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Date: Sun, 26 Jan 2020 14:30:32 +1100 (AEDT)
From: Damian McGuckin <>
Subject: Re: Considering x86-64 fenv.s to C

PowerPC 64:

  * powerpc64 ARCHITECTURE (tolterable reference is below)
  * PowerPC includes cause (or reason) bits which qualify why an INVALID
  * exception get raised. The cause can be regarded as just an explanation
  * in most cases. In fact, if only the ppropriate cause bit is set in the
  * control register and this is fed to in a 'Move to FPSCR', the INVALID
  * exception will be triggered automatically. The complete list is:
  *		VXSNAN	caused by a floating point operation on a SNAN
  *		VXISI	caused by the operation INFINITY - INFINITY
  *		VXIDI	caused by the operation INFINITY / INFINITY
  *		VXZDZ	caused by the operation 0.0 / 0.0
  *		VXIMZ	caused by the operation INFINITY * 0
  *		VXCVI	caused by an illegal integer conversion
  *		VXVC	caused by an ordered comparison involving a NaN
  *		VXSOFT	caused by an explicit sofware request
  *		VXSQRT	caused by a sqrt/rsqrt on a negative & non-zero number
  * The OR of all such causes is called FE_ALL_INVALID in the API.
  * One above, VXSOFT, is that caused 'By (An Explicit Software) Request'. The
  * documentation says that its purpose is to allow software to cause/raise an
  * Invalid Operation Exception for a condition that is not necessarily
  * associated with the execution of a floating-point instruction. It could
  * even be used by a program to complain about some negative and non-zero
  * operand long BEFORE it gets given to a SQRT or RSQRT imnstruction where
  * it would otherwise invoke the VXSQRT cause and trigger that bit to be set.
  * Note, when an FE_INVALID exception is cleared, all the causes including
  * the ones that other instructions can generate, i.e all of the bits in the
  * FE_ALL_INVALID bit mask, must be similarly also cleared. Otherwise, any
  * CAUSE bit left in the mask will, when fed back into the 'MOVE to FPSCR'
  * instruction (implicit in the 'fe_set_fpscr' routine) will retrigger the
  * FE_INVALID exception. So MUSL must still cater for these non-standard
  * causes, because to not do so will stab itself in the back.
  * So MUSL makes the assumption that at least for a PowerPC, any raising of
  * the INVALID exception must also raise a cause, that cause being by default,
  * that called 'By Explicit Software Request'. That makes sense as this is
  * beind done by the MUSL itself, not as a part, or side-effect, of some
  * arithmetic operation. Note also that if a user has explicitly (violated
  * the standard and) augmented an exception list to contain some other cause,
  * that extra cause gets trashed on entry to the fenv(3) routines and only
  * this default cause is added. The rejection of the benefits of the extra
  * information provided by a PowerPC could be addressed in the future as
  * the fix is trivial!
  * The FE_INVALID_SOFTWARE flag is relabelled as FE_INVALID_BY_REQUEST to
  * correctly explain its purpose and make any use remotely comprehensible.
  * This is questionable in terms of style but without this, the ability of
  * the comments to make sense is seriously compromised. For the same reason,
  * FE_ALL_INVALID is relabelled FE_ALL_INVALID_CAUSES. At some stage, it is
  * recommended that the IBM ISA documentation be enhanced to be more lucid.

#include	<fenv.h>
  * Remember that the floating point environment lives in a 'double' That
  * is locked by the ABI defined in the architecture dependent <fenv.h>.
  * The first routine pair gets and sets that environment for later use by
  * the MUSL mandated pair of fe_get_csr_arch/fe_set_csr_arch routines
static inline double fe_get_fpscr_f(void)
 	double fpscr;

 	__asm__ __volatile__ ("mffs %0" : "=d"(fpscr));
 	return fpscr;

static inline void fe_set_fpscr_f(double fpscr)
 	__asm__ __volatile__ ("mtfsf 255, %0" : : "d"(fpscr));
  * This routine pair move tos and from the double (of the environment) into
  * MUSL's unsigned int object in which it stores control & status registers
static inline unsigned int fe_get_csr_arch(void)
 	return (unsigned int) (union {double f; long i;}) {fe_get_fpscr_f()}.i;

static inline void fe_set_csr_arch(unsigned int fpscr)
 	fe_set_fpscr_f((union {long i; double f;}) {(long) fpscr}.f);
  * These two macros work with the environment directly
  * Should they eventually move to being inline routines?
#define	fe_get_e(e)	(*e = fe_get_fpscr_f())
#define	fe_set_e(e)	(fe_set_fpscr_f(*e))
  * the default value of a floating point environment of type fenv_t (double)
#define	FE_DFL_ENV_DATA	0.0
  * Eludicate the macros related to CAUSEs for improved readability.
  * While this is tolerable at the architecture definition stage, it
  * may need to change in the production code.

#include	"../fenv-generic.c"

Regards - Damian

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