* config/h8300/h8300.c: Revise comments about shift code.
From-SVN: r49030
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Kazu Hirata
Kazu Hirata
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@ -1,3 +1,7 @@
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2002-01-20 Kazu Hirata <kazu@hxi.com>
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* config/h8300/h8300.c: Revise comments about shift code.
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2002-01-20 Kazu Hirata <kazu@hxi.com>
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* config/h8300/h8300.c (function_arg): Update a comment.
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@ -1669,53 +1669,42 @@ output_logical_op (mode, code, operands)
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/* Shifts.
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We devote a fair bit of code to getting efficient shifts since we can only
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shift one bit at a time on the H8/300 and H8/300H and only one or two
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bits at a time on the H8/S.
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We devote a fair bit of code to getting efficient shifts since we
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can only shift one bit at a time on the H8/300 and H8/300H and only
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one or two bits at a time on the H8/S.
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The basic shift methods:
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All shift code falls into one of the following ways of
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implementation:
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* loop shifts -- emit a loop using one (or two on H8/S) bit shifts;
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this is the default. SHIFT_LOOP
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o SHIFT_INLINE: Emit straight line code for the shift; this is used
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when a straight line shift is about the same size or smaller than
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a loop.
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* inlined shifts -- emit straight line code for the shift; this is
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used when a straight line shift is about the same size or smaller
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than a loop. We allow the inline version to be slightly longer in
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some cases as it saves a register. SHIFT_INLINE
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o SHIFT_ROT_AND: Rotate the value the opposite direction, then mask
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off the bits we don't need. This is used when only a few of the
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bits in the original value will survive in the shifted value.
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* rotate + and -- rotate the value the opposite direction, then
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mask off the values we don't need. This is used when only a few
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of the bits in the original value will survive in the shifted value.
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Again, this is used when it's about the same size or smaller than
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a loop. We allow this version to be slightly longer as it is usually
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much faster than a loop. SHIFT_ROT_AND
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o SHIFT_SPECIAL: Often it's possible to move a byte or a word to
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simulate a shift by 8, 16, or 24 bits. Once moved, a few inline
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shifts can be added if the shift count is slightly more than 8 or
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16. This case also includes other oddballs that are not worth
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explaning here.
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* swap (+ shifts) -- often it's possible to swap bytes/words to
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simulate a shift by 8/16. Once swapped a few inline shifts can be
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added if the shift count is slightly more than 8 or 16. This is used
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when it's about the same size or smaller than a loop. We allow this
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version to be slightly longer as it is usually much faster than a loop.
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SHIFT_SPECIAL
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o SHIFT_LOOP: Emit a loop using one (or two on H8/S) bit shifts.
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* There other oddballs. Not worth explaining. SHIFT_SPECIAL
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Here are some thoughts on what the absolutely positively best code
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is. "Best" here means some rational trade-off between code size
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and speed, where speed is more preferred but not at the expense of
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generating 20 insns.
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Here are some thoughts on what the absolutely positively best code is.
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"Best" here means some rational trade-off between code size and speed,
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where speed is more preferred but not at the expense of generating 20 insns.
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A trailing '*' after the shift count indicates the "best" mode isn't
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implemented.
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Below, a trailing '*' after the shift count indicates the "best"
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mode isn't implemented. We only describe SHIFT_SPECIAL cases to
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simplify the table. For other cases, refer to shift_alg_[qhs]i.
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H8/300 QImode shifts
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1-4 - do them inline
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5-6 - ASHIFT | LSHIFTRT: rotate, mask off other bits
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ASHIFTRT: loop
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7 - ASHIFT | LSHIFTRT: rotate, mask off other bits
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ASHIFTRT: shll, subx (propagate carry bit to all bits)
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7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
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H8/300 HImode shifts
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1-4 - do them inline
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5-6 - loop
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7 - shift 2nd half other way into carry.
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copy 1st half into 2nd half
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rotate 2nd half other way with carry
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@ -1724,40 +1713,21 @@ output_logical_op (mode, code, operands)
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sign extend 1st half (ASHIFTRT)
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8 - move byte, zero (ASHIFT | LSHIFTRT) or sign extend other (ASHIFTRT)
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9-12 - do shift by 8, inline remaining shifts
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13-14* - ASHIFT | LSHIFTRT: rotate 3/2, mask, move byte, set other byte to 0
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- ASHIFTRT: loop
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15 - ASHIFT | LSHIFTRT: rotate 1, mask, move byte, set other byte to 0
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- ASHIFTRT: shll, subx, set other byte
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15 - ASHIFTRT: shll, subx, set other byte
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H8/300 SImode shifts
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1-2 - do them inline
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3-6 - loop
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7* - shift other way once, move bytes into place,
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move carry into place (possibly with sign extension)
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8 - move bytes into place, zero or sign extend other
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9-14 - loop
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15* - shift other way once, move word into place, move carry into place
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16 - move word, zero or sign extend other
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17-23 - loop
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24* - move bytes into place, zero or sign extend other
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25-27 - loop
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28-30* - ASHIFT | LSHIFTRT: rotate top byte, mask, move byte into place,
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zero others
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ASHIFTRT: loop
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31 - ASHIFT | LSHIFTRT: rotate top byte, mask, move byte into place,
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zero others
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ASHIFTRT: shll top byte, subx, copy to other bytes
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31 - ASHIFTRT: shll top byte, subx, copy to other bytes
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H8/300H QImode shifts (same as H8/300 QImode shifts)
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1-4 - do them inline
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5-6 - ASHIFT | LSHIFTRT: rotate, mask off other bits
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ASHIFTRT: loop
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7 - ASHIFT | LSHIFTRT: rotate, mask off other bits
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ASHIFTRT: shll, subx (propagate carry bit to all bits)
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7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
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H8/300H HImode shifts
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1-4 - do them inline
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5-6 - loop
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7 - shift 2nd half other way into carry.
