[LLVMbugs] [Bug 9187] New: LoopStrengthReduce violates LoopPass rules

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9187

           Summary: LoopStrengthReduce violates LoopPass rules
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: zwarich at apple.com
                CC: llvmbugs at cs.uiuc.edu


LoopStrengthReduce violates LoopPass rules. When processing a loop, it will
potentially modify its parent loop's preheader. For example, run llc on this
reduction:

===

; ModuleID = 'bugpoint-reduced-simplified.bc'
target triple = "x86_64-apple-darwin10"

define void @myquicksort(i8* %a) nounwind ssp {
entry:
  br i1 undef, label %loop1, label %return

loop1:                                            ; preds = %bb13.loopexit,
%entry
  %indvar419 = phi i64 [ %indvar.next420, %loop2.exit ], [ 0, %entry ]
  %tmp474 = shl i64 %indvar419, 2
  %tmp484 = add i64 %tmp474, 4
  br label %loop2

loop2:                                            ; preds = %loop1,
%loop2.backedge
  %indvar414 = phi i64 [ %indvar.next415, %loop2.backedge ], [ 0, %loop1 ]
  %tmp473 = mul i64 %indvar414, -4
  %tmp485 = add i64 %tmp484, %tmp473
  %storemerge4 = getelementptr i8* %a, i64 %tmp485
  %0 = icmp ugt i8* %storemerge4, %a
  br i1 false, label %loop2.exit, label %loop2.backedge

loop2.backedge:                                   ; preds = %loop2
  %indvar.next415 = add i64 %indvar414, 1
  br label %loop2

loop2.exit:                                       ; preds = %loop2
  %indvar.next420 = add i64 %indvar419, 1
  br i1 undef, label %loop1, label %return

return:                                           ; preds = %loop2.exit, %entry
  ret void
}

===

LSR gives this output:

===

define void @myquicksort(i8* %a) nounwind ssp {
entry:
  br i1 undef, label %loop1.preheader, label %return

loop1.preheader:                                  ; preds = %entry
  %scevgep = getelementptr i8* %a, i64 4
  br label %loop1

loop1:                                            ; preds = %loop1.preheader,
%loop2.exit
  %lsr.iv = phi i8* [ %scevgep, %loop1.preheader ], [ %scevgep1, %loop2.exit ]
  %indvar419 = phi i64 [ %indvar.next420, %loop2.exit ], [ 0, %loop1.preheader
]
  br label %loop2

loop2:                                            ; preds = %loop2.backedge,
%loop1
  %lsr.iv2 = phi i8* [ %scevgep3, %loop2.backedge ], [ %lsr.iv, %loop1 ]
  %0 = icmp ugt i8* %lsr.iv2, %a
  br i1 false, label %loop2.exit, label %loop2.backedge

loop2.backedge:                                   ; preds = %loop2
  %scevgep3 = getelementptr i8* %lsr.iv2, i64 -4
  br label %loop2

loop2.exit:                                       ; preds = %loop2
  %indvar.next420 = add i64 %indvar419, 1
  %scevgep1 = getelementptr i8* %lsr.iv, i64 4
  br i1 false, label %loop1, label %return.loopexit

return.loopexit:                                  ; preds = %loop2.exit
  br label %return

return:                                           ; preds = %return.loopexit,
%entry
  ret void
}

===

LSR inserts the definition of %scevgep when processing loop2, not when
processing loop1.

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