\f
/* Shifts.
- We devote a fair bit of code to getting efficient shifts since we can only
- shift one bit at a time on the H8/300 and H8/300H and only one or two
- bits at a time on the H8/S.
-
- The basic shift methods:
-
- * loop shifts -- emit a loop using one (or two on H8/S) bit shifts;
- this is the default. SHIFT_LOOP
-
- * inlined shifts -- emit straight line code for the shift; this is
- used when a straight line shift is about the same size or smaller
- than a loop. We allow the inline version to be slightly longer in
- some cases as it saves a register. SHIFT_INLINE
-
- * rotate + and -- rotate the value the opposite direction, then
- mask off the values we don't need. This is used when only a few
- of the bits in the original value will survive in the shifted value.
- Again, this is used when it's about the same size or smaller than
- a loop. We allow this version to be slightly longer as it is usually
- much faster than a loop. SHIFT_ROT_AND
-
- * swap (+ shifts) -- often it's possible to swap bytes/words to
- simulate a shift by 8/16. Once swapped a few inline shifts can be
- added if the shift count is slightly more than 8 or 16. This is used
- when it's about the same size or smaller than a loop. We allow this
- version to be slightly longer as it is usually much faster than a loop.
- SHIFT_SPECIAL
-
- * There other oddballs. Not worth explaining. SHIFT_SPECIAL
-
- Here are some thoughts on what the absolutely positively best code is.
- "Best" here means some rational trade-off between code size and speed,
- where speed is more preferred but not at the expense of generating 20 insns.
-
- A trailing '*' after the shift count indicates the "best" mode isn't
- implemented.
+ We devote a fair bit of code to getting efficient shifts since we
+ can only shift one bit at a time on the H8/300 and H8/300H and only
+ one or two bits at a time on the H8/S.
+
+ All shift code falls into one of the following ways of
+ implementation:
+
+ o SHIFT_INLINE: Emit straight line code for the shift; this is used
+ when a straight line shift is about the same size or smaller than
+ a loop.
+
+ o SHIFT_ROT_AND: Rotate the value the opposite direction, then mask
+ off the bits we don't need. This is used when only a few of the
+ bits in the original value will survive in the shifted value.
+
+ o SHIFT_SPECIAL: Often it's possible to move a byte or a word to
+ simulate a shift by 8, 16, or 24 bits. Once moved, a few inline
+ shifts can be added if the shift count is slightly more than 8 or
+ 16. This case also includes other oddballs that are not worth
+ explaning here.
+
+ o SHIFT_LOOP: Emit a loop using one (or two on H8/S) bit shifts.
+
+ Here are some thoughts on what the absolutely positively best code
+ is. "Best" here means some rational trade-off between code size
+ and speed, where speed is more preferred but not at the expense of
+ generating 20 insns.
+
+ Below, a trailing '*' after the shift count indicates the "best"
+ mode isn't implemented. We only describe SHIFT_SPECIAL cases to
+ simplify the table. For other cases, refer to shift_alg_[qhs]i.
H8/300 QImode shifts
- 1-4 - do them inline
- 5-6 - ASHIFT | LSHIFTRT: rotate, mask off other bits
- ASHIFTRT: loop
- 7 - ASHIFT | LSHIFTRT: rotate, mask off other bits
- ASHIFTRT: shll, subx (propagate carry bit to all bits)
+ 7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
H8/300 HImode shifts
- 1-4 - do them inline
- 5-6 - loop
7 - shift 2nd half other way into carry.
copy 1st half into 2nd half
rotate 2nd half other way with carry
sign extend 1st half (ASHIFTRT)
8 - move byte, zero (ASHIFT | LSHIFTRT) or sign extend other (ASHIFTRT)
9-12 - do shift by 8, inline remaining shifts
- 13-14* - ASHIFT | LSHIFTRT: rotate 3/2, mask, move byte, set other byte to 0
- - ASHIFTRT: loop
- 15 - ASHIFT | LSHIFTRT: rotate 1, mask, move byte, set other byte to 0
- - ASHIFTRT: shll, subx, set other byte
+ 15 - ASHIFTRT: shll, subx, set other byte
H8/300 SImode shifts
- 1-2 - do them inline
- 3-6 - loop
7* - shift other way once, move bytes into place,
move carry into place (possibly with sign extension)
8 - move bytes into place, zero or sign extend other
- 9-14 - loop
15* - shift other way once, move word into place, move carry into place
16 - move word, zero or sign extend other
- 17-23 - loop
24* - move bytes into place, zero or sign extend other
- 25-27 - loop
- 28-30* - ASHIFT | LSHIFTRT: rotate top byte, mask, move byte into place,
- zero others
- ASHIFTRT: loop
- 31 - ASHIFT | LSHIFTRT: rotate top byte, mask, move byte into place,
- zero others
- ASHIFTRT: shll top byte, subx, copy to other bytes
+ 31 - ASHIFTRT: shll top byte, subx, copy to other bytes
H8/300H QImode shifts (same as H8/300 QImode shifts)
- 1-4 - do them inline
- 5-6 - ASHIFT | LSHIFTRT: rotate, mask off other bits
- ASHIFTRT: loop
- 7 - ASHIFT | LSHIFTRT: rotate, mask off other bits
- ASHIFTRT: shll, subx (propagate carry bit to all bits)
+ 7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
H8/300H HImode shifts
- 1-4 - do them inline
- 5-6 - loop
7 - shift 2nd half other way into carry.
