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Commit 005e3e05 by Kazu Hirata Committed by Kazu Hirata
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* config/h8300/h8300.c: Revise comments about shift code. 

From-SVN: r49030
parent 64bead4c
Showing with 36 additions and 91 deletions
gcc/ChangeLog
2002-01-20 Kazu Hirata <kazu@hxi.com>
* config/h8300/h8300.c: Revise comments about shift code.
2002-01-20 Kazu Hirata <kazu@hxi.com>
* config/h8300/h8300.c (function_arg): Update a comment.
2002-01-20 Kazu Hirata <kazu@hxi.com>
......
gcc/config/h8300/h8300.c
......@@ -1669,53 +1669,42 @@ output_logical_op (mode, code, operands)
/* 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.
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:
All shift code falls into one of the following ways of
implementation:
* loop shifts -- emit a loop using one (or two on H8/S) bit shifts;
this is the default. SHIFT_LOOP
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.
* 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
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.
* 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
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.
* 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
o SHIFT_LOOP: Emit a loop using one (or two on H8/S) bit shifts.
* 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.
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.
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
......@@ -1724,40 +1713,21 @@ output_logical_op (mode, code, operands)
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
......@@ -1765,22 +1735,16 @@ output_logical_op (mode, code, operands)
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,
......@@ -1789,36 +1753,24 @@ output_logical_op (mode, code, operands)
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,
......@@ -1827,9 +1779,6 @@ output_logical_op (mode, code, operands)
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!!! */
......@@ -1878,15 +1827,7 @@ expand_a_shift (mode, code, operands)
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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