[LLVMbugs] [Bug 3174] New: GVN invalidates memdep

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3174

           Summary: GVN invalidates memdep
           Product: libraries
           Version: 1.0
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: clattner at apple.com
                CC: llvmbugs at cs.uiuc.edu


The GVN pass is invalidating memdep, which causes it to be recomputed for
memcpy optimizer.  We also run DSE substantially later, which means that extra
passes have to get recomputed:

        Memory Dependence Analysis
        Global Value Numbering
 *      Memory Dependence Analysis
        MemCpy Optimization
        Sparse Conditional Constant Propagation
        Combine redundant instructions
        Break critical edges in CFG
        Conditional Propagation
 *      Dominator Tree Construction
        Memory Dependence Analysis
        Dead Store Elimination

GVN would have to inform memdep about blocks it removes if it were to declare
that it preserved it though (MergeBlockIntoPredecessor) and we'd have to teach
memdep how to handle this.


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