Module Name: src
Committed By: dholland
Date: Sun Dec 31 04:20:40 UTC 2023
Modified Files:
src/sys/arch/m68k/include: float.h
src/sys/arch/x86/include: float.h
Log Message:
{x86,m68k}/float.h: document LDBL_MIN behavior
It seems that even though both these platforms have 12-byte floats
that are pretty much the same representation and both allegedly
IEEE-compliant, they manifest the top bit of the mantissa and then
differ slightly in the behavior of the extra encodings this permits.
Thanks to riastradh@ for helping sort this out.
To generate a diff of this commit:
cvs rdiff -u -r1.21 -r1.22 src/sys/arch/m68k/include/float.h
cvs rdiff -u -r1.6 -r1.7 src/sys/arch/x86/include/float.h
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/m68k/include/float.h
diff -u src/sys/arch/m68k/include/float.h:1.21 src/sys/arch/m68k/include/float.h:1.22
--- src/sys/arch/m68k/include/float.h:1.21 Tue Mar 18 18:20:41 2014
+++ src/sys/arch/m68k/include/float.h Sun Dec 31 04:20:40 2023
@@ -1,8 +1,31 @@
-/* $NetBSD: float.h,v 1.21 2014/03/18 18:20:41 riastradh Exp $ */
+/* $NetBSD: float.h,v 1.22 2023/12/31 04:20:40 dholland Exp $ */
#ifndef _M68K_FLOAT_H_
#define _M68K_FLOAT_H_
+/*
+ * LDBL_MIN is half the x86 LDBL_MIN, even though both are 12-byte
+ * floats with the same base properties and both allegedly
+ * IEEE-compliant, because both these representations materialize the
+ * top (integer-part) bit of the mantissa. But on m68k if the exponent
+ * is 0 and the integer bit is set, it's a regular number, whereas on
+ * x86 it's called a pseudo-denormal and apparently treated as a
+ * denormal, so it doesn't count as a valid value for LDBL_MIN.
+ *
+ * x86 citation: Intel 64 and IA-32 Architectures Software Developer's
+ * Manual, vol. 1 (Order Number: 253665-077US, April 2022), Sec. 8.2.2
+ * `Unsupported Double Extended-Precision Floating-Point Encodings
+ * and Pseudo-Denormals', p. 8-14.
+ *
+ * m86k citation: MC68881/MC68882 Floating-Point Coprocessor User's
+ * Manual, Second Edition (Prentice-Hall, 1989, apparently issued by
+ * Freescale), Section 3.2 `Binary Real Data formats', pg. 3-3 bottom
+ * in particular and pp. 3-2 to 3-5 in general.
+ *
+ * If anyone needs to update this comment please make sure the copy in
+ * m68k/float.h also gets updated.
+ */
+
#if defined(__LDBL_MANT_DIG__)
#define LDBL_MANT_DIG __LDBL_MANT_DIG__
#define LDBL_EPSILON __LDBL_EPSILON__
Index: src/sys/arch/x86/include/float.h
diff -u src/sys/arch/x86/include/float.h:1.6 src/sys/arch/x86/include/float.h:1.7
--- src/sys/arch/x86/include/float.h:1.6 Sat Apr 27 21:35:25 2013
+++ src/sys/arch/x86/include/float.h Sun Dec 31 04:20:40 2023
@@ -1,10 +1,33 @@
-/* $NetBSD: float.h,v 1.6 2013/04/27 21:35:25 joerg Exp $ */
+/* $NetBSD: float.h,v 1.7 2023/12/31 04:20:40 dholland Exp $ */
#ifndef _X86_FLOAT_H_
#define _X86_FLOAT_H_
#include <sys/featuretest.h>
+/*
+ * LDBL_MIN is twice the m68k LDBL_MIN, even though both are 12-byte
+ * floats with the same base properties and both allegedly
+ * IEEE-compliant, because both these representations materialize the
+ * top (integer-part) bit of the mantissa. But on m68k if the exponent
+ * is 0 and the integer bit is set, it's a regular number, whereas on
+ * x86 it's called a pseudo-denormal and apparently treated as a
+ * denormal, so it doesn't count as a valid value for LDBL_MIN.
+ *
+ * x86 citation: Intel 64 and IA-32 Architectures Software Developer's
+ * Manual, vol. 1 (Order Number: 253665-077US, April 2022), Sec. 8.2.2
+ * `Unsupported Double Extended-Precision Floating-Point Encodings
+ * and Pseudo-Denormals', p. 8-14.
+ *
+ * m86k citation: MC68881/MC68882 Floating-Point Coprocessor User's
+ * Manual, Second Edition (Prentice-Hall, 1989, apparently issued by
+ * Freescale), Section 3.2 `Binary Real Data formats', pg. 3-3 bottom
+ * in particular and pp. 3-2 to 3-5 in general.
+ *
+ * If anyone needs to update this comment please make sure the copy in
+ * x86/float.h also gets updated.
+ */
+
#define LDBL_MANT_DIG 64
#define LDBL_EPSILON 1.0842021724855044340E-19L
#define LDBL_DIG 18
