https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119681
--- Comment #12 from Artemiy Volkov <artemiy at synopsys dot com> --- (In reply to Andrew Pinski from comment #11) > (In reply to Artemiy Volkov from comment #10) > > > > CMIIW, but this live range splitting is done a bit later by web and works > > well in the case where those pseudos' live ranges can be split without > > changing the CFG. To handle the case when multiple definitions of a > > variable are visible at the exit BB, you need to create copies of the exit > > BB before you can rename the variable, which is what I do in this patch. > > I am not sure you need a copy of the exit BB but rather splitting the edge > and do an insert of a move on the new BB and allow the other parts of GCC do > the copy ... > > This is basically what out of SSA does rather than a full exit BB copy. Right. I am not sure why I said "copies of the exit BB". The patch splits the exit edge N times, where N is the unroll factor. In each of the new exit BBs it inserts a move instruction for each variable defined in the loop and live in at the old exit BB. This has the effect of splitting the DU chains of those variables. Then I rename the resulting DU chains when feasible. And yes, it is emulating an effect of a phi-function at the entry to the exit BB.
