Re: preprocessing & optimization

[billwolf%hazel.cs.clemson.edu@hubcap.clemson.edu (William Thomas Wolfe, 2847 )]

From stt@inmet.inmet.com:
> preprocessors create no optimization problem,
> since they operate at the lexical, or possibly syntactic, level,
> long before the optimizer takes a look at the program.

   Are you sure that this is true in the general case?  (Not just
   for current compiler products, but in the general case?)  If a 
   preprocessor takes a high-level construct and reduces it to a 
   series of lower-level constructs, then isn't there a certain 
   loss of exploitable semantic content?

   Perhaps many present optimizers are not engaging in this level
   of sophistication, but analytically speaking, the most heavy-duty 
   of optimizing compilers would squeeze the final drops from every 
   single bit of semantic knowledge available to it, using semantics 
   to develop sophisticated reasoning leading to proofs that certain
   optimizations can be safely performed.

   If something like Anna/TSL were incorporated into Ada, and the
   preprocessor were to preserve all the semantic information during
   transformation (inserting annotations to restore the semantics 
   lost during preprocessing), then under those conditions the actions
   of a preprocessor would probably not incur any penalty.  But if this
   is not the case, then it would seem, at least theoretically, that
   there would be considerable losses in the potential for optimization. 


   Bill Wolfe, wtwolfe@hubcap.clemson.edu