I think that it is time that we in the GCC community took some time to address the problem of compiling very large functions in a somewhat systematic manner.
GCC has two competing interests here: it needs to be able to provide state of the art optimization for modest sized functions and it needs to be able to properly process very large machine generated functions using reasonable resources. I believe that the default behavior for the compiler should be that certain non essential passes be skipped if a very large function is encountered. There are two problems here: 1) defining the set of optimizations that need to be skipped. 2) defining the set of functions that trigger the special processing. For (1) I would propose that three measures be made of each function. These measures should be made before inlining occurs. The three measures are the number of variables, the number of statements, and the number of basic blocks. Many of the gcc passes are non linear in one or more of these measures and these passes should be skipped if one or more of these measures exceeds some threshold. For (2) I would propose that we add three new fields to the compilation manager. These fields would be null or zero if the optimization is either essential or is only linear in the measure. Otherwise, some indication of either a threshold or the exponent of the growth is used as the field. The compilation manager could then look at the options, in particular the -O level and perhaps some new options to indicate that this is a small machine or in the other extreme "optimize all functions come hell or high water!!" and skip those passes which will cause performance problems. I do not claim to understand how sensitive every pass is to these measures. However, I could possibly make a good first cut on the rtl passes. However, I think that before anyone starts hacking anything, we should come to a consensus on an overall strategy and implement something consistent for the entire compiler rather than attack some particular pass in a manner that only gets us pass the next pr. Volunteer(s) to implement the compilation manager part of this would also be appreciated. Kenny
