[IVDesc] Check loop-preheader for loop-legality when pass-remarks enabled

When `-pass-remarks=loop-vectorize` is specified, the subsequent
logic is executed to display detailed debug messages even
if no PreHeader exists in the loop.

Therefore, an assert occurs when the `getLoopPreHeader()` function
is called. This commit resolves that issue.

Fixed: #165377

Author: ParkHanbum

llvm/lib/Analysis/IVDescriptors.cpp

@@ -222,7 +222,9 @@ bool RecurrenceDescriptor::AddReductionVar(
 
   // Obtain the reduction start value from the value that comes from the loop
   // preheader.
-  Value *RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
+  Value *RdxStart;
+  if (TheLoop->getLoopPreheader())
+    RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
 
   // ExitInstruction is the single value which is used outside the loop.
   // We only allow for a single reduction value to be used outside the loop.
@@ -587,6 +589,9 @@ bool RecurrenceDescriptor::AddReductionVar(
   collectCastInstrs(TheLoop, ExitInstruction, RecurrenceType, CastInsts,
                     MinWidthCastToRecurrenceType);
 
+  if (!RdxStart)
+    return false;
+
   // We found a reduction var if we have reached the original phi node and we
   // only have a single instruction with out-of-loop users.
 
@@ -1536,6 +1541,11 @@ bool InductionDescriptor::isInductionPHI(
     return false;
   }
 
+  if (!AR->getLoop()->getLoopPreheader()) {
+    LLVM_DEBUG(dbgs() << "LV: Loop doesn't have a legal pre-header.\n");
+    return false;
+  }
+
   // This function assumes that InductionPhi is called only on Phi nodes
   // present inside loop headers. Check for the same, and throw an assert if
   // the current Phi is not present inside the loop header.

llvm/test/Transforms/LoopVectorize/loop-legality-checks-remarks.ll

@@ -0,0 +1,25 @@
+; RUN: opt < %s -passes=loop-vectorize -pass-remarks=loop-vectorize -debug -disable-output 2>&1 | FileCheck %s
+; REQUIRES: asserts
+
+; Make sure LV legal bails out when the loop doesn't have a legal pre-header.
+; CHECK-LABEL: 'not_exist_preheader'
+; CHECK: LV: Not vectorizing: Loop doesn't have a legal pre-header.
+define void @not_exist_preheader(ptr %currMB, i1 %arg, ptr %arg2) nounwind uwtable {
+
+start.exit:
+  indirectbr ptr %arg2, [label %0, label %for.bodyprime]
+
+0:
+  unreachable
+
+for.bodyprime:
+  %i.057375 = phi i32 [0, %start.exit], [%1, %for.bodyprime]
+  %arrayidx8prime = getelementptr inbounds i32, ptr %currMB, i32 %i.057375
+  store i32 0, ptr %arrayidx8prime, align 4
+  %1 = add i32 %i.057375, 1
+  %cmp5prime = icmp slt i32 %1, 4
+  br i1 %cmp5prime, label %for.bodyprime, label %exit
+
+exit:
+  ret void
+}