diff options
Diffstat (limited to 'apps/codecs/lib')
| -rw-r--r-- | apps/codecs/lib/udiv32_armv4.S | 55 |
1 files changed, 31 insertions, 24 deletions
diff --git a/apps/codecs/lib/udiv32_armv4.S b/apps/codecs/lib/udiv32_armv4.S index 2cb6fc8..6b34cae 100644 --- a/apps/codecs/lib/udiv32_armv4.S +++ b/apps/codecs/lib/udiv32_armv4.S @@ -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 |