[LLVMbugs] [Bug 2495] New: Dominator queries should be lazy

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=2495

           Summary: Dominator queries should be lazy
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Keywords: slow-compile
          Severity: normal
          Priority: P2
         Component: Global Analyses
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org
                CC: llvmbugs at cs.uiuc.edu


llvm-ld runs mem2reg after jump threading because jump threading introduces
allocas in some cases.  However, the number of cases it does this is pretty
low.

Because mem2reg is run after jump threading, the pass manager computes
dominator info and dom frontier info.  However, if there are no promotable
allocas in the function this information is just thrown away again and never
used.

We should make domfrontier and domtree be lazy: compute their info for the
function on the first meaningful query, instead of computing everything in
runOnFunction.

One minor issue with this is that the time to compute this dom info will be
charged (in -time-passes) to the pass that uses it, instead of to the analysis
pass.  You could argue that this is actually a good thing though.

-Chris


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