Message ID  20200803175504.8447383keithp@keithp.com 

State  Superseded 
Headers  show 
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On Aug 3 10:55, Keith Packard via Newlib wrote: > math_errhandling is specified to contain two bits of information: > > 1. MATH_ERRNO  Set when the library sets errno > 2. MATH_ERREXCEPT  Set when math operations report exceptions > > MATH_ERRNO should match whether the original math code is compiled in > _IEEE_LIBM mode and the new math code has WANT_ERRNO == 1. > > MATH_ERREXCEPT should match whether the underlying hardware has > exception support. This patch adds configurations of this value for > RISCV, ARM, Aarch64, x86 and x86_64 when using HW float. > > Signedoffby: Keith Packard <keithp@keithp.com> >  > newlib/libc/include/machine/ieeefp.h  11 +++++++++++ > newlib/libc/include/math.h  12 +++++++++++ > 2 files changed, 22 insertions(+), 1 deletion() Prior to this patch, math_errhandling on Cygwin was set to MATH_ERRNO. With this patch, math_errhandling on Cygwin is set to MATH_ERREXCEPT. That may break backward compatibility. From your patch I take it that not setting MATH_ERREXCEPT before was just missing, but actually changing the MATH_ERRNO bit looks wrong to me. Does that mean Cygwin just has to define _IEEE_LIBM somehwere? I don't see that any target is setting _IEEE_LIBM anywhere... Corinna  Corinna Vinschen Cygwin Maintainer Red Hat
Corinna Vinschen <vinschen@redhat.com> writes: > Prior to this patch, math_errhandling on Cygwin was set to MATH_ERRNO. > With this patch, math_errhandling on Cygwin is set to MATH_ERREXCEPT. Oh, my test is *backwards*. _IEEE_LIBM means that the original math functions never set errno. However, even inverted, my test is not complete. In the non_IEEE_LIBM case, it needs to check _LIB_VERSION != _IEEE_ as much of the old math library uses that to decide whether to set errno or not. Thanks for catching this! I didn't create a test case for this, which I have done now. > That may break backward compatibility. From your patch I take it > that not setting MATH_ERREXCEPT before was just missing, but actually > changing the MATH_ERRNO bit looks wrong to me. Yes, MATH_ERREXCEPT should be set on any platform that supports exceptions. What that means for hardware that only supports exceptions on a subset of floating point formats is unclear. > Does that mean Cygwin just has to define _IEEE_LIBM somehwere? I don't > see that any target is setting _IEEE_LIBM anywhere... I don't know  right now, Cygwin is not setting _POSIX_MODE and so applications are not getting errno set unless they set _LIB_VERSION=_POSIX_, which I suspect no applications are even aware of. The test I wrote uncovered some additional information about errno handling in libm. There are three different mechanisms for controlling errno in various parts of the library: 1) _IEEE_LIBM. Used by some routines the old math library to only support IEEE (754 I assume) standards for math functions, which means not setting errno and (sometimes) applying slightly different semantics to the functions (!). Not universally supported (i.e., some functions still set errno even when this flag is set). 2) _LIB_VERSION. This global variable (named __fdlib_version) selects whether the library supports IEEE semantics or posix semantics at run time. The default value for this is _IEEE_ unless the configuration sets _POSIX_MODE, which is only set on the 'spu' host. As a result, the default math library configuration is to run in _IEEE_ mode, which doesn't set errno in lots of functions. 3) WANT_ERRNO. This is used in most of the libm/common code to select whether math routines should set errno or not. All of the relevant code is found in math_err.c and math_errf.c. This talks about using math_errhandling but it doesn't actually do it. This is enabled by default, and no configurations appear to disable it. I think we need to fix things so that the library is internally consistent  right now, there's no way to configure the library to disable errno entirely, nor is there a way to control errno at runtime in all cases. It seems that there are three possible ways we might want to configure newlib: 1) No errno ever. Use exceptions to report errors back to applications. 