There were some questions regarding the SNaN code, e.g. in
<https://lists.gnu.org/archive/html/bug-gnulib/2013-12/msg00092.html>:
"I'm a little bit
confused about the original intentions of the test, however. Why does it
attempt to move to another word? If it simply attempted to set the
*second* highest mantissa bit, everything would work out OK. And if it
does move to another word, why the NWORDS / 2 test?"
This patch adds some clarifications and other comments.
2023-10-12 Bruno Haible <br...@clisp.org>
snan: Comments.
* lib/snan.h: Add more comments.
diff --git a/lib/snan.h b/lib/snan.h
index 5dd4dd6f9e..88950437c8 100644
--- a/lib/snan.h
+++ b/lib/snan.h
@@ -24,6 +24,23 @@
#include "nan.h"
+/* The bit that distinguishes a quiet NaN from a signalling NaN is, according
to
+ <https://en.wikipedia.org/wiki/NaN#Encoding>, the most significant bit of
the
+ mantissa field.
+ According to <https://en.wikipedia.org/wiki/IEEE_754#Formats>, this is the
+ next bit, right below the bit 0 of the exponent.
+ This bit is
+ * == 0 to indicate a quiet NaN or Infinity,
+ == 1 to indicate a signalling NaN,
+ on these CPUs: hppa, mips.
+ * == 1 to indicate a quiet NaN,
+ == 0 to indicate a signalling NaN or Infinity,
+ on all other CPUs.
+ On these platforms, additionally a signalling NaN must have some other
+ mantissa bit == 1, because when all exponent bits are == 1 and all
+ mantissa bits are == 0, the number denotes ±Infinity. */
+
+
#if defined FLT_EXPBIT0_WORD && defined FLT_EXPBIT0_BIT
# define HAVE_SNANF 1
@@ -37,15 +54,17 @@ SNaNf ()
typedef union { float value; unsigned int word[NWORDS]; } memory_float;
memory_float m;
m.value = NaNf ();
- #if FLT_EXPBIT0_BIT > 0
+ /* Turn the quiet NaN into a signalling NaN. */
+ #if FLT_EXPBIT0_BIT > 0
m.word[FLT_EXPBIT0_WORD] ^= (unsigned int) 1 << (FLT_EXPBIT0_BIT - 1);
- #else
+ #else
m.word[FLT_EXPBIT0_WORD + (FLT_EXPBIT0_WORD < NWORDS / 2 ? 1 : - 1)]
^= (unsigned int) 1 << (sizeof (unsigned int) * CHAR_BIT - 1);
- #endif
- if (FLT_EXPBIT0_WORD < NWORDS / 2)
+ #endif
+ /* Set some arbitrary mantissa bit. */
+ if (FLT_EXPBIT0_WORD < NWORDS / 2) /* NWORDS > 1 and big endian */
m.word[FLT_EXPBIT0_WORD + 1] |= (unsigned int) 1 << FLT_EXPBIT0_BIT;
- else
+ else /* NWORDS == 1 or little endian */
m.word[0] |= (unsigned int) 1;
#undef NWORDS
return m.value;
@@ -67,12 +86,14 @@ SNaNd ()
typedef union { double value; unsigned int word[NWORDS]; } memory_double;
memory_double m;
m.value = NaNd ();
+ /* Turn the quiet NaN into a signalling NaN. */
#if DBL_EXPBIT0_BIT > 0
m.word[DBL_EXPBIT0_WORD] ^= (unsigned int) 1 << (DBL_EXPBIT0_BIT - 1);
#else
m.word[DBL_EXPBIT0_WORD + (DBL_EXPBIT0_WORD < NWORDS / 2 ? 1 : - 1)]
^= (unsigned int) 1 << (sizeof (unsigned int) * CHAR_BIT - 1);
#endif
+ /* Set some arbitrary mantissa bit. */
m.word[DBL_EXPBIT0_WORD + (DBL_EXPBIT0_WORD < NWORDS / 2 ? 1 : - 1)]
|= (unsigned int) 1 << DBL_EXPBIT0_BIT;
#undef NWORDS
@@ -107,12 +128,14 @@ SNaNl ()
#else
#define HNWORDS NWORDS
#endif
+ /* Turn the quiet NaN into a signalling NaN. */
#if LDBL_EXPBIT0_BIT > 0
m.word[LDBL_EXPBIT0_WORD] ^= (unsigned int) 1 << (LDBL_EXPBIT0_BIT - 1);
#else
m.word[LDBL_EXPBIT0_WORD + (LDBL_EXPBIT0_WORD < HNWORDS / 2 ? 1 : - 1)]
^= (unsigned int) 1 << (sizeof (unsigned int) * CHAR_BIT - 1);
#endif
+ /* Set some arbitrary mantissa bit. */
m.word[LDBL_EXPBIT0_WORD + (LDBL_EXPBIT0_WORD < HNWORDS / 2 ? 1 : - 1)]
|= (unsigned int) 1 << LDBL_EXPBIT0_BIT;
#undef HNWORDS
