[Cfp-interest] outline of directives pragmas

[David Hough CFP]

I've prepared an outline of directives pragmas content - but not filled
in as much content as I'd hoped.      
Tomorrow we can discuss specific issues evident so far.
Much of the text is from the existing standard and reports, to provide
a framework for new material.






Directives Pragmas

754-2008 specifies:

==========
 4.1 Attribute specification

 An attribute is logically associated with a program block to modify
 its numerical and exception semantics.  A user can specify a constant
 value for an attribute parameter.

 Some attributes have the effect of an implicit parameter to most
 individual operations of this standard; language standards shall
 specify

 * rounding-direction attributes

 and should specify

 * alternate exception handling attributes

 Other attributes change the mapping of language expressions into
 operations of this standard; language standards that permit more than
 one such mapping should provide support for:

 * preferredWidth attributes

 * value-changing optimization attributes

 * reproducibility attributes

 For attribute specification, the implementation shall provide
 language-defined means, such as compiler directives, to specify a
 constant value for the attribute parameter for all standard operations
 in a block; the scope of the attribute value is the block with which
 it is associated. Language standards shall provide for constant
 specification of the default and each specific value of the
 attribute.
==========

ANSI-C specifies standard pragmas thus:

==========
 6.10.6 Pragma directive

 Semantics A preprocessing directive of the form
  # pragma pp-tokensopt new-line where the preprocessing token STDC
 does not immediately follow pragma in the directive (prior to any
 macro replacement)174) causes the implementation to behave in an
 implementation-defined manner. The behavior might cause translation to
 fail or cause the translator or the resulting program to behave in a
 non-conforming manner. Any such pragma that is not recognized by the
 implementation is ignored.

 If the preprocessing token STDC does immediately follow pragma in the
 directive (prior to any macro replacement), then no macro replacement
 is performed on the directive, and the directive shall have one of the
 following forms175) whose meanings are described elsewhere:

  #pragma STDC FP_CONTRACT on-off-switch

  #pragma STDC FENV_ACCESS on-off-switch

  #pragma STDC CX_LIMITED_RANGE on-off-switch

 on-off-switch: one of

  ON OFF DEFAULT
==========

Scope (adapted from C 7.12.2 for FP_CONTRACT) :

==========
 Each FP_CONTRACT pragma can occur either outside external declarations
 or preceding all explicit declarations and statements inside a
 compound statement. When outside external declarations, the pragma
 takes effect from its occurrence until another FP_CONTRACT pragma is
 encountered, or until the end of the translation unit. When inside a
 compound statement, the pragma takes effect from its occurrence until
 another FP_CONTRACT pragma is encountered (including within a nested
 compound statement), or until the end of the compound statement; at
 the end of a compound statement the state for the pragma is restored
 to its condition just before the compound statement. If this pragma is
 used in any other context, the behavior is undefined.
==========

This existing pragma STDC syntax and scope will be extended to
encompass the attributes required by 754-2008.


(existing)
#pragma STDC FP_CONTRACT on-off-switch
#pragma STDC FENV_ACCESS on-off-switch
#pragma STDC CX_LIMITED_RANGE on-off-switch
 on-off-switch: one of
  	ON 
  	OFF 
	DEFAULT

(part 1)
#pragma STDC FENV_ROUND bdirection
 bdirection: 
	FE_DOWNWARD
	FE_TONEAREST 
	FE_TOWARDZERO
	FE_UPWARD
	FE_DYNAMIC

(part2 )
#pragma STDC FENV_DEC_ROUND ddirection
 ddirection:
	FE_DEC_DOWNWARD
	FE_DEC_TONEAREST
	FE_DEC_TONEARESTFROMZERO
	FE_DEC_TOWARDZERO
	FE_DEC_UPWARD
	FE_DEC_DYNAMIC

#pragma STDC FENV_WIDTH format
 for generic floating-point types, format is:
	genericnone
	float
	double
	long double
 for binary floating-point types, format is:
	_Floatnone
	_Float32 
	_Float64
	_Float128
	_Float32x
	_Float64x
	_Float128x
 for decimal floating-point types, format is:
	_Decimalnone
	_Decimal64
	_Decimal128
	_Decimal64x
	_Decimal128x
FENV_WIDTH specifies the minimum evaluation width for expressions involving
generic, or binary, or decimal floating-point types.    In a particular
implementation, generic types might be the same as binary types
or might be the same as decimal types

#pragma STDC FENV_VALUE_CHANGING_OPTIMIZATIONS switch
 switch:
  ENABLE
  DISABLE
  DEFAULT

and specific optimizations under this umbrella...

such as

#pragma STDC FENV_FAST_SUBNORMAL switch
 allows abrupt underflow and zero treatment of subnormal operands if faster,
 implementation defined

#pragma STDC FENV_REPRODUCIBLE switch
  ENABLE
  DISABLE
  DEFAULT

and specific optimizations under this umbrella...

