(g77.info.gz) Context-Sensitive Constants

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(g77.info.gz) Context-Sensitive Intrinsicness

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(g77.info.gz) Equivalence Versus Equality

 
 Context-Sensitive Constants
 ---------------------------
 
    `g77' does not use context to determine the types of constants or
 named constants (`PARAMETER'), except for (non-standard) typeless
 constants such as `'123'O'.
 
    For example, consider the following statement:
 
      PRINT *, 9.435784839284958 * 2D0
 
 `g77' will interpret the (truncated) constant `9.435784839284958' as a
 `REAL(KIND=1)', not `REAL(KIND=2)', constant, because the suffix `D0'
 is not specified.
 
    As a result, the output of the above statement when compiled by
 `g77' will appear to have "less precision" than when compiled by other
 compilers.
 
    In these and other cases, some compilers detect the fact that a
 single-precision constant is used in a double-precision context and
 therefore interpret the single-precision constant as if it was
 *explicitly* specified as a double-precision constant.  (This has the
 effect of appending *decimal*, not *binary*, zeros to the fractional
 part of the number--producing different computational results.)
 
    The reason this misfeature is dangerous is that a slight, apparently
 innocuous change to the source code can change the computational
 results.  Consider:
 
      REAL ALMOST, CLOSE
      DOUBLE PRECISION FIVE
      PARAMETER (ALMOST = 5.000000000001)
      FIVE = 5
      CLOSE = 5.000000000001
      PRINT *, 5.000000000001 - FIVE
      PRINT *, ALMOST - FIVE
      PRINT *, CLOSE - FIVE
      END
 
 Running the above program should result in the same value being printed
 three times.  With `g77' as the compiler, it does.
 
    However, compiled by many other compilers, running the above program
 would print two or three distinct values, because in two or three of
 the statements, the constant `5.000000000001', which on most systems is
 exactly equal to `5.'  when interpreted as a single-precision constant,
 is instead interpreted as a double-precision constant, preserving the
 represented precision.  However, this "clever" promotion of type does
 not extend to variables or, in some compilers, to named constants.
 
    Since programmers often are encouraged to replace manifest constants
 or permanently-assigned variables with named constants (`PARAMETER' in
 Fortran), and might need to replace some constants with variables
 having the same values for pertinent portions of code, it is important
 that compilers treat code so modified in the same way so that the
 results of such programs are the same.  `g77' helps in this regard by
 treating constants just the same as variables in terms of determining
 their types in a context-independent way.
 
    Still, there is a lot of existing Fortran code that has been written
 to depend on the way other compilers freely interpret constants' types
 based on context, so anything `g77' can do to help flag cases of this
 in such code could be very helpful.
 

Info Catalog

(g77.info.gz) Context-Sensitive Intrinsicness

(g77.info.gz) Non-bugs

(g77.info.gz) Equivalence Versus Equality