf.z != 100) {
+        printf ("Got e7ucbf.z = %u, expected 100.\n", e7ucbf.z);
+        failures++;
+    }
+
+    e7ucbf.x = -1;  /* Will store 1. */
+    e7ucbf.y = -1;  /* Will store 15. */
+    e7ucbf.z = 127;
+
+    /* Both signed char and unsigned char are converted to int in arithmetic expressions,
+    ** so we write this test differently to enum_bitfield_int.
+    */
+    if (e7ucbf.x != 1) {
+        printf ("Got e7ucbf.x = %u, expected 1.\n", e7ucbf.x);
+        failures++;
+    }
+
+    if (e7ucbf.y != 15) {
+        printf ("Got e7ucbf.y = %u, expected 15.\n", e7ucbf.y);
+        failures++;
+    }
+    if (e7ucbf.z != 127) {
+        printf ("Got e7ucbf.z = %u, expected 127.\n", e7ucbf.z);
+        failures++;
+    }
+}
+
+/* Enum with underlying type signed char. */
+enum e8sc {
+    E8SC_M1 = -1,
+    E8SC_100 = 100,
+};
+
+static struct enum_bitfield_char {
+    enum e8sc x : 1;
+    enum e8sc y : 4;
+    enum e8sc z : 8;
+} e8scbf = {0, 5, E8SC_100};
+
+static void test_enum_bitfield_char(void)
+{
+    if (sizeof (struct enum_bitfield_char) != 2) {
+        printf ("Got sizeof(struct enum_bitfield_char) = %zu, expected 2.\n",
+                sizeof(struct enum_bitfield_char));
+        failures++;
+    }
+
+    if (e8scbf.x != 0) {
+        printf ("Got e8scbf.x = %d, expected 0.\n", e8scbf.x);
+        failures++;
+    }
+    if (e8scbf.y != 5) {
+        printf ("Got e8scbf.y = %d, expected 10.\n", e8scbf.y);
+        failures++;
+    }
+    if (e8scbf.z != 100) {
+        printf ("Got e8scbf.z = %d, expected 100.\n", e8scbf.z);
+        failures++;
+    }
+
+    e8scbf.x = -1;
+    e8scbf.y = -3;
+    e8scbf.z = 127;
+
+    if (e8scbf.x != -1) {
+        printf ("Got e8scbf.x = %d, expected -1.\n", e8scbf.x);
+        failures++;
+    }
+    if (e8scbf.y != -3) {
+        printf ("Got e8scbf.y = %d, expected -3.\n", e8scbf.y);
+        failures++;
+    }
+    if (e8scbf.z != 127) {
+        printf ("Got e8scbf.z = %d, expected 127.\n", e8scbf.z);
+        failures++;
+    }
+}
+
 int main(void)
 {
     test_enum_bitfield_uint();
     test_enum_bitfield_int();
+    test_enum_bitfield_uchar();
+    test_enum_bitfield_char();
     printf("failures: %u\n", failures);
     return failures;
 }