#pragma STDC FENV_EXCEPT exception-list action 
 exception list is a list of exceptions and subexceptions
 separated by commas
 exception:
	FE_INEXACT
	FE_UNDERFLOW
	FE_OVERFLOW 
	FE_DIVBYZERO
	FE_INVALID
	FE_ALL			shorthand for all

and the following subexceptions of invalid and divbyzero:
 subexception:
	FE_SNAN			7.2a
	FE_INVALID_MUL		7.2b
	FE_INVALID_FMA		7.2c
	FE_INVALID_ADD		7.2d
	FE_INVALID_DIV		7.2e
	FE_INVALID_REM		7.2f
	FE_INVALID_SQRT		7.2g
	FE_INVALID_QUANTIZE	7.2h
	FE_INVALID_INT		7.2i
	FE_UNORDERED		7.2j
	FE_INVALID_LOGB		7.2k
	FE_DIV_ZERO		7.3 divisor zero
	FE_DIV_LOG		7.3 log(0) or logb(0)
	
 actions that don't change control flow:
	DEFAULT				754 default
 	NOFLAG				default result, but no flag raised
	OPTFLAG				implementation may raise flag or not
	ABRUPT 				underflow only
	SUBSTITUTE expression
	SUBSTITUTEXOR expression 	multiplication or division only
	RECORD				record the exception in a "debugger"
					and continue; same as default if
					no debugger active

 actions that change control flow:
	BREAK				to just past end of pragma scope
	ASAP label 			goto
	DELAYED label   		goto
	ASAP function(...) 		call, no return
	DELAYED function(...) 		call, no return

ASAP means as soon as possible - implementation defined.
DELAYED means at exit from the pragma scope

labels, functions, and expressions raise scope issues: is the relevant
scope defined at the site of the pragma or at the site of the exception?

Because of scope issues, pragmas specifying
actions that change control flow are only allowed within compound statements,
and not at the beginning of a file.

What about functions invoked within compound statements controlled by
an alternate exception handling pragma?    To follow the lead of
FENV_ROUND, a list of functions corresponding to 754 operations
specifies those affected by rounding and alternate exception handling pragmas.
Inlining does not affect whether an operation is affected.
	


Functions affected by constant rounding modes and alternate exception
handling modes

Within the scope of an FENV directive establishing a constant rounding mode
or an alternate exception handling mode,
all floating-point operators, 
implicit conversions (including the conversion of a value represented in a
format wider than its semantic types to its semantic type, 
as done by classification macros), and
invocations of functions indicated in the table below, 
for which macro replacement has not been
suppressed, 
shall be evaluated according to the specified mode. 
Invocations of functions for which macro replacement has
been suppressed and invocations of functions other than those indicated in the
table below shall not
be affected by constant rounding modes or alternate exception handling modes.

<Header> Function groups

<math.h> acos, asin, atan, atan2
<math.h> cos, sin, tan
<math.h> acosh, asinh, atanh
<math.h> cosh, sinh, tanh
<math.h> exp, exp2, expm1
<math.h> log, log10, log1p, log2
<math.h> scalbn, scalbln, ldexp
<math.h> cbrt, hypot, pow, sqrt
<math.h> erf, erfc
<math.h> lgamma, tgamma
<math.h> rint, nearbyint, lrint, llrint
<math.h> fdim
<math.h> fma
<math.h> fadd, daddl, fsub, dsubl, fmul, dmull, fdiv, ddivl, ffma, dfmal
<math.h> fsqrt, dsqrtl 
<stdlib.h> atof, strfromd, strfromf, strfroml, strtod, strtof, strtold
<wchar.h> wcstod, wcstof, wcstold
<stdio.h> printf and scanf families
<wchar.h> wprintf and wscanf families

Each <math.h> functon listed in the table above indicates the family of functions of all supported types (for example, acosf and acosl as well as acos).