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LoadExpr: Optimize <= 8-bit bit-field loads

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Set CF_FORCECHAR to do as many operations as char ops as possible.
Clear high byte at the end.
This commit is contained in:
Jesse Rosenstock 2020-08-02 09:35:33 +02:00 committed by Oliver Schmidt
parent 0c72647edd
commit d8f9201ecd
1 changed files with 27 additions and 13 deletions
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40
src/cc65/loadexpr.c
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@ -117,12 +117,27 @@ void LoadExpr (unsigned Flags, struct ExprDesc* Expr)
*/
unsigned EndBit = 0; /* End bit for bit-fields, or zero if non-bit-field. */
int AdjustBitField = 0;
unsigned BitFieldFullWidthFlags = 0;
if (ED_IsBitField (Expr)) {
EndBit = Expr->BitOffs + Expr->BitWidth;
AdjustBitField = Expr->BitOffs != 0 || (EndBit != CHAR_BITS && EndBit != INT_BITS);
/* TODO: This probably needs to be guarded by AdjustBitField when long bit-fields are
** supported.
*/
Flags |= (EndBit <= CHAR_BITS) ? CF_CHAR : CF_INT;
Flags |= CF_UNSIGNED;
/* Flags we need operate on the whole bit-field, without CF_FORCECHAR. */
BitFieldFullWidthFlags = Flags;
/* If we're adjusting, then only load a char (not an int) and do only char ops;
** We will clear the high byte in the adjustment. CF_FORCECHAR does nothing if the
** type is not CF_CHAR.
*/
if (AdjustBitField) {
Flags |= CF_FORCECHAR;
}
} else if ((Flags & CF_TYPEMASK) == 0) {
Flags |= TypeOf (Expr->Type);
}
@ -211,10 +226,7 @@ void LoadExpr (unsigned Flags, struct ExprDesc* Expr)
** so be sure to always use unsigned ints for the operations.
*/
if (AdjustBitField) {
/* 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.
*/
unsigned F = Flags | CF_FORCECHAR | CF_CONST;
unsigned F = Flags | CF_CONST;
/* We always need to do something with the low byte, so there is no opportunity
** for optimization by skipping it.
@ -242,19 +254,21 @@ void LoadExpr (unsigned Flags, struct ExprDesc* Expr)
/* Shift right by the bit offset; no code is emitted if BitOffs is zero */
g_asr (F, Expr->BitOffs);
/* Since we have now shifted down, we can do char ops as long as the width fits in
** a char.
/* Since we have now shifted down, we could do char ops when the width fits in
** a char, but we also need to clear the high byte since we've been using
** CF_FORCECHAR up to now.
*/
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 it does end on a boundary, then zeros have already been shifted in,
** but we need to clear the high byte for char. g_and emits no code if the mask
** is all ones.
*/
if (EndBit != CHAR_BITS && EndBit != INT_BITS) {
g_and (F, (0x0001U << Expr->BitWidth) - 1U);
if (EndBit == CHAR_BITS) {
/* We need to clear the high byte, since CF_FORCECHAR was set. */
g_and (BitFieldFullWidthFlags | CF_CONST, 0xFF);
} else if (EndBit != INT_BITS) {
g_and (BitFieldFullWidthFlags | CF_CONST, (0x0001U << Expr->BitWidth) - 1U);
}
}
}