[LLVMbugs] [Bug 7620] New: MachineLICM should hoist ImplicitDefs.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=7620

           Summary: MachineLICM should hoist ImplicitDefs.
           Product: libraries
           Version: 1.0
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: clattner at apple.com
                CC: llvmbugs at cs.uiuc.edu


MachineLICM::IsLICMCandidate makes it not hoist implicit defs, however, as the
fixme I added indicates, hoisting an IMPLICIT_DEF may allow hoisting other
instructions that use the impdef as an operand.

Hoisting it should not affect register pressure at all (assuming the RA is
doing the right thing with implicitdef), so just removing that check should be
fine.

However, doing this causes other RA fallout, e.g. test/CodeGen/ARM/spill-q.ll
crashes in the two-address pass.

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