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registers it defines then interfere with an existing preg live range. For instance, if we had something like these machine instructions: BB#0 ... = imul ... EFLAGS<imp-def,dead> test ..., EFLAGS<imp-def> jcc BB#2 EFLAGS<imp-use> BB#1 ... ; fallthrough to BB#2 BB#2 ... ; No code that defines EFLAGS jcc ... EFLAGS<imp-use> Machine sink will come along, see that imul implicitly defines EFLAGS, but because it's "dead", it assumes that it can move imul into BB#2. But when it does, imul's "dead" imp-def of EFLAGS is raised from the dead (a zombie) and messes up the condition code for the jump (and pretty much anything else which relies upon it being correct). The solution is to know which pregs are live going into a basic block. However, that information isn't calculated at this point. Nor does the LiveVariables pass take into account non-allocatable physical registers. In lieu of this, we do a *very* conservative pass through the basic block to determine if a preg is live coming out of it. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105387 91177308-0d34-0410-b5e6-96231b3b80d8
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