[PATCH] m68k FPU emulator

From: Geert Uytterhoeven (geert@linux-m68k.org)
Date: Sat Jul 26 2003 - 09:52:01 EST

M68k FPU emulator: Add fgetman, fgetexp and fsqrt (from Michael Müller and
Roman Zippel)

--- linux-2.6.x/arch/m68k/math-emu/fp_emu.h Mon Apr 1 13:00:29 2002
+++ linux-m68k-2.6.x/arch/m68k/math-emu/fp_emu.h Wed Jul 23 22:15:29 2003
@@ -115,6 +115,15 @@
         __res; \
 })
 
+#define fp_conv_long2ext(dest, src) ({ \
+ register struct fp_ext *__dest asm ("a0") = dest; \
+ register int __src asm ("d0") = src; \
+ \
+ asm volatile ("jsr fp_conv_ext2long" \
+ : : "d" (__src), "a" (__dest) \
+ : "a1", "d1", "d2", "memory"); \
+})
+
 #else /* __ASSEMBLY__ */
 
 /*
--- linux-2.6.x/arch/m68k/math-emu/fp_log.c Sun Aug 15 20:47:29 1999
+++ linux-m68k-2.6.x/arch/m68k/math-emu/fp_log.c Wed Jul 23 22:15:29 2003
@@ -17,10 +17,22 @@
 
 #include "fp_emu.h"
 
+static const struct fp_ext fp_one =
+{
+ 0, 0, 0x3fff, { 0 }
+};
+
+extern struct fp_ext *fp_fadd(struct fp_ext *dest, const struct fp_ext *src);
+extern struct fp_ext *fp_fdiv(struct fp_ext *dest, const struct fp_ext *src);
+extern struct fp_ext *fp_fmul(struct fp_ext *dest, const struct fp_ext *src);
+
 struct fp_ext *
 fp_fsqrt(struct fp_ext *dest, struct fp_ext *src)
 {
- uprint("fsqrt\n");
+ struct fp_ext tmp, src2;
+ int i, exp;
+
+ dprint(PINSTR, "fsqrt\n");
 
         fp_monadic_check(dest, src);
 
@@ -34,6 +46,56 @@
         if (IS_INF(dest))
                 return dest;
 
+ /*
+ * sqrt(m) * 2^(p) , if e = 2*p
+ * sqrt(m*2^e) =
+ * sqrt(2*m) * 2^(p) , if e = 2*p + 1
+ *
+ * So we use the last bit of the exponent to decide wether to
+ * use the m or 2*m.
+ *
+ * Since only the fractional part of the mantissa is stored and
+ * the integer part is assumed to be one, we place a 1 or 2 into
+ * the fixed point representation.
+ */
+ exp = dest->exp;
+ dest->exp = 0x3FFF;
+ if (!(exp & 1)) /* lowest bit of exponent is set */
+ dest->exp++;
+ fp_copy_ext(&src2, dest);
+
+ /*
+ * The taylor row arround a for sqrt(x) is:
+ * sqrt(x) = sqrt(a) + 1/(2*sqrt(a))*(x-a) + R
+ * With a=1 this gives:
+ * sqrt(x) = 1 + 1/2*(x-1)
+ * = 1/2*(1+x)
+ */
+ fp_fadd(dest, &fp_one);
+ dest->exp--; /* * 1/2 */
+
+ /*
+ * We now apply the newton rule to the function
+ * f(x) := x^2 - r
+ * which has a null point on x = sqrt(r).
+ *
+ * It gives:
+ * x' := x - f(x)/f'(x)
+ * = x - (x^2 -r)/(2*x)
+ * = x - (x - r/x)/2
+ * = (2*x - x + r/x)/2
+ * = (x + r/x)/2
+ */
+ for (i = 0; i < 9; i++) {
+ fp_copy_ext(&tmp, &src2);
+
+ fp_fdiv(&tmp, dest);
+ fp_fadd(dest, &tmp);
+ dest->exp--;
+ }
+
+ dest->exp += (exp - 0x3FFF) / 2;
+
         return dest;
 }
 
@@ -123,19 +185,38 @@
 struct fp_ext *
 fp_fgetexp(struct fp_ext *dest, struct fp_ext *src)
 {
- uprint("fgetexp\n");
+ dprint(PINSTR, "fgetexp\n");
 
         fp_monadic_check(dest, src);
 
+ if (IS_INF(dest)) {
+ fp_set_nan(dest);
+ return dest;
+ }
+ if (IS_ZERO(dest))
+ return dest;
+
+ fp_conv_long2ext(dest, (int)dest->exp - 0x3FFF);
+
+ fp_normalize_ext(dest);
+
         return dest;
 }
 
 struct fp_ext *
 fp_fgetman(struct fp_ext *dest, struct fp_ext *src)
 {
- uprint("fgetman\n");
+ dprint(PINSTR, "fgetman\n");
 
         fp_monadic_check(dest, src);
+
+ if (IS_ZERO(dest))
+ return dest;
+
+ if (IS_INF(dest))
+ return dest;
+
+ dest->exp = 0x3FFF;
 
         return dest;
 }

Gr{oetje,eeting}s,

                                                Geert 

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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