src/runtime/vlop_arm.s

// Inferno's libkern/vlop-arm.s
// https://bitbucket.org/inferno-os/inferno-os/src/default/libkern/vlop-arm.s
//
//         Copyright © 1994-1999 Lucent Technologies Inc. All rights reserved.
//         Revisions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com).  All rights reserved.
//         Portions Copyright 2009 The Go Authors. All rights reserved.
//
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// of this software and associated documentation files (the "Software"), to deal
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// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// THE SOFTWARE.

#include "go_asm.h"
#include "go_tls.h"
#include "funcdata.h"
#include "textflag.h"

// func runtime·udiv(n, d uint32) (q, r uint32)
// compiler knowns the register usage of this function
// Reference: 
// Sloss, Andrew et. al; ARM System Developer's Guide: Designing and Optimizing System Software
// Morgan Kaufmann; 1 edition (April 8, 2004), ISBN 978-1558608740
#define Rq	R0 // input d, output q
#define Rr	R1 // input n, output r
#define Rs	R2 // three temporary variables
#define RM	R3
#define Ra	R11

// Be careful: Ra == R11 will be used by the linker for synthesized instructions.
// Note: this function does not have a frame. If it ever needs a frame,
// the RET instruction will clobber R12 on nacl, and the compiler's register
// allocator needs to know.
TEXT runtime·udiv(SB),NOSPLIT|NOFRAME,$0
	MOVBU	runtime·hardDiv(SB), Ra
	CMP	$0, Ra
	BNE	udiv_hardware

	CLZ 	Rq, Rs // find normalizing shift
	MOVW.S	Rq<<Rs, Ra
	MOVW	$fast_udiv_tab<>-64(SB), RM
	ADD.NE	Ra>>25, RM, Ra // index by most significant 7 bits of divisor
	MOVBU.NE	(Ra), Ra

	SUB.S	$7, Rs
	RSB 	$0, Rq, RM // M = -q
	MOVW.PL	Ra<<Rs, Rq

	// 1st Newton iteration
	MUL.PL	RM, Rq, Ra // a = -q*d
	BMI 	udiv_by_large_d
	MULAWT	Ra, Rq, Rq, Rq // q approx q-(q*q*d>>32)
	TEQ 	RM->1, RM // check for d=0 or d=1

	// 2nd Newton iteration
	MUL.NE	RM, Rq, Ra
	MOVW.NE	$0, Rs
	MULAL.NE Rq, Ra, (Rq,Rs)
	BEQ 	udiv_by_0_or_1

	// q now accurate enough for a remainder r, 0<=r<3*d
	MULLU	Rq, Rr, (Rq,Rs) // q = (r * q) >> 32
	ADD 	RM, Rr, Rr // r = n - d
	MULA	RM, Rq, Rr, Rr // r = n - (q+1)*d

	// since 0 <= n-q*d < 3*d; thus -d <= r < 2*d
	CMN 	RM, Rr // t = r-d
	SUB.CS	RM, Rr, Rr // if (t<-d || t>=0) r=r+d
	ADD.CC	$1, Rq
	ADD.PL	RM<<1, Rr
	ADD.PL	$2, Rq
	RET

// use hardware divider
udiv_hardware:
	DIVUHW	Rq, Rr, Rs
	MUL	Rs, Rq, RM
	RSB	Rr, RM, Rr
	MOVW	Rs, Rq
	RET

udiv_by_large_d:
	// at this point we know d>=2^(31-6)=2^25
	SUB 	$4, Ra, Ra
	RSB 	$0, Rs, Rs
	MOVW	Ra>>Rs, Rq
	MULLU	Rq, Rr, (Rq,Rs)
	MULA	RM, Rq, Rr, Rr

	// q now accurate enough for a remainder r, 0<=r<4*d
	CMN 	Rr>>1, RM // if(r/2 >= d)
	ADD.CS	RM<<1, Rr
	ADD.CS	$2, Rq
	CMN 	Rr, RM
	ADD.CS	RM, Rr
	ADD.CS	$1, Rq
	RET

udiv_by_0_or_1:
	// carry set if d==1, carry clear if d==0
	BCC udiv_by_0
	MOVW	Rr, Rq
	MOVW	$0, Rr
	RET

udiv_by_0:
	MOVW	$runtime·panicdivide(SB), R11
	B	(R11)

// var tab [64]byte
// tab[0] = 255; for i := 1; i <= 63; i++ { tab[i] = (1<<14)/(64+i) }
// laid out here as little-endian uint32s
DATA fast_udiv_tab<>+0x00(SB)/4, $0xf4f8fcff
DATA fast_udiv_tab<>+0x04(SB)/4, $0xe6eaedf0
DATA fast_udiv_tab<>+0x08(SB)/4, $0xdadde0e3
DATA fast_udiv_tab<>+0x0c(SB)/4, $0xcfd2d4d7
DATA fast_udiv_tab<>+0x10(SB)/4, $0xc5c7cacc
DATA fast_udiv_tab<>+0x14(SB)/4, $0xbcbec0c3
DATA fast_udiv_tab<>+0x18(SB)/4, $0xb4b6b8ba
DATA fast_udiv_tab<>+0x1c(SB)/4, $0xacaeb0b2
DATA fast_udiv_tab<>+0x20(SB)/4, $0xa5a7a8aa
DATA fast_udiv_tab<>+0x24(SB)/4, $0x9fa0a2a3
DATA fast_udiv_tab<>+0x28(SB)/4, $0x999a9c9d
DATA fast_udiv_tab<>+0x2c(SB)/4, $0x93949697
DATA fast_udiv_tab<>+0x30(SB)/4, $0x8e8f9092
DATA fast_udiv_tab<>+0x34(SB)/4, $0x898a8c8d
DATA fast_udiv_tab<>+0x38(SB)/4, $0x85868788
DATA fast_udiv_tab<>+0x3c(SB)/4, $0x81828384
GLOBL fast_udiv_tab<>(SB), RODATA, $64

