softfloat: Move floatN_log2 to softfloat-parts.c.inc

Rename to parts$N_log2.  Though this is partly a ruse, since I do not
believe the code will succeed for float128 without work.  Which is ok
for now, because we do not need this for more than float32 and float64.

Since berkeley-testfloat-3 doesn't support log2, compare float64_log2
vs the system log2.  Fix the errors for inputs near 1.0:

test: 3ff00000000000b0  +0x1.00000000000b0p+0
  sf: 3d2fa00000000000  +0x1.fa00000000000p-45
libm: 3d2fbd422b1bd36f  +0x1.fbd422b1bd36fp-45
Error in fraction: 32170028290927 ulp

test: 3feec24f6770b100  +0x1.ec24f6770b100p-1
  sf: bfad3740d13c9ec0  -0x1.d3740d13c9ec0p-5
libm: bfad3740d13c9e98  -0x1.d3740d13c9e98p-5
Error in fraction: 40 ulp

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/fp/fp-test-log2.c

@@ -0,0 +1,118 @@
+/*
+ * fp-test-log2.c - test QEMU's softfloat log2
+ *
+ * Copyright (C) 2020, Linaro, Ltd.
+ *
+ * License: GNU GPL, version 2 or later.
+ *   See the COPYING file in the top-level directory.
+ */
+#ifndef HW_POISON_H
+#error Must define HW_POISON_H to work around TARGET_* poisoning
+#endif
+
+#include "qemu/osdep.h"
+#include "qemu/cutils.h"
+#include <math.h>
+#include "fpu/softfloat.h"
+
+typedef union {
+    double d;
+    float64 i;
+} ufloat64;
+
+static int errors;
+
+static void compare(ufloat64 test, ufloat64 real, ufloat64 soft, bool exact)
+{
+    int msb;
+    uint64_t ulp = UINT64_MAX;
+
+    if (real.i == soft.i) {
+        return;
+    }
+    msb = 63 - __builtin_clzll(real.i ^ soft.i);
+
+    if (msb < 52) {
+        if (real.i > soft.i) {
+            ulp = real.i - soft.i;
+        } else {
+            ulp = soft.i - real.i;
+        }
+    }
+
+    /* glibc allows 3 ulp error in its libm-test-ulps; allow 4 here */
+    if (!exact && ulp <= 4) {
+        return;
+    }
+
+    printf("test: %016" PRIx64 "  %+.13a\n"
+           "  sf: %016" PRIx64 "  %+.13a\n"
+           "libm: %016" PRIx64 "  %+.13a\n",
+           test.i, test.d, soft.i, soft.d, real.i, real.d);
+
+    if (msb == 63) {
+        printf("Error in sign!\n\n");
+    } else if (msb >= 52) {
+        printf("Error in exponent: %d\n\n",
+               (int)(soft.i >> 52) - (int)(real.i >> 52));
+    } else {
+        printf("Error in fraction: %" PRIu64 " ulp\n\n", ulp);
+    }
+
+    if (++errors == 20) {
+        exit(1);
+    }
+}
+
+int main(int ac, char **av)
+{
+    ufloat64 test, real, soft;
+    float_status qsf = {0};
+    int i;
+
+    set_float_rounding_mode(float_round_nearest_even, &qsf);
+
+    test.d = 0.0;
+    real.d = -__builtin_inf();
+    soft.i = float64_log2(test.i, &qsf);
+    compare(test, real, soft, true);
+
+    test.d = 1.0;
+    real.d = 0.0;
+    soft.i = float64_log2(test.i, &qsf);
+    compare(test, real, soft, true);
+
+    test.d = 2.0;
+    real.d = 1.0;
+    soft.i = float64_log2(test.i, &qsf);
+    compare(test, real, soft, true);
+
+    test.d = 4.0;
+    real.d = 2.0;
+    soft.i = float64_log2(test.i, &qsf);
+    compare(test, real, soft, true);
+
+    test.d = 0x1p64;
+    real.d = 64.0;
+    soft.i = float64_log2(test.i, &qsf);
+    compare(test, real, soft, true);
+
+    test.d = __builtin_inf();
+    real.d = __builtin_inf();
+    soft.i = float64_log2(test.i, &qsf);
+    compare(test, real, soft, true);
+
+    for (i = 0; i < 10000; ++i) {
+        test.d = drand48() + 1.0;    /* [1.0, 2.0) */
+        real.d = log2(test.d);
+        soft.i = float64_log2(test.i, &qsf);
+        compare(test, real, soft, false);
+
+        test.d = drand48() * 100;    /* [0.0, 100) */
+        real.d = log2(test.d);
+        soft.i = float64_log2(test.i, &qsf);
+        compare(test, real, soft, false);
+    }
+
+    return 0;
+}

tests/fp/meson.build

@@ -634,3 +634,14 @@ fpbench = executable(
   include_directories: [sfinc, include_directories(tfdir)],
   c_args: fpcflags,
 )
+
+fptestlog2 = executable(
+  'fp-test-log2',
+  ['fp-test-log2.c', '../../fpu/softfloat.c'],
+  link_with: [libsoftfloat],
+  dependencies: [qemuutil],
+  include_directories: [sfinc],
+  c_args: fpcflags,
+)
+test('fp-test-log2', fptestlog2,
+     suite: ['softfloat', 'softfloat-ops'])