Mark Mitchell wrote:
Kenneth Zadeck wrote:
The majority of the new bugs were places where the rest of the
compiler was just not expecting to see auto inc or dec instructions.
If you want to take on doing this kind of extension, be prepared for
the additional cost.
Kenny, do you have any pointers to autoincrement algorithms in the
literature? I can see how using some of the obvious global analysis
can spot things like a store followed on all paths by an add to the
pointer, and prove that there are no intervening uses of the pointer,
and therefore note that could could merge the addition with the store
to do a post-increment -- but is there anything especially clever out
there?
Thanks,
Remember that we are not really talking about pointers here, even though
the values that are computed are used as pointers. The things that are
combined are pseudo register expressions, so there is no rocket science
here, not any involvement with aliases.
Given that kind of simplification, this is a simple meet over all paths
problem. If all paths that reach some "suitable point" (the load or
store that can accommodate the inc or dec) increment or decrement the
register in the same way and moving that inc or dec will not create an
inc or dec free path, you are free to move combine the inc into the
suitable point.
This is a very simplified instance of code motion or commoning. The
simplification is that it is goal directed. The goal is to move the inc
insns to the "suitable points".
As far as literature, i do not know any off the top of my head, but this
is a classic meet over all paths kind of dataflow problem. I would
rather solve it in ssa form than using dataflow equations but that is my
bias and unfortunately is not an option until we get fuds put into the
back end.
Kenny