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copy 1st half into 2nd half
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rotate entire word other way using carry
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@ -1765,22 +1735,16 @@ output_logical_op (mode, code, operands)
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sign extend remaining bits (ASHIFTRT)
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8 - move byte, zero (ASHIFT | LSHIFTRT) or sign extend other (ASHIFTRT)
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9-12 - do shift by 8, inline remaining shifts
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13-14 - ASHIFT | LSHIFTRT: rotate 3/2, mask, move byte, set other byte to 0
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- ASHIFTRT: loop
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15 - ASHIFT | LSHIFTRT: rotate 1, mask, move byte, set other byte to 0
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- ASHIFTRT: shll, subx, set other byte
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15 - ASHIFTRT: shll, subx, set other byte
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H8/300H SImode shifts
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(These are complicated by the fact that we don't have byte level access to
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the top word.)
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A word is: bytes 3,2,1,0 (msb -> lsb), word 1,0 (msw -> lsw)
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1-4 - do them inline
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5-14 - loop
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15* - shift other way once, move word into place, move carry into place
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(with sign extension for ASHIFTRT)
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16 - move word into place, zero or sign extend other
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17-20 - do 16bit shift, then inline remaining shifts
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20-23 - loop
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24* - ASHIFT: move byte 0(msb) to byte 1, zero byte 0,
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move word 0 to word 1, zero word 0
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LSHIFTRT: move word 1 to word 0, move byte 1 to byte 0,
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@ -1789,36 +1753,24 @@ output_logical_op (mode, code, operands)
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sign extend byte 0, sign extend word 0
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25-27* - either loop, or
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do 24 bit shift, inline rest
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28-30 - ASHIFT: rotate 4/3/2, mask
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LSHIFTRT: rotate 4/3/2, mask
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ASHIFTRT: loop
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31 - shll, subx byte 0, sign extend byte 0, sign extend word 0
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H8/S QImode shifts
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1-6 - do them inline
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7 - ASHIFT | LSHIFTRT: rotate, mask off other bits
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ASHIFTRT: shll, subx (propagate carry bit to all bits)
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7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
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H8/S HImode shifts
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1-7 - do them inline
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8 - move byte, zero (ASHIFT | LSHIFTRT) or sign extend other (ASHIFTRT)
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9-12 - do shift by 8, inline remaining shifts
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13-14 - ASHIFT | LSHIFTRT: rotate 3/2, mask, move byte, set other byte to 0
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- ASHIFTRT: loop
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15 - ASHIFT | LSHIFTRT: rotate 1, mask, move byte, set other byte to 0
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- ASHIFTRT: shll, subx, set other byte
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15 - ASHIFTRT: shll, subx, set other byte
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H8/S SImode shifts
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(These are complicated by the fact that we don't have byte level access to
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the top word.)
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A word is: bytes 3,2,1,0 (msb -> lsb), word 1,0 (msw -> lsw)
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1-10 - do them inline
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11-14 - loop
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15* - shift other way once, move word into place, move carry into place
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(with sign extension for ASHIFTRT)
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16 - move word into place, zero or sign extend other
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17-20 - do 16bit shift, then inline remaining shifts
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21-23 - loop
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24* - ASHIFT: move byte 0(msb) to byte 1, zero byte 0,
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move word 0 to word 1, zero word 0
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LSHIFTRT: move word 1 to word 0, move byte 1 to byte 0,
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@ -1827,9 +1779,6 @@ output_logical_op (mode, code, operands)
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sign extend byte 0, sign extend word 0
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25-27* - either loop, or
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do 24 bit shift, inline rest
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28-30 - ASHIFT: rotate 4/3/2, mask
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LSHIFTRT: rotate 4/3/2, mask
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ASHIFTRT: loop
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31 - shll, subx byte 0, sign extend byte 0, sign extend word 0
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Panic!!! */
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@ -1878,15 +1827,7 @@ expand_a_shift (mode, code, operands)
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return 1;
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}
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/* Shift algorithm determination.
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There are various ways of doing a shift:
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SHIFT_INLINE: If the amount is small enough, just generate as many one-bit
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shifts as we need.
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SHIFT_ROT_AND: If the amount is large but close to either end, rotate the
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necessary bits into position and then set the rest to zero.
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SHIFT_SPECIAL: Hand crafted assembler.
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SHIFT_LOOP: If the above methods fail, just loop. */
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/* See above for explanation of this enum. */
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enum shift_alg
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{
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