| Message ID | Pine.LNX.4.64.1409172134470.17727@digraph.polyomino.org.uk |
|---|---|
| State | New |
| Headers | show |
Ping. This patch <https://sourceware.org/ml/libc-alpha/2014-09/msg00442.html> is pending review.
Ping^2. This patch <https://sourceware.org/ml/libc-alpha/2014-09/msg00442.html> is still pending review.
Ping^3. This patch <https://sourceware.org/ml/libc-alpha/2014-09/msg00442.html> is still pending review.
On 09/17/2014 05:35 PM, Joseph S. Myers wrote: > 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). Slightly unrelated, but why isn't everything using TFmode? What benefit is there to XFmode? > 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 (and an sfp-machine.h bug fix > <https://gcc.gnu.org/ml/gcc-patches/2014-09/msg01415.html> - > FP_TRAPPING_EXCEPTIONS was wrongly defined in libgcc for x86) with no > regressions on x86_64-unknown-linux-gnu and verified that a test of > the relevant case passes where it failed before. > > 2014-09-17 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. Looks good to me. > diff --git a/soft-fp/extenddftf2.c b/soft-fp/extenddftf2.c > index 6984b41..1765532 100644 > --- a/soft-fp/extenddftf2.c > +++ b/soft-fp/extenddftf2.c > @@ -28,6 +28,7 @@ > License along with the GNU C Library; if not, see > <http://www.gnu.org/licenses/>. */ > > +#define FP_NO_EXACT_UNDERFLOW OK. > #include "soft-fp.h" > #include "double.h" > #include "quad.h" > diff --git a/soft-fp/extendsfdf2.c b/soft-fp/extendsfdf2.c > index a9b6bfb..6224195 100644 > --- a/soft-fp/extendsfdf2.c > +++ b/soft-fp/extendsfdf2.c > @@ -28,6 +28,7 @@ > License along with the GNU C Library; if not, see > <http://www.gnu.org/licenses/>. */ > > +#define FP_NO_EXACT_UNDERFLOW OK. > #include "soft-fp.h" > #include "single.h" > #include "double.h" > diff --git a/soft-fp/extendsftf2.c b/soft-fp/extendsftf2.c > index 07fc367..f67d614 100644 > --- a/soft-fp/extendsftf2.c > +++ b/soft-fp/extendsftf2.c > @@ -28,6 +28,7 @@ > License along with the GNU C Library; if not, see > <http://www.gnu.org/licenses/>. */ > > +#define FP_NO_EXACT_UNDERFLOW OK. > #include "soft-fp.h" > #include "single.h" > #include "quad.h" > diff --git a/soft-fp/extendxftf2.c b/soft-fp/extendxftf2.c > index 67b9095..a1386a6 100644 > --- a/soft-fp/extendxftf2.c > +++ b/soft-fp/extendxftf2.c > @@ -39,7 +39,7 @@ __extendxftf2 (XFtype a) > FP_DECL_Q (R); > TFtype r; > > - FP_INIT_ROUNDMODE; > + FP_INIT_TRAPPING_EXCEPTIONS; OK. > FP_UNPACK_RAW_E (A, a); > #if (2 * _FP_W_TYPE_SIZE) < _FP_FRACBITS_Q > FP_EXTEND (Q, E, 4, 4, R, A); > diff --git a/soft-fp/op-common.h b/soft-fp/op-common.h > index 5f37a32..d55a1e5 100644 > --- a/soft-fp/op-common.h > +++ b/soft-fp/op-common.h > @@ -1538,6 +1538,8 @@ > _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); \ OK. Here we signal underflow if the trapping flag was set. > } \ > else \ > { \ > diff --git a/soft-fp/soft-fp.h b/soft-fp/soft-fp.h > index 8d0efa5..807f847 100644 > --- a/soft-fp/soft-fp.h > +++ b/soft-fp/soft-fp.h > @@ -108,9 +108,15 @@ > #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 OK. Default to what we did before. > +#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 OK. > #endif > > #ifndef FP_HANDLE_EXCEPTIONS > @@ -166,6 +172,16 @@ > > #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 > + OK. > #define _FP_ROUND_NEAREST(wc, X) \ > do \ > { \ > Cheers, Carlos.
On Wed, 8 Oct 2014, Carlos O'Donell wrote: > On 09/17/2014 05:35 PM, Joseph S. Myers wrote: > > 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). > > Slightly unrelated, but why isn't everything using TFmode? > What benefit is there to XFmode? As a hardware floating-point format, I expect XFmode is a lot faster (even if x87 floating point is probably slower than SSE), though I don't have any benchmarks comparing long double and __float128 on x86. (And the fact that it's long double - on x86 / x86_64 / ia64 - whereas TFmode is __float128 - no doubt also means it's much more widely used. And so far getting libm functions and string conversions for __float128 requires you to use libquadmath shipped with GCC. __float128 support in glibc was previously rejected in <https://sourceware.org/ml/libc-alpha/2004-03/msg00326.html> and <https://sourceware.org/ml/libc-alpha/2004-05/msg00055.html>, but now there are proposed standard C bindings for a _Float128 type with IEEE binary128 format - draft TS 18661-3 - so I think it makes sense to revisit the principle of adding such support to glibc now the standard bindings exist.)
diff --git a/soft-fp/extenddftf2.c b/soft-fp/extenddftf2.c index 6984b41..1765532 100644 --- a/soft-fp/extenddftf2.c +++ b/soft-fp/extenddftf2.c @@ -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" diff --git a/soft-fp/extendsfdf2.c b/soft-fp/extendsfdf2.c index a9b6bfb..6224195 100644 --- a/soft-fp/extendsfdf2.c +++ b/soft-fp/extendsfdf2.c @@ -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" diff --git a/soft-fp/extendsftf2.c b/soft-fp/extendsftf2.c index 07fc367..f67d614 100644 --- a/soft-fp/extendsftf2.c +++ b/soft-fp/extendsftf2.c @@ -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" diff --git a/soft-fp/extendxftf2.c b/soft-fp/extendxftf2.c index 67b9095..a1386a6 100644 --- a/soft-fp/extendxftf2.c +++ b/soft-fp/extendxftf2.c @@ -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); diff --git a/soft-fp/op-common.h b/soft-fp/op-common.h index 5f37a32..d55a1e5 100644 --- a/soft-fp/op-common.h +++ b/soft-fp/op-common.h @@ -1538,6 +1538,8 @@ _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 \ { \ diff --git a/soft-fp/soft-fp.h b/soft-fp/soft-fp.h index 8d0efa5..807f847 100644 --- a/soft-fp/soft-fp.h +++ b/soft-fp/soft-fp.h @@ -108,9 +108,15 @@ #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 @@ -166,6 +172,16 @@ #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 \ { \