[LLVMbugs] [Bug 23143] New: MachineLICM register pressure calculation incorrect

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23143

            Bug ID: 23143
           Summary: MachineLICM register pressure calculation incorrect
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: djasper at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The new test test/CodeGen/X86/licm-regpressure.ll shows this behavior
(currently marked as XFAIL). MachineLICM assumes that it is hoisting all of the
GEPs out of the loop under in a low pressure situation, when indeed there is
register pressure.

The cause here is that the RegLimit (for the RegClass GR64) is 12, but each
instruction is assumed to have a cost of only 1.

The problem seems to be that MachineLICM isn't considering the regmasks, i.e.
the registers clobbered by the call.

IR leading to the bad behavior:

%struct.A = type { i32, i32, i32, i32, i32, i32, i32 }

define void @test(i1 %b, %struct.A* %a) nounwind {
entry:
  br label %loop-header

loop-header:                                       
  br label %loop-body

loop-body:
  %0 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 0
  %1 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 1
  %2 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 2
  %3 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 3
  %4 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 4
  %5 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 5
  %6 = getelementptr inbounds %struct.A, %struct.A* %a, i64 0, i32 6
  call void @assign(i32* %0)
  call void @assign(i32* %1)
  call void @assign(i32* %2)
  call void @assign(i32* %3)
  call void @assign(i32* %4)
  call void @assign(i32* %5)
  call void @assign(i32* %6)
  br i1 %b, label %loop-body, label %loop-exit

loop-exit:
  ret void
}

declare void @assign(i32*)

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