copy 1st half into 2nd half
rotate entire word other way using carry
sign extend remaining bits (ASHIFTRT)
8 - move byte, zero (ASHIFT | LSHIFTRT) or sign extend other (ASHIFTRT)
9-12 - do shift by 8, inline remaining shifts
- 13-14 - ASHIFT | LSHIFTRT: rotate 3/2, mask, move byte, set other byte to 0
- - ASHIFTRT: loop
- 15 - ASHIFT | LSHIFTRT: rotate 1, mask, move byte, set other byte to 0
- - ASHIFTRT: shll, subx, set other byte
+ 15 - ASHIFTRT: shll, subx, set other byte
H8/300H SImode shifts
(These are complicated by the fact that we don't have byte level access to
the top word.)
A word is: bytes 3,2,1,0 (msb -> lsb), word 1,0 (msw -> lsw)
- 1-4 - do them inline
- 5-14 - loop
15* - shift other way once, move word into place, move carry into place
(with sign extension for ASHIFTRT)
16 - move word into place, zero or sign extend other
17-20 - do 16bit shift, then inline remaining shifts
- 20-23 - loop
24* - ASHIFT: move byte 0(msb) to byte 1, zero byte 0,
move word 0 to word 1, zero word 0
LSHIFTRT: move word 1 to word 0, move byte 1 to byte 0,
sign extend byte 0, sign extend word 0
25-27* - either loop, or
do 24 bit shift, inline rest
- 28-30 - ASHIFT: rotate 4/3/2, mask
- LSHIFTRT: rotate 4/3/2, mask
- ASHIFTRT: loop
31 - shll, subx byte 0, sign extend byte 0, sign extend word 0
H8/S QImode shifts
- 1-6 - do them inline
- 7 - ASHIFT | LSHIFTRT: rotate, mask off other bits
- ASHIFTRT: shll, subx (propagate carry bit to all bits)
+ 7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
H8/S HImode shifts
- 1-7 - do them inline
8 - move byte, zero (ASHIFT | LSHIFTRT) or sign extend other (ASHIFTRT)
9-12 - do shift by 8, inline remaining shifts
- 13-14 - ASHIFT | LSHIFTRT: rotate 3/2, mask, move byte, set other byte to 0
- - ASHIFTRT: loop
- 15 - ASHIFT | LSHIFTRT: rotate 1, mask, move byte, set other byte to 0
- - ASHIFTRT: shll, subx, set other byte
+ 15 - ASHIFTRT: shll, subx, set other byte
H8/S SImode shifts
(These are complicated by the fact that we don't have byte level access to
the top word.)
A word is: bytes 3,2,1,0 (msb -> lsb), word 1,0 (msw -> lsw)
- 1-10 - do them inline
- 11-14 - loop
15* - shift other way once, move word into place, move carry into place
(with sign extension for ASHIFTRT)
16 - move word into place, zero or sign extend other
17-20 - do 16bit shift, then inline remaining shifts
- 21-23 - loop
24* - ASHIFT: move byte 0(msb) to byte 1, zero byte 0,
move word 0 to word 1, zero word 0
LSHIFTRT: move word 1 to word 0, move byte 1 to byte 0,
sign extend byte 0, sign extend word 0
25-27* - either loop, or
do 24 bit shift, inline rest
- 28-30 - ASHIFT: rotate 4/3/2, mask
- LSHIFTRT: rotate 4/3/2, mask
- ASHIFTRT: loop
31 - shll, subx byte 0, sign extend byte 0, sign extend word 0
Panic!!! */
return 1;
}
-/* Shift algorithm determination.
-
- There are various ways of doing a shift:
- SHIFT_INLINE: If the amount is small enough, just generate as many one-bit
- shifts as we need.
- SHIFT_ROT_AND: If the amount is large but close to either end, rotate the
- necessary bits into position and then set the rest to zero.
- SHIFT_SPECIAL: Hand crafted assembler.
- SHIFT_LOOP: If the above methods fail, just loop. */
+/* See above for explanation of this enum. */
enum shift_alg
{
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