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To: gdb-patches@sources.redhat.com
Subject: Simulation of MIPS recip and rsqrt instructions
From: Richard Sandiford <rsandifo@redhat.com>
Date: Tue, 26 Nov 2002 11:41:00 -0000
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X-SW-Source: 2002-11/txt/msg00658.txt.bz2

The mips simulator uses sim_fpu_inv() as a reciprocal function
but all it really does is negate the exponent.  So we end up
calculating 1 / (x*2^y) as x*2^-y.

Test case:

    #include <stdio.h>

    int
    main ()
    {
      double d;
      float f;

      asm ("recip.s %0,%1" : "=f" (f) : "f" (20.0f));
      printf ("%g\n", f);

      asm ("recip.d %0,%1" : "=f" (d) : "f" (10.0));
      printf ("%g\n", d);

      asm ("rsqrt.s %0,%1" : "=f" (f) : "f" (0.01f));
      printf ("%g\n", f);

      asm ("rsqrt.d %0,%1" : "=f" (d) : "f" (25.0));
      printf ("%g\n", d);

      return 0;
    }

The simulator says:

    0.078125
    0.15625
    25.6
    0.3125

while a vr5500 says:

    0.05
    0.1
    10
    0.2

Unfortunately, sim_fpu_inv isn't commented and cp1.c seems to be
its only active user.  Which should change?

FWIW, the patch below fixes the test case.  Please install if OK.

Richard


	* cp1.c (inner_sqrt): Use sim_fpu_div instead of sim_fpu_inv.
	(fpu_inv): New function.
	(fp_recip): Use it.

Index: sim/mips/cp1.c
===================================================================
RCS file: /cvs/src/src/sim/mips/cp1.c,v
retrieving revision 1.19
diff -u -d -p -F^[(a-zA-Z0-9_^#] -r1.19 cp1.c
--- sim/mips/cp1.c	29 Jul 2002 23:17:10 -0000	1.19
+++ sim/mips/cp1.c	26 Nov 2002 19:13:24 -0000
@@ -900,7 +900,7 @@ inner_rsqrt(unsigned64 op1,
 	status |= sim_fpu_sqrt (&ans, &wop1);
 	status |= sim_fpu_round_32 (&ans, status, round);
 	wop1 = ans;
-	op_status = sim_fpu_inv (&ans, &wop1);
+	op_status = sim_fpu_div (&ans, &sim_fpu_one, &wop1);
 	op_status |= sim_fpu_round_32 (&ans, round, denorm);
 	sim_fpu_to32 (&res, &ans);
 	temp = res;
@@ -914,7 +914,7 @@ inner_rsqrt(unsigned64 op1,
 	status |= sim_fpu_sqrt (&ans, &wop1);
 	status |= sim_fpu_round_64 (&ans, round, denorm);
 	wop1 = ans;
-	op_status = sim_fpu_inv (&ans, &wop1);
+	op_status = sim_fpu_div (&ans, &sim_fpu_one, &wop1);
 	op_status |= sim_fpu_round_64 (&ans, round, denorm);
 	sim_fpu_to64 (&res, &ans);
 	temp = res;
@@ -1027,13 +1027,19 @@ fp_div(sim_cpu *cpu,
   return fp_binary(cpu, cia, &sim_fpu_div, op1, op2, fmt);
 }
 
+static int
+fpu_inv(sim_fpu *f, const sim_fpu *l)
+{
+  return sim_fpu_div (f, &sim_fpu_one, l);
+}
+
 unsigned64
 fp_recip(sim_cpu *cpu,
          address_word cia,
          unsigned64 op,
          FP_formats fmt)
 {
-  return fp_unary(cpu, cia, &sim_fpu_inv, op, fmt);
+  return fp_unary(cpu, cia, &fpu_inv, op, fmt);
 }
 
 unsigned64