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Used TAY/TYA instead of PHA/PLA when extracting a bit-field.

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Without optimization, it saves a few CPU cycles.  With optimization, it saves more cycles and a few bytes.
This commit is contained in:
Greg King 2020-09-18 16:33:12 -04:00
parent ea95728330
commit 81ac28ff71
1 changed files with 9 additions and 9 deletions
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18
src/cc65/codegen.c
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@ -4511,8 +4511,8 @@ void g_extractbitfield (unsigned Flags, unsigned FullWidthFlags, int IsSigned,
/* Handle signed bit-fields. */ /* Handle signed bit-fields. */
if (IsSigned) { if (IsSigned) {
/* Push A, since the sign bit test will destroy it. */ /* Save .A because the sign-bit test will destroy it. */
AddCodeLine ("pha"); AddCodeLine ("tay");
/* Check sign bit */ /* Check sign bit */
unsigned SignBitPos = BitWidth - 1U; unsigned SignBitPos = BitWidth - 1U;
@ -4520,7 +4520,7 @@ void g_extractbitfield (unsigned Flags, unsigned FullWidthFlags, int IsSigned,
unsigned SignBitPosInByte = SignBitPos % CHAR_BITS; unsigned SignBitPosInByte = SignBitPos % CHAR_BITS;
unsigned SignBitMask = 1U << SignBitPosInByte; unsigned SignBitMask = 1U << SignBitPosInByte;
/* Move the correct byte to A. This can only be X for now, /* Move the correct byte to .A. This can be only .X for now,
** but more cases will be needed to support long. ** but more cases will be needed to support long.
*/ */
switch (SignBitByte) { switch (SignBitByte) {
@ -4538,10 +4538,10 @@ void g_extractbitfield (unsigned Flags, unsigned FullWidthFlags, int IsSigned,
unsigned ZeroExtendLabel = GetLocalLabel (); unsigned ZeroExtendLabel = GetLocalLabel ();
AddCodeLine ("beq %s", LocalLabelName (ZeroExtendLabel)); AddCodeLine ("beq %s", LocalLabelName (ZeroExtendLabel));
/* Pop A back and sign-extend if required; operating on the full result needs /* Get back .A and sign-extend if required; operating on the full result needs
** to sign-extend into high byte, too. ** to sign-extend into the high byte, too.
*/ */
AddCodeLine ("pla"); AddCodeLine ("tya");
g_or (FullWidthFlags | CF_CONST, ~Mask); g_or (FullWidthFlags | CF_CONST, ~Mask);
/* We can generate a branch, instead of a jump, here because we know /* We can generate a branch, instead of a jump, here because we know
@ -4551,11 +4551,11 @@ void g_extractbitfield (unsigned Flags, unsigned FullWidthFlags, int IsSigned,
unsigned DoneLabel = GetLocalLabel (); unsigned DoneLabel = GetLocalLabel ();
g_branch (DoneLabel); g_branch (DoneLabel);
/* Pop A back, then zero-extend. We need to duplicate the PLA, rather than move it before /* Get back .A, then zero-extend. We need to duplicate the TYA, rather than move it before
** the branch to share with the other label, because PLA changes some condition codes. ** the branch to share with the other label, because TYA changes some condition codes.
*/ */
g_defcodelabel (ZeroExtendLabel); g_defcodelabel (ZeroExtendLabel);
AddCodeLine ("pla"); AddCodeLine ("tya");
/* Zero the upper bits, the same as the unsigned path. */ /* Zero the upper bits, the same as the unsigned path. */
if (ZeroExtendMask != 0) { if (ZeroExtendMask != 0) {