* config/h8300/h8300.c: Update the comments related to shifts.
From-SVN: r59836
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Kazu Hirata
Kazu Hirata
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2 changed files with 20 additions and 97 deletions
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@ -1,3 +1,7 @@
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2002-12-04 Kazu Hirata <kazu@cs.umass.edu>
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* config/h8300/h8300.c: Update the comments related to shifts.
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2002-12-04 Chris Demetriou <cgd@broadcom.com>
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* config/mips/mips.md (get_fnaddr): Correct length attribute.
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@ -2148,96 +2148,16 @@ compute_logical_op_cc (mode, operands)
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o SHIFT_LOOP: Emit a loop using one (or two on H8S) bit shifts.
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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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For each shift count, we try to use code that has no trade-off
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between code size and speed whenever possible.
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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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If the trade-off is unavoidable, we try to be reasonable.
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Specifically, the fastest version is one instruction longer than
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the shortest version, we take the fastest version. We also provide
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the use a way to switch back to the shortest version with -Os.
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H8/300 QImode shifts
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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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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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rotate 1st half other way (no carry)
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mask off bits in 1st half (ASHIFT | LSHIFTRT).
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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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15 - ASHIFTRT: shll, subx, set other byte
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H8/300 SImode shifts
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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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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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24* - move bytes into place, zero or sign extend other
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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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7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
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H8/300H HImode shifts
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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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mask off remaining bits (ASHIFT | LSHIFTRT)
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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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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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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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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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zero word 1, zero byte 1
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ASHIFTRT: move word 1 to word 0, move byte 1 to byte 0,
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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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31 - shll, subx byte 0, sign extend byte 0, sign extend word 0
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H8S QImode shifts
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7 - ASHIFTRT: shll, subx (propagate carry bit to all bits)
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H8S HImode shifts
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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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15 - ASHIFTRT: shll, subx, set other byte
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H8S 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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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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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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zero word 1, zero byte 1
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ASHIFTRT: move word 1 to word 0, move byte 1 to byte 0,
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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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31 - shll, subx byte 0, sign extend byte 0, sign extend word 0
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Panic!!! */
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For the details of the shift algorithms for various shift counts,
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refer to shift_alg_[qhs]i. */
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int
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nshift_operator (x, mode)
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@ -2474,16 +2394,15 @@ static void get_shift_alg PARAMS ((enum shift_type,
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/* Given SHIFT_TYPE, SHIFT_MODE, and shift count COUNT, determine the
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best algorithm for doing the shift. The assembler code is stored
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in the pointers in INFO. We don't achieve maximum efficiency in
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all cases, but the hooks are here to do so.
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in the pointers in INFO. We achieve the maximum efficiency in most
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cases when !TARGET_H8300. In case of TARGET_H8300, shifts in
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SImode in particular have a lot of room to optimize.
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For now we just use lots of switch statements. Since we don't even come
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close to supporting all the cases, this is simplest. If this function ever
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gets too big, perhaps resort to a more table based lookup. Of course,
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at this point you may just wish to do it all in rtl.
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WARNING: The constraints on insns shiftbyn_QI/HI/SI assume shifts of
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1,2,3,4 will be inlined (1,2 for SI). */
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We first determine the strategy of the shift algorithm by a table
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lookup. If that tells us to use a hand crafted assembly code, we
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go into the big switch statement to find what that is. Otherwise,
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we resort to a generic way, such as inlining. In either case, the
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result is returned through INFO. */
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static void
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get_shift_alg (shift_type, shift_mode, count, info)
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