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soft-fp: Make extensions of subnormals from XFmode to TFmode signal underflow if traps enabled.

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This patch fixes a soft-fp corner case I previously noted in
<https://sourceware.org/ml/libc-alpha/2013-10/msg00349.html>: when
trapping on underflow is enabled, extensions of subnormals from XFmode
to TFmode need to signal underflow because the result is tiny (but
exact, so the underflow flag is not raised unless trapping is
enabled).

To avoid any excess initialization or tests for other cases of
floating-point extensions, a new FP_INIT_TRAPPING_EXCEPTIONS is added
that does the initialization required for this particular case (more
than FP_INIT_EXCEPTIONS, less than FP_INIT_ROUNDMODE, in general), and
FP_NO_EXACT_UNDERFLOW is added to stub out FP_TRAPPING_EXCEPTIONS
tests for those cases of extensions where the test would be dead code,
to avoid any uninitialized variable warnings.

As the relevant case only applies in libgcc, not to any use of soft-fp
in glibc, there is no bug report in Bugzilla and no non-default
definitions of FP_INIT_TRAPPING_EXCEPTIONS are added by the patch.  A
testcase will be added to GCC as part of an update of soft-fp in
libgcc once this patch is in libc.

Tested for powerpc-nofpu that the disassembly of installed shared
libraries is unchanged by this patch.  Bootstrapped GCC with updated
soft-fp with no regressions on x86_64-unknown-linux-gnu and verified
that a test of the relevant case passes where it failed before.

	* soft-fp/op-common.h (FP_EXTEND): When a subnormal input produces
	a subnormal result, set the underflow exception if trapping on
	underflow is enabled.
	* soft-fp/soft-fp.h (FP_INIT_TRAPPING_EXCEPTIONS): New macro.
	(FP_INIT_EXCEPTIONS): Default to FP_INIT_TRAPPING_EXCEPTIONS.
	[FP_NO_EXACT_UNDERFLOW] (FP_TRAPPING_EXCEPTIONS): Undefine and
	redefine to 0.
	* soft-fp/extenddftf2.c (FP_NO_EXACT_UNDERFLOW): Define.
	* soft-fp/extendsfdf2.c (FP_NO_EXACT_UNDERFLOW): Likewise.
	* soft-fp/extendsftf2.c (FP_NO_EXACT_UNDERFLOW): Likewise.
	* soft-fp/extendxftf2.c (__extendxftf2): Use
	FP_INIT_TRAPPING_EXCEPTIONS instead of FP_INIT_ROUNDMODE.
This commit is contained in:
Joseph Myers 2014-10-09 01:00:41 +00:00
parent 2d6a47ba4c
commit 43059f42db
7 changed files with 36 additions and 2 deletions
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13
ChangeLog
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@ -1,5 +1,18 @@
2014-10-09 Joseph Myers <joseph@codesourcery.com>
* soft-fp/op-common.h (FP_EXTEND): When a subnormal input produces
a subnormal result, set the underflow exception if trapping on
underflow is enabled.
* soft-fp/soft-fp.h (FP_INIT_TRAPPING_EXCEPTIONS): New macro.
(FP_INIT_EXCEPTIONS): Default to FP_INIT_TRAPPING_EXCEPTIONS.
[FP_NO_EXACT_UNDERFLOW] (FP_TRAPPING_EXCEPTIONS): Undefine and
redefine to 0.
* soft-fp/extenddftf2.c (FP_NO_EXACT_UNDERFLOW): Define.
* soft-fp/extendsfdf2.c (FP_NO_EXACT_UNDERFLOW): Likewise.
* soft-fp/extendsftf2.c (FP_NO_EXACT_UNDERFLOW): Likewise.
* soft-fp/extendxftf2.c (__extendxftf2): Use
FP_INIT_TRAPPING_EXCEPTIONS instead of FP_INIT_ROUNDMODE.
* soft-fp/soft-fp.h (FP_CLEAR_EXCEPTIONS): Remove macro.
* sysdeps/sparc/sparc32/soft-fp/q_itoq.c: Define FP_NO_EXCEPTIONS.
(_Q_itoq): Do not use FP_DECL_EX, FP_CLEAR_EXCEPTIONS or

1
soft-fp/extenddftf2.c
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@ -28,6 +28,7 @@
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#define FP_NO_EXACT_UNDERFLOW
#include "soft-fp.h"
#include "double.h"
#include "quad.h"

1
soft-fp/extendsfdf2.c
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@ -28,6 +28,7 @@
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#define FP_NO_EXACT_UNDERFLOW
#include "soft-fp.h"
#include "single.h"
#include "double.h"

1
soft-fp/extendsftf2.c
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@ -28,6 +28,7 @@
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#define FP_NO_EXACT_UNDERFLOW
#include "soft-fp.h"
#include "single.h"
#include "quad.h"

2
soft-fp/extendxftf2.c
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@ -39,7 +39,7 @@ __extendxftf2 (XFtype a)
FP_DECL_Q (R);
TFtype r;
FP_INIT_ROUNDMODE;
FP_INIT_TRAPPING_EXCEPTIONS;
FP_UNPACK_RAW_E (A, a);
#if (2 * _FP_W_TYPE_SIZE) < _FP_FRACBITS_Q
FP_EXTEND (Q, E, 4, 4, R, A);

2
soft-fp/op-common.h
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@ -1522,6 +1522,8 @@
_FP_FRAC_SLL_##dwc (D, (_FP_FRACBITS_##dfs \
- _FP_FRACBITS_##sfs)); \
D##_e = 0; \
if (FP_TRAPPING_EXCEPTIONS & FP_EX_UNDERFLOW) \
FP_SET_EXCEPTION (FP_EX_UNDERFLOW); \
} \
else \
{ \

18
soft-fp/soft-fp.h
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@ -107,10 +107,16 @@
# define FP_INIT_ROUNDMODE do {} while (0)
#endif
/* Initialize any machine-specific state used in
FP_TRAPPING_EXCEPTIONS or FP_HANDLE_EXCEPTIONS. */
#ifndef FP_INIT_TRAPPING_EXCEPTIONS
# define FP_INIT_TRAPPING_EXCEPTIONS FP_INIT_ROUNDMODE
#endif
/* Initialize any machine-specific state used in
FP_HANDLE_EXCEPTIONS. */
#ifndef FP_INIT_EXCEPTIONS
# define FP_INIT_EXCEPTIONS FP_INIT_ROUNDMODE
# define FP_INIT_EXCEPTIONS FP_INIT_TRAPPING_EXCEPTIONS
#endif
#ifndef FP_HANDLE_EXCEPTIONS
@ -162,6 +168,16 @@
#endif
/* A file using soft-fp may define FP_NO_EXACT_UNDERFLOW before
including soft-fp.h to indicate that, although a macro used there
could allow for the case of exact underflow requiring the underflow
exception to be raised if traps are enabled, for the particular
arguments used in that file no exact underflow can occur. */
#ifdef FP_NO_EXACT_UNDERFLOW
# undef FP_TRAPPING_EXCEPTIONS
# define FP_TRAPPING_EXCEPTIONS 0
#endif
#define _FP_ROUND_NEAREST(wc, X) \
do \
{ \