[llvm-bugs] [Bug 26952] New: LoopVectorizer does not properly preserve LoopInfo

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26952

            Bug ID: 26952
           Summary: LoopVectorizer does not properly preserve LoopInfo
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: anemet at apple.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16040
  --> https://llvm.org/bugs/attachment.cgi?id=16040&action=edit
testcase

On the attached testcase based on MultiSource/Benchmarks/nbench/nbench1.c, LV
fails to preserve LoopInfo.  -verify-loop-info fails with:

$ opt before-vect-func.ll -targetlibinfo -tti -tbaa -scoped-noalias
-assumption-cache-tracker -forceattrs -inferattrs -loop-vectorize
-verify-loop-info

Assertion failed: (HasInsideLoopSuccs && "Loop block has no in-loop
successors!"), function verifyLoop, file
../include/llvm/Analysis/LoopInfoImpl.h, line 279.

This is the difference between what -loops and what LV produces:

@@ -21 +20,0 @@
-    Loop at depth 2 containing:
%vector.body<header>,%pred.store.if,%13,%pred.store.continue,%pred.store.if2,%15,%pred.store.continue3<latch><exiting>
@@ -31,0 +31 @@
+    Loop at depth 2 containing:
%vector.body<header><exiting>,%pred.store.continue<exiting>,%pred.store.continue3<latch><exiting>

Bunch of blocks are missing, looks like all related to store predication.

I triggered this latent bug after making LoopLoadElimination dependent on
LoopSimplify.  LoopSimplify trims many blocks because of corrupt LI:

LoopSimplify: Deleting edge from dead predecessor
LoopSimplify: Deleting edge from dead predecessor
LoopSimplify: Creating dedicated exit block .loopexit
LoopSimplify: Creating dedicated exit block .loopexit4
LoopSimplify: Creating dedicated exit block for.end31.loopexit

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