Re: constant folding at parse time

markh@csd4.csd.uwm.edu (Hunk) writes:
>A expression evaluation routine I wrote for an expression parser in a
>recently released assembler (C-expression syntax is handled) is set up in
>such a way that all constant subexpressions will be reduced to leaf nodes
>during parsing. The question about whether a given expression is constant
>or not doesn't even arise because it's already been handled by the parser.
>Basically, if the expression tree returned by the parser is a leaf node
>then it's a constant, otherwise it's not.


It all depends on what you call a constant. The address of a global
object is a constant and may be used in an initializer list:


      char c[10];
      char *cp = &c[2];


The address of c[2] is a constant but cannot be folded by the compiler.
Because of this, we have had to use a flag in a parse tree node to
indicate the 'constant' status of the sub-expression.


--
Scott Amspoker, Basis International, Albuquerque, NM
scott@bbx.basis.com
[I've worked with compilers that considered a constant to be a numeric
offset plus an optional static symbol address. This can be very convenient
for generating addressing code. -John]
--

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