On Fri, Mar 18, 2011 at 17:31, Jakub Jelinek <ja...@redhat.com> wrote: > On Fri, Mar 18, 2011 at 12:23:11PM -0400, Kenneth Zadeck wrote: >> I believe that this is not the right way to go. >> >> if someone specifies -fcrossjumping, then the pass should turn on >> live for the duration of the pass just as ifcvt does. If they ask >> for crossjumping you should give them crossjumping and not some >> crippled version of it. > > Such (untested) patch is in > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48156#c7 > But Paolo just added comment there that he prefers this version. > > Is the live problem so much more useful for crossjumping than lr problem? > All it wants to prove is if it can safely move a couple of instructions > across some other instructions, and the live_union bitmap that is computed > using the live/lr problem is used to find out if registers set by the > moved instructions are actually live at the end of those instructions > or not. What would be an example where live problem would allow optimizing > more than lr?
LIVE == LR except when you have uninitialized uses. Unless it is needed for correctness, I see no reason to prefer LIVE to LR at -O1. Paolo
