jlg@lanl.gov (Jim Giles) (09/22/88)
By the way, I've found the passage in Fortran that allows the following
expression optimization:
A = F(I) + F(I)
is the same as:
tmp = F(I)
A = 2*tmp
Section 6.6.4 of the ANSI Fortran standard:
6.6.4 Evaluation of Arithmetic Expressions. The rules
given in 6.1.2 specify the interpretation of an arithmetic
expression. ...
Section 6.1.2 give the normal left-to-right evaluation order with precedence.
... Once the interpretation has been established in
accordance with those rules, the processor may evaluate any
mathematically equivalent expression, provided that the
integrity of parenthesis is not violated.
Provided that the function F obeys all the prohibitions on side effects
given secion 6.6, the result of my optimization is mathematically (and
computationally, in this case) the same. So it's legal.
J. Giles
Los alamosags@h.cc.purdue.edu (Dave Seaman) (09/22/88)
In article <3979@lanl.gov> jlg@lanl.gov (Jim Giles) writes: >By the way, I've found the passage in Fortran that allows the following >expression optimization: > A = F(I) + F(I) >is the same as: > tmp = F(I) > A = 2*tmp [Quotes rule on expression evaluation]. That is true, but in Fortran (unlike C), assignment is a statement, not an expression. Therefore the rule you quoted does not apply to A(INVERT(I)) = A(INVERT(I)) + 1 -- Dave Seaman ags@j.cc.purdue.edu
jlg@lanl.gov (Jim Giles) (09/23/88)
From article <4002@h.cc.purdue.edu>, by ags@h.cc.purdue.edu (Dave Seaman): > That is true, but in Fortran (unlike C), assignment is a statement, not an > expression. Therefore the rule you quoted does not apply to > > A(INVERT(I)) = A(INVERT(I)) + 1 True. This case is ambiguous (as I've said before). But in C (unlike Fortran) this construct is deliberately ambiguous. I suspect that the Fortran committee would prefer not to have left this hole in the standard. The C committee is aparently going to leave a lot of ambiguous stuff in the ANSI C standard. Myself, I would prefer that the ambiguity be removed. In the meantime, I will continue to regard that construct as dangerous as if it were illegal. J. Giles Los Alamos