Avoid excess range in results from i386 scalb functions (bug 18981) [committed]

[Joseph Myers <joseph at codesourcery dot com>]

i386 scalb / scalbn / scalbln (and thus ldexp) functions for float and
double can return results with excess range (and consequently excess
precision for subnormal results).  As the results of these functions
are fully determined by reference to IEEE 754 operations, this is
unambiguously a bug, apart from the testsuite failures it causes.

This patch makes those functions store their results on the stack and
load them back to eliminate the excess range.  Double rounding is not
a problem, as the only cases where it could occur are when the result
overflows or underflows for extended precision, and then the
double-rounded results are the same as the single-rounded results.

The new macros will be used for more functions, more such macros
added, and existing code refactored to use such macros, in subsequent
patches.

Tested for x86.  Committed.

2015-09-18  Joseph Myers  <joseph@codesourcery.com>

	[BZ #18981]
	* sysdeps/i386/fpu/i386-math-asm.h: New file.
	* sysdeps/i386/fpu/e_scalb.S: Include <i386-math-asm.h>.
	(__ieee754_scalb): Use DBL_NARROW_EVAL.
	* sysdeps/i386/fpu/e_scalbf.S: Include <i386-math-asm.h>.
	(__ieee754_scalbf): Use FLT_NARROW_EVAL.
	* sysdeps/i386/fpu/s_scalbn.S: Include <i386-math-asm.h>.
	(__scalbn): Use DBL_NARROW_EVAL.
	* sysdeps/i386/fpu/s_scalbnf.S: Include <i386-math-asm.h>.
	(__scalbnf): Use FLT_NARROW_EVAL.

diff --git a/sysdeps/i386/fpu/e_scalb.S b/sysdeps/i386/fpu/e_scalb.S
index 6a01aa6..370924c 100644
--- a/sysdeps/i386/fpu/e_scalb.S
+++ b/sysdeps/i386/fpu/e_scalb.S
@@ -6,6 +6,7 @@
  */
 
 #include <machine/asm.h>
+#include <i386-math-asm.h>
 
 	.section .rodata
 
@@ -52,6 +53,7 @@ ENTRY(__ieee754_scalb)
 	jne	4f
 	fscale
 	fstp	%st(1)
+	DBL_NARROW_EVAL
 	ret
 
 	/* y is -inf */
diff --git a/sysdeps/i386/fpu/e_scalbf.S b/sysdeps/i386/fpu/e_scalbf.S
index 12b2515..4f2dfa3 100644
--- a/sysdeps/i386/fpu/e_scalbf.S
+++ b/sysdeps/i386/fpu/e_scalbf.S
@@ -7,6 +7,7 @@
  */
 
 #include <machine/asm.h>
+#include <i386-math-asm.h>
 
 	.section .rodata
 
@@ -54,6 +55,7 @@ ENTRY(__ieee754_scalbf)
 	jne	4f
 	fscale
 	fstp	%st(1)
+	FLT_NARROW_EVAL
 	ret
 
 	/* y is -inf */
diff --git a/sysdeps/i386/fpu/i386-math-asm.h b/sysdeps/i386/fpu/i386-math-asm.h
new file mode 100644
index 0000000..fd43130
--- /dev/null
+++ b/sysdeps/i386/fpu/i386-math-asm.h
@@ -0,0 +1,39 @@
+/* Helper macros for x86 libm functions.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef _I386_MATH_ASM_H
+#define _I386_MATH_ASM_H 1
+
+/* Remove excess range and precision by storing a value on the stack
+   and loading it back.  */
+#define FLT_NARROW_EVAL				\
+	subl	$4, %esp;			\
+	cfi_adjust_cfa_offset (4);		\
+	fstps	(%esp);				\
+	flds	(%esp);				\
+	addl	$4, %esp;			\
+	cfi_adjust_cfa_offset (-4);
+#define DBL_NARROW_EVAL				\
+	subl	$8, %esp;			\
+	cfi_adjust_cfa_offset (8);		\
+	fstpl	(%esp);				\
+	fldl	(%esp);				\
+	addl	$8, %esp;			\
+	cfi_adjust_cfa_offset (-8);
+
+#endif /* i386-math-asm.h.  */
diff --git a/sysdeps/i386/fpu/s_scalbn.S b/sysdeps/i386/fpu/s_scalbn.S
index dab7580..4e90903 100644
--- a/sysdeps/i386/fpu/s_scalbn.S
+++ b/sysdeps/i386/fpu/s_scalbn.S
@@ -4,6 +4,7 @@
  */
 
 #include <machine/asm.h>
+#include <i386-math-asm.h>
 
 RCSID("$NetBSD: s_scalbn.S,v 1.4 1995/05/09 00:19:06 jtc Exp $")
 
@@ -12,6 +13,7 @@ ENTRY(__scalbn)
 	fldl	4(%esp)
 	fscale
 	fstp	%st(1)
+	DBL_NARROW_EVAL
 	ret
 END (__scalbn)
 strong_alias (__scalbn, __scalbln)
diff --git a/sysdeps/i386/fpu/s_scalbnf.S b/sysdeps/i386/fpu/s_scalbnf.S
index ad97be8..f8353c4 100644
--- a/sysdeps/i386/fpu/s_scalbnf.S
+++ b/sysdeps/i386/fpu/s_scalbnf.S
@@ -4,6 +4,7 @@
  */
 
 #include <machine/asm.h>
+#include <i386-math-asm.h>
 
 RCSID("$NetBSD: s_scalbnf.S,v 1.3 1995/05/09 00:19:59 jtc Exp $")
 
@@ -12,6 +13,7 @@ ENTRY(__scalbnf)
 	flds	4(%esp)
 	fscale
 	fstp	%st(1)
+	FLT_NARROW_EVAL
 	ret
 END (__scalbnf)
 strong_alias (__scalbnf, __scalblnf)

-- 
Joseph S. Myers
joseph@codesourcery.com