2) Always set errno. All exceptions always set errno. 3) Allow applications to enable/disable errno setting at runtime. Right now, you can get 1) by defining _IEEE_LIBM, setting WANT_ERRNO=0 and never touching _LIB_VERSION. Applications may still end up referencing errno if they use functions which only check _LIB_VERSION. You can get 2) by by defining _POSIX_MODE, not defining _IEEE_LIBM and never setting _LIB_VERSION. You can't get 3) because math/common doesn't use _LIB_VERSION. Given that only SPU ever sets _POSIX_MODE, I'm tempted to suggest that newlib should never set errno and report errors only through the exception mechanism. Applications could detect this by checking the value of math_errhandling, which would not include MATH_ERRNO. I don't think there's any reasonable standard way to enable 3). The _LIB_VERSION mechanism isn't great  it's a global setting (not perthread), and it's not supported in either musl or glibc. Plus, newlib doesn't support the 'matherr' function anyways, which is tightly associated with _LIB_VERSION. The math_errhandling 'replacement' doesn't offer any defined way to change the current settings as math_errhandling may well be constant. If we want to support 2) in addition to 1), we'll want a single global definition that covers that mode. I'd suggest using the existing _IEEE_LIBM value as that is used in a lot more places than the somewhat ambiguously named WANT_ERRNO, and has a nice '_' prefix so we can expose it to applications (as is required if we want to use it when defining math_errhandling).  keith
diff git a/newlib/libc/include/machine/ieeefp.h b/newlib/libc/include/machine/ieeefp.h index aa8a1903b..b0042bbf6 100644  a/newlib/libc/include/machine/ieeefp.h +++ b/newlib/libc/include/machine/ieeefp.h @@ 87,6 +87,9 @@ # define __IEEE_BYTES_LITTLE_ENDIAN # endif #endif +#ifndef __SOFTFP__ +# define _SUPPORTS_ERREXCEPT +#endif #endif #if defined (__aarch64__) @@ 96,6 +99,9 @@ #define __IEEE_BIG_ENDIAN #endif #define __OBSOLETE_MATH_DEFAULT 0 +#ifdef __ARM_FP +# define _SUPPORTS_ERREXCEPT +#endif #endif #ifdef __epiphany__ @@ 189,10 +195,14 @@ #ifdef __i386__ #define __IEEE_LITTLE_ENDIAN +# define _SUPPORTS_ERREXCEPT #endif #ifdef __riscv #define __IEEE_LITTLE_ENDIAN +#ifdef __riscv_flen +# define _SUPPORTS_ERREXCEPT +#endif #endif #ifdef __i960__ @@ 386,6 +396,7 @@ #ifdef __x86_64__ #define __IEEE_LITTLE_ENDIAN +# define _SUPPORTS_ERREXCEPT #endif #ifdef __mep__ diff git a/newlib/libc/include/math.h b/newlib/libc/include/math.h index 1efc5b92c..4a99b322e 100644  a/newlib/libc/include/math.h +++ b/newlib/libc/include/math.h @@ 188,7 +188,17 @@ extern int isnan (double); # define MATH_ERREXCEPT 2 #endif #ifndef math_errhandling # define math_errhandling MATH_ERRNO +# ifdef _IEEE_LIBM +# define _MATH_ERRHANDLING_ERRNO MATH_ERRNO +# else +# define _MATH_ERRHANDLING_ERRNO 0 +# endif +# ifdef _SUPPORTS_ERREXCEPT +# define _MATH_ERRHANDLING_ERREXCEPT MATH_ERREXCEPT +# else +# define _MATH_ERRHANDLING_ERREXCEPT 0 +# endif +# define math_errhandling (_MATH_ERRHANDLING_ERRNO  _MATH_ERRHANDLING_ERREXCEPT) #endif extern int __isinff (float x);
math_errhandling is specified to contain two bits of information: 1. MATH_ERRNO  Set when the library sets errno 2. MATH_ERREXCEPT  Set when math operations report exceptions MATH_ERRNO should match whether the original math code is compiled in _IEEE_LIBM mode and the new math code has WANT_ERRNO == 1. MATH_ERREXCEPT should match whether the underlying hardware has exception support. This patch adds configurations of this value for RISCV, ARM, Aarch64, x86 and x86_64 when using HW float. Signedoffby: Keith Packard <keithp@keithp.com>  newlib/libc/include/machine/ieeefp.h  11 +++++++++++ newlib/libc/include/math.h  12 +++++++++++ 2 files changed, 22 insertions(+), 1 deletion()  2.28.0