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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /arch/alpha/lib/divide.S
downloadlinux-stericsson-2.6.12-rc2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+/*
+ * arch/alpha/lib/divide.S
+ *
+ * (C) 1995 Linus Torvalds
+ *
+ * Alpha division..
+ */
+
+/*
+ * The alpha chip doesn't provide hardware division, so we have to do it
+ * by hand. The compiler expects the functions
+ *
+ * __divqu: 64-bit unsigned long divide
+ * __remqu: 64-bit unsigned long remainder
+ * __divqs/__remqs: signed 64-bit
+ * __divlu/__remlu: unsigned 32-bit
+ * __divls/__remls: signed 32-bit
+ *
+ * These are not normal C functions: instead of the normal
+ * calling sequence, these expect their arguments in registers
+ * $24 and $25, and return the result in $27. Register $28 may
+ * be clobbered (assembly temporary), anything else must be saved.
+ *
+ * In short: painful.
+ *
+ * This is a rather simple bit-at-a-time algorithm: it's very good
+ * at dividing random 64-bit numbers, but the more usual case where
+ * the divisor is small is handled better by the DEC algorithm
+ * using lookup tables. This uses much less memory, though, and is
+ * nicer on the cache.. Besides, I don't know the copyright status
+ * of the DEC code.
+ */
+
+/*
+ * My temporaries:
+ * $0 - current bit
+ * $1 - shifted divisor
+ * $2 - modulus/quotient
+ *
+ * $23 - return address
+ * $24 - dividend
+ * $25 - divisor
+ *
+ * $27 - quotient/modulus
+ * $28 - compare status
+ */
+
+#define halt .long 0
+
+/*
+ * Select function type and registers
+ */
+#define mask $0
+#define divisor $1
+#define compare $28
+#define tmp1 $3
+#define tmp2 $4
+
+#ifdef DIV
+#define DIV_ONLY(x,y...) x,##y
+#define MOD_ONLY(x,y...)
+#define func(x) __div##x
+#define modulus $2
+#define quotient $27
+#define GETSIGN(x) xor $24,$25,x
+#define STACK 48
+#else
+#define DIV_ONLY(x,y...)
+#define MOD_ONLY(x,y...) x,##y
+#define func(x) __rem##x
+#define modulus $27
+#define quotient $2
+#define GETSIGN(x) bis $24,$24,x
+#define STACK 32
+#endif
+
+/*
+ * For 32-bit operations, we need to extend to 64-bit
+ */
+#ifdef INTSIZE
+#define ufunction func(lu)
+#define sfunction func(l)
+#define LONGIFY(x) zapnot x,15,x
+#define SLONGIFY(x) addl x,0,x
+#else
+#define ufunction func(qu)
+#define sfunction func(q)
+#define LONGIFY(x)
+#define SLONGIFY(x)
+#endif
+
+.set noat
+.align 3
+.globl ufunction
+.ent ufunction
+ufunction:
+ subq $30,STACK,$30
+ .frame $30,STACK,$23
+ .prologue 0
+
+7: stq $1, 0($30)
+ bis $25,$25,divisor
+ stq $2, 8($30)
+ bis $24,$24,modulus
+ stq $0,16($30)
+ bis $31,$31,quotient
+ LONGIFY(divisor)
+ stq tmp1,24($30)
+ LONGIFY(modulus)
+ bis $31,1,mask
+ DIV_ONLY(stq tmp2,32($30))
+ beq divisor, 9f /* div by zero */
+
+#ifdef INTSIZE
+ /*
+ * shift divisor left, using 3-bit shifts for
+ * 32-bit divides as we can't overflow. Three-bit
+ * shifts will result in looping three times less
+ * here, but can result in two loops more later.
+ * Thus using a large shift isn't worth it (and
+ * s8add pairs better than a sll..)
+ */
+1: cmpult divisor,modulus,compare
+ s8addq divisor,$31,divisor
+ s8addq mask,$31,mask
+ bne compare,1b
+#else
+1: cmpult divisor,modulus,compare
+ blt divisor, 2f
+ addq divisor,divisor,divisor
+ addq mask,mask,mask
+ bne compare,1b
+ unop
+#endif
+
+ /* ok, start to go right again.. */
+2: DIV_ONLY(addq quotient,mask,tmp2)
+ srl mask,1,mask
+ cmpule divisor,modulus,compare
+ subq modulus,divisor,tmp1
+ DIV_ONLY(cmovne compare,tmp2,quotient)
+ srl divisor,1,divisor
+ cmovne compare,tmp1,modulus
+ bne mask,2b
+
+9: ldq $1, 0($30)
+ ldq $2, 8($30)
+ ldq $0,16($30)
+ ldq tmp1,24($30)
+ DIV_ONLY(ldq tmp2,32($30))
+ addq $30,STACK,$30
+ ret $31,($23),1
+ .end ufunction
+
+/*
+ * Uhh.. Ugly signed division. I'd rather not have it at all, but
+ * it's needed in some circumstances. There are different ways to
+ * handle this, really. This does:
+ * -a / b = a / -b = -(a / b)
+ * -a % b = -(a % b)
+ * a % -b = a % b
+ * which is probably not the best solution, but at least should
+ * have the property that (x/y)*y + (x%y) = x.
+ */
+.align 3
+.globl sfunction
+.ent sfunction
+sfunction:
+ subq $30,STACK,$30
+ .frame $30,STACK,$23
+ .prologue 0
+ bis $24,$25,$28
+ SLONGIFY($28)
+ bge $28,7b
+ stq $24,0($30)
+ subq $31,$24,$28
+ stq $25,8($30)
+ cmovlt $24,$28,$24 /* abs($24) */
+ stq $23,16($30)
+ subq $31,$25,$28
+ stq tmp1,24($30)
+ cmovlt $25,$28,$25 /* abs($25) */
+ unop
+ bsr $23,ufunction
+ ldq $24,0($30)
+ ldq $25,8($30)
+ GETSIGN($28)
+ subq $31,$27,tmp1
+ SLONGIFY($28)
+ ldq $23,16($30)
+ cmovlt $28,tmp1,$27
+ ldq tmp1,24($30)
+ addq $30,STACK,$30
+ ret $31,($23),1
+ .end sfunction