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Use char ops if possible for bit-field loads

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

This saves some unnecessary ldx #0 instructions.
This commit is contained in:
Jesse Rosenstock
2020-07-30 12:00:21 +02:00
committed by Oliver Schmidt
parent e38f601fcc
commit 2d5fd0fc63
2 changed files with 110 additions and 2 deletions
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20
src/cc65/loadexpr.c
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@@ -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);