Add a probe for signbit and use it to implement MVM_num_isnegzero

MoarVM · May 8, 2021 · d602912 · d602912
1 parent 86e1289
commit d602912
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Showing 3 changed files with 19 additions and 2 deletions.
diff --git a/build/config.h.in b/build/config.h.in
@@ -101,6 +101,10 @@
 #define MVM_HAS_ISNAN
 #endif
 
+#if @has_signbit@
+#define MVM_HAS_SIGNBIT
+#endif
+
 /* Should we translate \n to \r\n on output? */
 #define MVM_TRANSLATE_NEWLINE_OUTPUT @translate_newline_output@
 

diff --git a/build/probe.pm b/build/probe.pm
@@ -387,7 +387,8 @@ EOT
 }
 
 my @floating_point_vals = qw(zero one nan inf neg_inf neg_zero
-                             int_less_than_minus_one int_more_than_one);
+                             int_less_than_minus_one int_more_than_one
+                             roughly_ten roughly_minus_ten);
 
 my $floating_point_main = <<'EOT';
 #include <math.h>
@@ -402,12 +403,14 @@ int main(int argc, char **argv) {
      */
     double zero = argv[0][0] == '\0';
     double one = zero + 1.0;
-    double nan = sqrt(-1);
     double int_less_than_minus_one = 34 - argv[0][0];
     double int_more_than_one = argv[0][0] - 30;
     double inf = int_more_than_one * pow(10.0, pow(10.0, 100));
     double neg_inf = int_less_than_minus_one * pow(10.0, pow(10.0, 100));
     double neg_zero = one / neg_inf;
+    double roughly_ten = atan2(zero, -one) * atan2(zero, -one);
+    double roughly_minus_ten = -roughly_ten;
+    double nan = sqrt(roughly_minus_ten);
 
     if (zero) {
 #ifdef CHATTY
@@ -734,6 +737,7 @@ sub has_isinf_and_isnan {
     my @probes = (
         [ isnan => 'nan' ],
         [ isinf => qw(inf neg_inf) ],
+        [ signbit => qw(neg_inf neg_zero int_less_than_minus_one roughly_minus_ten)],
     );
 
     if ($config->{crossconf}) {
@@ -754,6 +758,11 @@ sub has_isinf_and_isnan {
         my $exit = 11;
 
         for my $val (@floating_point_vals) {
+            # NaNs can actually have sign bits, so the sign bit might/might not
+            # be set, and we can't know (and shouldn't care):
+            next
+                if $func eq 'signbit' && $val eq 'nan';
+
             my $want = $true{$val} ? 'T' : 'f';
 
             $code .= <<"EOT";

diff --git a/src/math/num.h b/src/math/num.h
@@ -50,7 +50,11 @@ MVM_STATIC_INLINE MVMnum64 MVM_num_nan(MVMThreadContext *tc) {
 }
 
 MVM_STATIC_INLINE MVMnum64 MVM_num_isnegzero(MVMThreadContext *tc, MVMnum64 n) {
+#ifdef MVM_HAS_SIGNBIT
+    return n == 0 && signbit(n);
+#else
     return n == 0 && 1.0 / n == MVM_NUM_NEGINF;
+#endif
 }
 
 #endif