FPU: Implement fre[s]

This just returns the value from the inverse lookup table.  The result
is accurate to better than one part in 512 (the architecture requires
1/256).

This also adds a simple test, which relies on the particular values in
the inverse lookup table, so it is not a general test.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

decode1.vhdl

@@ -419,6 +419,7 @@ architecture behaviour of decode1 is
         2#10010#  =>  (FPU,   OP_FPOP,       FRA,  FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '1', '0', RC,   '0', '0'), -- fdivs
         2#10100#  =>  (FPU,   OP_FPOP,       FRA,  FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '1', '0', RC,   '0', '0'), -- fsubs
         2#10101#  =>  (FPU,   OP_FPOP,       FRA,  FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '1', '0', RC,   '0', '0'), -- fadds
+        2#11000#  =>  (FPU,   OP_FPOP,       NONE, FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '1', '0', RC,   '0', '0'), -- fres
         2#11001#  =>  (FPU,   OP_FPOP,       FRA,  NONE, FRC,  FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '1', '0', RC,   '0', '0'), -- fmuls
         others => illegal_inst
         );
@@ -473,6 +474,7 @@ architecture behaviour of decode1 is
         2#0010#  =>  (FPU,   OP_FPOP,       FRA,  FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', RC,   '0', '0'), -- fdiv
         2#0100#  =>  (FPU,   OP_FPOP,       FRA,  FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', RC,   '0', '0'), -- fsub
         2#0101#  =>  (FPU,   OP_FPOP,       FRA,  FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', RC,   '0', '0'), -- fadd
+        2#1000#  =>  (FPU,   OP_FPOP,       NONE, FRB,  NONE, FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', RC,   '0', '0'), -- fre
         2#1001#  =>  (FPU,   OP_FPOP,       FRA,  NONE, FRC,  FRT,  '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', RC,   '0', '0'), -- fmul
         others => illegal_inst
         );

fpu.vhdl

@@ -41,11 +41,13 @@ architecture behaviour of fpu is
                      DO_FCFID, DO_FCTI,
                      DO_FRSP, DO_FRI,
                      DO_FADD, DO_FMUL, DO_FDIV,
+                     DO_FRE,
                      FRI_1,
                      ADD_SHIFT, ADD_2, ADD_3,
                      MULT_1,
                      LOOKUP,
                      DIV_2, DIV_3, DIV_4, DIV_5, DIV_6,
+                     FRE_1,
                      INT_SHIFT, INT_ROUND, INT_ISHIFT,
                      INT_FINAL, INT_CHECK, INT_OFLOW,
                      FINISH, NORMALIZE,
@@ -639,6 +641,8 @@ begin
                             v.state := DO_FDIV;
                         when "10100" | "10101" =>
                             v.state := DO_FADD;
+                        when "11000" =>
+                            v.state := DO_FRE;
                         when "11001" =>
                             v.is_multiply := '1';
                             v.state := DO_FMUL;
@@ -1041,6 +1045,36 @@ begin
                     arith_done := '1';
                 end if;
 
+            when DO_FRE =>
+                opsel_a <= AIN_B;
+                v.result_class := r.b.class;
+                v.result_sign := r.b.negative;
+                v.fpscr(FPSCR_FR) := '0';
+                v.fpscr(FPSCR_FI) := '0';
+                if r.b.class = NAN and r.b.mantissa(53) = '0' then
+                    v.fpscr(FPSCR_VXSNAN) := '1';
+                    invalid := '1';
+                end if;
+                case r.b.class is
+                    when FINITE =>
+                        v.result_exp := - r.b.exponent;
+                        if r.b.mantissa(54) = '0' then
+                            v.state := RENORM_B;
+                        else
+                            v.state := FRE_1;
+                        end if;
+                    when NAN =>
+                        -- result is B
+                        arith_done := '1';
+                    when INFINITY =>
+                        v.result_class := ZERO;
+                        arith_done := '1';
+                    when ZERO =>
+                        v.result_class := INFINITY;
+                        zero_divide := '1';
+                        arith_done := '1';
+                end case;
+
             when RENORM_A =>
                 renormalize := '1';
                 v.state := RENORM_A2;
@@ -1149,7 +1183,11 @@ begin
                 opsel_a <= AIN_B;
                 -- wait one cycle for inverse_table[B] lookup
                 v.first := '1';
+                if r.insn(4) = '0' then
                     v.state := DIV_2;
+                else
+                    v.state := FRE_1;
+                end if;
 
             when DIV_2 =>
                 -- compute Y = inverse_table[B] (when count=0); P = 2 - B * Y
@@ -1221,6 +1259,12 @@ begin
                 end if;
                 v.state := FINISH;
 
+            when FRE_1 =>
+                opsel_r <= RES_MISC;
+                misc_sel <= "0111";
+                v.shift := to_signed(1, EXP_BITS);
+                v.state := NORMALIZE;
+
             when INT_SHIFT =>
                 opsel_r <= RES_SHIFT;
                 set_x := '1';
@@ -1609,6 +1653,8 @@ begin
                     when "0110" =>
                         -- fmrgew result
                         misc := r.a.mantissa(63 downto 32) & r.b.mantissa(63 downto 32);
+                    when "0111" =>
+                        misc := 10x"000" & inverse_est & 35x"000000000";
                     when "1000" =>
                         -- max positive result for fctiw[z]
                         misc := x"000000007fffffff";

tests/fpu/fpu.c

@@ -1111,6 +1111,43 @@ int fpu_test_17(void)
 	return trapit(0, test17);
 }
 
+struct recipvals {
+	unsigned long val;
+	unsigned long inv;
+} recipvals[] = {
+	{ 0x0000000000000000, 0x7ff0000000000000 },
+	{ 0xfff0000000000000, 0x8000000000000000 },
+	{ 0x3ff0000000000000, 0x3feff00400000000 },
+	{ 0xbff0000000000000, 0xbfeff00400000000 },
+	{ 0x4008000000000000, 0x3fd54e3800000000 },
+	{ 0xc03ffffffdffffbf, 0xbfa0040000000000 },
+};
+
+int test18(long arg)
+{
+	long i;
+	unsigned long result;
+	struct recipvals *vp = recipvals;
+
+	set_fpscr(FPS_RN_NEAR);
+	for (i = 0; i < sizeof(recipvals) / sizeof(recipvals[0]); ++i, ++vp) {
+		asm("lfd 6,0(%0); fre 7,6; stfd 7,0(%1)"
+		    : : "b" (&vp->val), "b" (&result) : "memory");
+		if (result != vp->inv) {
+			print_hex(i, 2, " ");
+			print_hex(result, 16, " ");
+			return i + 1;
+		}
+	}
+	return 0;
+}
+
+int fpu_test_18(void)
+{
+	enable_fp();
+	return trapit(0, test18);
+}
+
 int fail = 0;
 
 void do_test(int num, int (*test)(void))
@@ -1153,6 +1190,7 @@ int main(void)
 	do_test(15, fpu_test_15);
 	do_test(16, fpu_test_16);
 	do_test(17, fpu_test_17);
+	do_test(18, fpu_test_18);
 
 	return fail;
 }

tests/test_fpu.console_out

@@ -15,3 +15,4 @@ test 14:PASS
 test 15:PASS
 test 16:PASS
 test 17:PASS
+test 18:PASS