204 lines
3.8 KiB
ArmAsm
204 lines
3.8 KiB
ArmAsm
/*
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* linux/arch/arm/lib/div64.S
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*
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* Optimized computation of 64-bit dividend / 32-bit divisor
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*
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* Author: Nicolas Pitre
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* Created: Oct 5, 2003
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* Copyright: Monta Vista Software, Inc.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/linkage.h>
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#ifdef __ARMEB__
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#define xh r0
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#define xl r1
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#define yh r2
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#define yl r3
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#else
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#define xl r0
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#define xh r1
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#define yl r2
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#define yh r3
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#endif
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/*
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* __do_div64: perform a division with 64-bit dividend and 32-bit divisor.
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*
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* Note: Calling convention is totally non standard for optimal code.
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* This is meant to be used by do_div() from include/asm/div64.h only.
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*
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* Input parameters:
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* xh-xl = dividend (clobbered)
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* r4 = divisor (preserved)
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*
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* Output values:
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* yh-yl = result
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* xh = remainder
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*
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* Clobbered regs: xl, ip
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*/
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ENTRY(__do_div64)
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@ Test for easy paths first.
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subs ip, r4, #1
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bls 9f @ divisor is 0 or 1
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tst ip, r4
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beq 8f @ divisor is power of 2
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@ See if we need to handle upper 32-bit result.
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cmp xh, r4
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mov yh, #0
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blo 3f
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@ Align divisor with upper part of dividend.
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@ The aligned divisor is stored in yl preserving the original.
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@ The bit position is stored in ip.
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#if __LINUX_ARM_ARCH__ >= 5
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clz yl, r4
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clz ip, xh
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sub yl, yl, ip
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mov ip, #1
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mov ip, ip, lsl yl
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mov yl, r4, lsl yl
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#else
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mov yl, r4
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mov ip, #1
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1: cmp yl, #0x80000000
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cmpcc yl, xh
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movcc yl, yl, lsl #1
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movcc ip, ip, lsl #1
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bcc 1b
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#endif
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@ The division loop for needed upper bit positions.
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@ Break out early if dividend reaches 0.
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2: cmp xh, yl
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orrcs yh, yh, ip
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subcss xh, xh, yl
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movnes ip, ip, lsr #1
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mov yl, yl, lsr #1
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bne 2b
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@ See if we need to handle lower 32-bit result.
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3: cmp xh, #0
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mov yl, #0
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cmpeq xl, r4
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movlo xh, xl
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movlo pc, lr
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@ The division loop for lower bit positions.
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@ Here we shift remainer bits leftwards rather than moving the
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@ divisor for comparisons, considering the carry-out bit as well.
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mov ip, #0x80000000
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4: movs xl, xl, lsl #1
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adcs xh, xh, xh
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beq 6f
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cmpcc xh, r4
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5: orrcs yl, yl, ip
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subcs xh, xh, r4
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movs ip, ip, lsr #1
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bne 4b
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mov pc, lr
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@ The top part of remainder became zero. If carry is set
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@ (the 33th bit) this is a false positive so resume the loop.
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@ Otherwise, if lower part is also null then we are done.
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6: bcs 5b
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cmp xl, #0
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moveq pc, lr
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@ We still have remainer bits in the low part. Bring them up.
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#if __LINUX_ARM_ARCH__ >= 5
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clz xh, xl @ we know xh is zero here so...
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add xh, xh, #1
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mov xl, xl, lsl xh
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mov ip, ip, lsr xh
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#else
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7: movs xl, xl, lsl #1
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mov ip, ip, lsr #1
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bcc 7b
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#endif
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@ Current remainder is now 1. It is worthless to compare with
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@ divisor at this point since divisor can not be smaller than 3 here.
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@ If possible, branch for another shift in the division loop.
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@ If no bit position left then we are done.
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movs ip, ip, lsr #1
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mov xh, #1
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bne 4b
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mov pc, lr
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8: @ Division by a power of 2: determine what that divisor order is
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@ then simply shift values around
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#if __LINUX_ARM_ARCH__ >= 5
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clz ip, r4
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rsb ip, ip, #31
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#else
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mov yl, r4
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cmp r4, #(1 << 16)
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mov ip, #0
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movhs yl, yl, lsr #16
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movhs ip, #16
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cmp yl, #(1 << 8)
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movhs yl, yl, lsr #8
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addhs ip, ip, #8
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cmp yl, #(1 << 4)
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movhs yl, yl, lsr #4
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addhs ip, ip, #4
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cmp yl, #(1 << 2)
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addhi ip, ip, #3
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addls ip, ip, yl, lsr #1
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#endif
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mov yh, xh, lsr ip
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mov yl, xl, lsr ip
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rsb ip, ip, #32
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ARM( orr yl, yl, xh, lsl ip )
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THUMB( lsl xh, xh, ip )
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THUMB( orr yl, yl, xh )
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mov xh, xl, lsl ip
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mov xh, xh, lsr ip
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mov pc, lr
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@ eq -> division by 1: obvious enough...
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9: moveq yl, xl
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moveq yh, xh
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moveq xh, #0
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moveq pc, lr
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@ Division by 0:
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str lr, [sp, #-8]!
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bl __div0
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@ as wrong as it could be...
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mov yl, #0
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mov yh, #0
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mov xh, #0
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ldr pc, [sp], #8
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ENDPROC(__do_div64)
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