On Sat, Nov 26, 2011 at 3:41 PM, Amker.Cheng <amker.ch...@gmail.com> wrote: > Hi, > I looked into PR43491 a while and found in this case the gimple > generated before pre > is like: > > reg.0_12 = reg > ... > c() > reg.0_1 = reg > D.xxx = MEM[reg.0_1 + 8B] > > The pre pass transforms it into: > > reg.0_12 = reg > ... > c() > reg.0_1 = reg.0_12 > D.xxx = MEM[reg.0_1 + 8B] > > From now on, following passes(like copy_prop) can not transform it back and > resulting in an additional mov instruction as the bug reported. > > The flow is like: > 1, when rewriting gimple into ssa, reg is treated as a memory use; > 2, seems pre noticed that reg is const and replace reg with reg.0_12, > by this pre thinks it has eliminated an additional memory load operation; > 3, following passes do not transform it back either because reg is treated > as mem use or the const attribute is ignored. > > I think pre does the right thing given the information it knows, so wondering > at which pass thing starts going wrong and how could this issue be handled? >
Should PRE be changed to global register variable aware, thus it does not do the mentioned unnecessary elimination? -- Best Regards.
