Kenneth Zadeck wrote:
I suppose reg-live-at-start / reg-live-at-end information is actually easier
to maintain during if-conversion that def-use chains.
This is true, certainly in theory, a lot less so in practice.
The way that you order things is this.
while (something changes and we have not done too many passes)
do the analysis
do one properly ordered pass over the cfg changing what you can
change without using information that has been corrupted during this pass.
Except that with the current def-use chains updating def-use chains is
pointless since they
don't have the required information in the first place.
In my problem, the gen set is the set of uses, and the kill set are
defs, clobbers and uses. Hence, it's still backwards problem.
You could define it either way. However, the backwards problem is
already there in both the mainline and the dataflow branch. It is
called reaching uses or DF_RU.
When the block has one or more uses, but no def of a register, the question
to ask is 'which uses of this register reaches this *use* site'
(backwards). That's
different from DF_RU, which calculates this information for def sites
instead.
I think you should look closely are DF_RU.
What more is there to see? When the block I'm looking at only has uses of a
register, I don't want to know about uses in 'siblings' in the control
flow graph -
what I want to know is if the register is live at the end of the block.
LR should
answer that, and I can keep this information up-to-date inasmuch as it will
mention all live registers, and remain self-consistent.
