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ARMv4 unsigned integer division: Using an overflow-safe comparison method in the main calculation allows to put back the 1.5 cyle (average) optimisation. Shaved off another instruction, as we don't need the remainder. * Use the very efficient ffs algorithm from ffs-arm.S for dividing by a power of 2.

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git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19032 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Jens Arnold 2008-11-06 21:21:33 +00:00
parent 35823422c2
commit 545b51e2e4
1 changed files with 31 additions and 24 deletions
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55
apps/codecs/lib/udiv32_armv4.S
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@ -33,7 +33,7 @@
.macro ARM_DIV_BODY dividend, divisor, result, curbit
mov \result, \dividend
mov \curbit, #93 @ 3 * 31, (calculating branch dest)
mov \curbit, #90 @ 3 * 30, (calculating branch dest)
cmp \divisor, \result, lsr #16
movls \result,\result, lsr #16
subls \curbit, \curbit, #48
@ -44,40 +44,35 @@
movls \result,\result, lsr #4
subls \curbit, \curbit, #12
cmp \divisor, \result, lsr #2
movls \result,\result, lsr #2
subls \curbit, \curbit, #6
cmp \divisor, \result, lsr #1
subls \curbit, \curbit, #3
@ Calculation is only done down to shift=2, because the shift=1 step
@ would need 3 more cycles, but would only gain 1.5 cycles on average.
mov \result, #0
add pc, pc, \curbit, lsl #2
nop
.set shift, 32
.rept 32
.rept 31
.set shift, shift - 1
cmp \dividend, \divisor, lsl #shift
adc \result, \result, \result
subcs \dividend, \dividend, \divisor, lsl #shift
.endr
cmp \divisor, \dividend, lsr #shift
orrls \result, \result, #(1 << shift)
subls \dividend, \dividend, \divisor, lsl #shift
.endr @ shift==0 in the .rept would cause a warning for lsr #0
cmp \divisor, \dividend
orrls \result, \result, #1
@subls \dividend, \dividend, \divisor @ correct remainder not needed
.endm
.macro ARM_DIV2_ORDER divisor, order
cmp \divisor, #(1 << 16)
movhs \divisor, \divisor, lsr #16
movhs \order, #16
movlo \order, #0
@ There's exactly one bit set in the divisor, so ffs() can be used
@ This is the ffs algorithm devised by D.Seal and posted to
@ comp.sys.arm on 16 Feb 1994.
adr \order, L_ffs_table
orr \divisor, \divisor, \divisor, lsl #4 @ = X * 0x11
orr \divisor, \divisor, \divisor, lsl #6 @ = X * 0x451
rsb \divisor, \divisor, \divisor, lsl #16 @ = X * 0x0450fbaf
cmp \divisor, #(1 << 8)
movhs \divisor, \divisor, lsr #8
addhs \order, \order, #8
cmp \divisor, #(1 << 4)
movhs \divisor, \divisor, lsr #4
addhs \order, \order, #4
cmp \divisor, #(1 << 2)
addhi \order, \order, #3
addls \order, \order, \divisor, lsr #1
ldrb \order, [\order, \divisor, lsr #26]
.endm
@ -113,3 +108,15 @@ udiv32_arm:
ARM_DIV2_ORDER r1, r2
mov r0, r0, lsr r2
bx lr
L_ffs_table:
@ 0 1 2 3 4 5 6 7
@----------------------------------------------
.byte 32, 0, 1, 12, 2, 6, 0, 13 @ 0- 7
.byte 3, 0, 7, 0, 0, 0, 0, 14 @ 8-15
.byte 10, 4, 0, 0, 8, 0, 0, 25 @ 16-23
.byte 0, 0, 0, 0, 0, 21, 27, 15 @ 24-31
.byte 31, 11, 5, 0, 0, 0, 0, 0 @ 32-39
.byte 9, 0, 0, 24, 0, 0, 20, 26 @ 40-47
.byte 30, 0, 0, 0, 0, 23, 0, 19 @ 48-55
.byte 29, 0, 22, 18, 28, 17, 16, 0 @ 56-63