[llvm-bugs] [Bug 48126] New: GEP inbound with negative offset generated by loop vectorizer

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48126

            Bug ID: 48126
           Summary: GEP inbound with negative offset generated by loop
                    vectorizer
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: liuz at cs.utah.edu
                CC: llvm-bugs at lists.llvm.org

Test case from test/Transforms/LoopVectorize/gcc-examples.ll

;CHECK-LABEL: @example21(
;CHECK: load <4 x i32>
;CHECK: shufflevector {{.*}} <i32 3, i32 2, i32 1, i32 0>
;CHECK: ret i32
define i32 @example21(i32* nocapture %b, i32 %n) nounwind uwtable readonly ssp
{
  %1 = icmp sgt i32 %n, 0
  br i1 %1, label %.lr.ph, label %._crit_edge

.lr.ph:                                           ; preds = %0
  %2 = sext i32 %n to i64
  br label %3

; <label>:3                                       ; preds = %.lr.ph, %3
  %indvars.iv = phi i64 [ %2, %.lr.ph ], [ %indvars.iv.next, %3 ]
  %a.02 = phi i32 [ 0, %.lr.ph ], [ %6, %3 ]
  %indvars.iv.next = add i64 %indvars.iv, -1
  %4 = getelementptr inbounds i32, i32* %b, i64 %indvars.iv.next
  %5 = load i32, i32* %4, align 4
  %6 = add nsw i32 %5, %a.02
  %7 = trunc i64 %indvars.iv.next to i32
  %8 = icmp sgt i32 %7, 0
  br i1 %8, label %3, label %._crit_edge

._crit_edge:                                      ; preds = %3, %0
  %a.0.lcssa = phi i32 [ 0, %0 ], [ %6, %3 ]
  ret i32 %a.0.lcssa
}

Below is the vectorized function after calling opt with the arguments listed in
the test case (-basic-aa -loop-vectorize -force-vector-width=4
-force-vector-interleave=1 -dce -instcombine).

; Function Attrs: nounwind readonly ssp uwtable
define i32 @example21(i32* nocapture %b, i32 %n) #0 {
  %1 = icmp sgt i32 %n, 0
  br i1 %1, label %.lr.ph, label %._crit_edge

.lr.ph:                                           ; preds = %0
  %2 = sext i32 %n to i64
  %3 = add i32 %n, -1
  %4 = zext i32 %3 to i64
  %5 = add nuw nsw i64 %4, 1
  %min.iters.check = icmp ult i32 %3, 3
  br i1 %min.iters.check, label %scalar.ph, label %vector.ph

vector.ph:                                        ; preds = %.lr.ph
  %n.vec = and i64 %5, 8589934588
  %ind.end = sub nsw i64 %2, %n.vec
  br label %vector.body

vector.body:                                      ; preds = %vector.body,
%vector.ph
  %index = phi i64 [ 0, %vector.ph ], [ %index.next, %vector.body ]
  %vec.phi = phi <4 x i32> [ zeroinitializer, %vector.ph ], [ %11, %vector.body
]
  %6 = xor i64 %index, -1
  %7 = add i64 %6, %2
  %8 = getelementptr inbounds i32, i32* %b, i64 -3
  %9 = getelementptr inbounds i32, i32* %8, i64 %7
  %10 = bitcast i32* %9 to <4 x i32>*
  %wide.load = load <4 x i32>, <4 x i32>* %10, align 4
  %reverse = shufflevector <4 x i32> %wide.load, <4 x i32> undef, <4 x i32>
<i32 3, i32 2, i32 1, i32 0>
  %11 = add <4 x i32> %reverse, %vec.phi
  %index.next = add i64 %index, 4
  %12 = icmp eq i64 %index.next, %n.vec
  br i1 %12, label %middle.block, label %vector.body, !llvm.loop !0

middle.block:                                     ; preds = %vector.body
  %rdx.shuf = shufflevector <4 x i32> %11, <4 x i32> undef, <4 x i32> <i32 2,
i32 3, i32 undef, i32 undef>
  %bin.rdx = add <4 x i32> %11, %rdx.shuf
  %rdx.shuf1 = shufflevector <4 x i32> %bin.rdx, <4 x i32> undef, <4 x i32>
<i32 1, i32 undef, i32 undef, i32 undef>
  %bin.rdx2 = add <4 x i32> %bin.rdx, %rdx.shuf1
  %13 = extractelement <4 x i32> %bin.rdx2, i32 0
  %cmp.n = icmp eq i64 %5, %n.vec
  br i1 %cmp.n, label %._crit_edge.loopexit, label %scalar.ph

scalar.ph:                                        ; preds = %middle.block,
%.lr.ph
  %bc.resume.val = phi i64 [ %ind.end, %middle.block ], [ %2, %.lr.ph ]
  %bc.merge.rdx = phi i32 [ %13, %middle.block ], [ 0, %.lr.ph ]
  br label %14

14:                                               ; preds = %14, %scalar.ph
  %indvars.iv = phi i64 [ %bc.resume.val, %scalar.ph ], [ %indvars.iv.next, %14
]
  %a.02 = phi i32 [ %bc.merge.rdx, %scalar.ph ], [ %17, %14 ]
  %indvars.iv.next = add i64 %indvars.iv, -1
  %15 = getelementptr inbounds i32, i32* %b, i64 %indvars.iv.next
  %16 = load i32, i32* %15, align 4
  %17 = add nsw i32 %16, %a.02
  %18 = trunc i64 %indvars.iv.next to i32
  %19 = icmp sgt i32 %18, 0
  br i1 %19, label %14, label %._crit_edge.loopexit, !llvm.loop !2

._crit_edge.loopexit:                             ; preds = %middle.block, %14
  %.lcssa = phi i32 [ %17, %14 ], [ %13, %middle.block ]
  br label %._crit_edge

._crit_edge:                                      ; preds =
%._crit_edge.loopexit, %0
  %a.0.lcssa = phi i32 [ 0, %0 ], [ %.lcssa, %._crit_edge.loopexit ]
  ret i32 %a.0.lcssa
}

attributes #0 = { nounwind readonly ssp uwtable }

!0 = distinct !{!0, !1}
!1 = !{!"llvm.loop.isvectorized", i32 1}
!2 = distinct !{!2, !3, !1}
!3 = !{!"llvm.loop.unroll.runtime.disable"}


Here %8 is a generated gep inbound with negative offset. The source in this
transformation is more defined than target.

Alive2: https://alive2.llvm.org/ce/z/g2VueK

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