Use char ops if possible for bit-field loads

Set CF_FORCECHAR and change type to char once we have
shifted into a char.

This saves some unnecessary ldx #0 instructions.

src/cc65/loadexpr.c

@@ -193,10 +193,26 @@ void LoadExpr (unsigned Flags, struct ExprDesc* Expr)
         ** operations.
         */
         if (ED_IsBitField (Expr)) {
-            unsigned F = CF_INT | CF_UNSIGNED | CF_CONST | (Flags & CF_TEST);
+            /* If the field was loaded as a char, force the shift/mask ops to be char ops.
+            ** If it is a char, the load has already put 0 in the upper byte, so that can remain.
+            ** CF_FORCECHAR does nothing if the type is not CF_CHAR.
+            */
+            unsigned F = Flags | CF_FORCECHAR | CF_CONST;
+
             /* Shift right by the bit offset */
             g_asr (F, Expr->BitOffs);
-            /* And by the width if the field doesn't end on an int boundary */
+
+            /* Since we have now shifted down, we can do char ops as long as the width fits in
+            ** a char.
+            */
+            if (Expr->BitWidth <= CHAR_BITS) {
+                F |= CF_CHAR;
+            }
+
+            /* And by the width if the field doesn't end on a char or int boundary.  If it does
+            ** end on a boundary, then zeros have already been shifted in.  g_and emits no code
+            ** if the mask is all ones.
+            */
             if (Expr->BitOffs + Expr->BitWidth != CHAR_BITS &&
                 Expr->BitOffs + Expr->BitWidth != INT_BITS) {
                 g_and (F, (0x0001U << Expr->BitWidth) - 1U);

test/val/bitfield.c

@@ -182,12 +182,104 @@ static void test_overlap_with_int(void)
     }
 }
 
+static struct full_width {
+    unsigned int x : 8;
+    unsigned int y : 16;
+} fw = {255, 17};
+
+static void test_full_width(void)
+{
+    if (sizeof(struct full_width) != 3) {
+        printf("Got sizeof(struct full_width) = %zu, expected 3.\n",
+               sizeof(struct full_width));
+        failures++;
+    }
+
+    if (fw.x != 255) {
+        printf("Got fw.x = %u, expected 255.\n", fw.x);
+        failures++;
+    }
+
+    if (fw.y != 17) {
+        printf("Got fw.y = %u, expected 17.\n", fw.y);
+        failures++;
+    }
+
+    fw.x = 42;
+    fw.y = 1023;
+
+    if (fw.x != 42) {
+        printf("Got fw.x = %u, expected 42.\n", fw.x);
+        failures++;
+    }
+
+    if (fw.y != 1023) {
+        printf("Got fw.y = %u, expected 1023.\n", fw.y);
+        failures++;
+    }
+}
+
+static struct aligned_end {
+    unsigned int : 2;
+    unsigned int x : 6;
+    unsigned int : 3;
+    unsigned int y : 13;
+    /* z crosses a byte boundary, but fits in a byte when shifted. */
+    unsigned int : 6;
+    unsigned int z : 7;
+} ae = {63, 17, 100};
+
+static void test_aligned_end(void)
+{
+    if (sizeof(struct aligned_end) != 5) {
+        printf("Got sizeof(struct aligned_end) = %zu, expected 5.\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++;
+    }
+
+    if (ae.z != 100) {
+        printf("Got ae.z = %u, expected 100.\n", ae.z);
+        failures++;
+    }
+
+    ae.x = 42;
+    ae.y = 1023;
+    ae.z = 66;
+
+    if (ae.x != 42) {
+        printf("Got ae.x = %u, expected 42.\n", ae.x);
+        failures++;
+    }
+
+    if (ae.y != 1023) {
+        printf("Got ae.y = %u, expected 1023.\n", ae.y);
+        failures++;
+    }
+
+    if (ae.z != 66) {
+        printf("Got ae.z = %u, expected 66.\n", ae.z);
+        failures++;
+    }
+}
+
 int main(void)
 {
     test_four_bits();
     test_four_bits_with_int();
     test_overlap();
     test_overlap_with_int();
+    test_full_width();
+    test_aligned_end();
     printf("failures: %u\n", failures);
     return failures;
 }