[llvm-bugs] [Bug 51866] New: Miscompile by -loop-bound-split (load from wrong address after split)

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https://bugs.llvm.org/show_bug.cgi?id=51866

            Bug ID: 51866
           Summary: Miscompile by -loop-bound-split (load from wrong
                    address after split)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: mikael.holmen at ericsson.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 25259
  --> https://bugs.llvm.org/attachment.cgi?id=25259&action=edit
bbi-60288.ll reproducer

llvm commit: 0dc461441eed

Reproduce with:
 opt -passes='module(loop-bound-split)' -S -o - bbi-60288.ll

In the input the loop executes 10 rounds, and for each round we read in B with
the induction variable "i" as offset.

After loop-bound-split, the loop is split in two, one for i < 5 and one for
larger i. However, in the second loop we starts over reading at offset 0 in B
instead of offset 5.

I.e. in the second loop we do

for.cond.split:                                   ; preds =
%for.cond.split.preheader, %for.inc.split
  %i.0.split = phi i16 [ %inc.split, %for.inc.split ], [ 0,
%for.cond.split.preheader ]
  %exitcond.not.split = icmp eq i16 %i.0.split, 10
  br i1 %exitcond.not.split, label %for.end.loopexit, label %for.body.split

for.body.split:                                   ; preds = %for.cond.split
  %cmp1.split = icmp ult i16 %i.0.split, 5
  %arrayidx.split = getelementptr inbounds [10 x i16], [10 x i16]* @B, i16 0,
i16 %i.0.split
  %2 = load i16, i16* %arrayidx.split, align 1
  br i1 false, label %if.then.split, label %if.else.split

where %i.0.split "starts over" from 0 in the PHI and %i.0.split is used in the
gep determining where to read.
I think the PHI should be

  %i.0.split = phi i16 [ %inc.split, %for.inc.split ], [ 5,
%for.cond.split.preheader ]

instead.

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