// The linker will pass numerator in R8
#define Rn R8
// The linker expects the result in RTMP
#define RTMP R11

TEXT runtime·_divu(SB), NOSPLIT, $16-0
	// It's not strictly true that there are no local pointers.
	// It could be that the saved registers Rq, Rr, Rs, and Rm
	// contain pointers. However, the only way this can matter
	// is if the stack grows (which it can't, udiv is nosplit)
	// or if a fault happens and more frames are added to
	// the stack due to deferred functions.
	// In the latter case, the stack can grow arbitrarily,
	// and garbage collection can happen, and those
	// operations care about pointers, but in that case
	// the calling frame is dead, and so are the saved
	// registers. So we can claim there are no pointers here.
	NO_LOCAL_POINTERS
	MOVW	Rq, 4(R13)
	MOVW	Rr, 8(R13)
	MOVW	Rs, 12(R13)
	MOVW	RM, 16(R13)

	MOVW	Rn, Rr			/* numerator */
	MOVW	g_m(g), Rq
	MOVW	m_divmod(Rq), Rq	/* denominator */
	BL  	runtime·udiv(SB)
	MOVW	Rq, RTMP
	MOVW	4(R13), Rq
	MOVW	8(R13), Rr
	MOVW	12(R13), Rs
	MOVW	16(R13), RM
	RET

TEXT runtime·_modu(SB), NOSPLIT, $16-0
	NO_LOCAL_POINTERS
	MOVW	Rq, 4(R13)
	MOVW	Rr, 8(R13)
	MOVW	Rs, 12(R13)
	MOVW	RM, 16(R13)

	MOVW	Rn, Rr			/* numerator */
	MOVW	g_m(g), Rq
	MOVW	m_divmod(Rq), Rq	/* denominator */
	BL  	runtime·udiv(SB)
	MOVW	Rr, RTMP
	MOVW	4(R13), Rq
	MOVW	8(R13), Rr
	MOVW	12(R13), Rs
	MOVW	16(R13), RM
	RET

TEXT runtime·_div(SB),NOSPLIT,$16-0
	NO_LOCAL_POINTERS
	MOVW	Rq, 4(R13)
	MOVW	Rr, 8(R13)
	MOVW	Rs, 12(R13)
	MOVW	RM, 16(R13)
	MOVW	Rn, Rr			/* numerator */
	MOVW	g_m(g), Rq
	MOVW	m_divmod(Rq), Rq	/* denominator */
	CMP 	$0, Rr
	BGE 	d1
	RSB 	$0, Rr, Rr
	CMP 	$0, Rq
	BGE 	d2
	RSB 	$0, Rq, Rq
d0:
	BL  	runtime·udiv(SB)  	/* none/both neg */
	MOVW	Rq, RTMP
	B	out1
d1:
	CMP 	$0, Rq
	BGE 	d0
	RSB 	$0, Rq, Rq
d2:
	BL  	runtime·udiv(SB)  	/* one neg */
	RSB	$0, Rq, RTMP
out1:
	MOVW	4(R13), Rq
	MOVW	8(R13), Rr
	MOVW	12(R13), Rs
	MOVW	16(R13), RM
	RET

TEXT runtime·_mod(SB),NOSPLIT,$16-0
	NO_LOCAL_POINTERS
	MOVW	Rq, 4(R13)
	MOVW	Rr, 8(R13)
	MOVW	Rs, 12(R13)
	MOVW	RM, 16(R13)
	MOVW	Rn, Rr			/* numerator */
	MOVW	g_m(g), Rq
	MOVW	m_divmod(Rq), Rq	/* denominator */
	CMP 	$0, Rq
	RSB.LT	$0, Rq, Rq
	CMP 	$0, Rr
	BGE 	m1
	RSB 	$0, Rr, Rr
	BL  	runtime·udiv(SB)  	/* neg numerator */
	RSB 	$0, Rr, RTMP
	B   	out
m1:
	BL  	runtime·udiv(SB)  	/* pos numerator */
	MOVW	Rr, RTMP
out:
	MOVW	4(R13), Rq
	MOVW	8(R13), Rr
	MOVW	12(R13), Rs
	MOVW	16(R13), RM
	RET

// _mul64by32 and _div64by32 not implemented on arm
TEXT runtime·_mul64by32(SB), NOSPLIT, $0
	MOVW	$0, R0
	MOVW	(R0), R1 // crash

TEXT runtime·_div64by32(SB), NOSPLIT, $0
	MOVW	$0, R0
	MOVW	(R0), R1 // crash