[IndVars] Check if WideInc available before trying to use it

WideInc/WideIncExpr can be null. Previously this worked out
because the comparison with WideIncExpr would fail. Now we have
accesses to WideInc prior to that. Avoid the issue with an
explicit check.

Fixes #106239.

(cherry picked from commit c9a5e1b)

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -1928,18 +1928,24 @@ Instruction *WidenIV::widenIVUse(WidenIV::NarrowIVDefUse DU,
     if (!WideAddRec.first)
       return nullptr;
 
-    // Reuse the IV increment that SCEVExpander created. Recompute flags, unless
-    // the flags for both increments agree and it is safe to use the ones from
-    // the original inc. In that case, the new use of the wide increment won't
-    // be more poisonous.
-    bool NeedToRecomputeFlags =
-        !SCEVExpander::canReuseFlagsFromOriginalIVInc(OrigPhi, WidePhi,
-                                                      DU.NarrowUse, WideInc) ||
-        DU.NarrowUse->hasNoUnsignedWrap() != WideInc->hasNoUnsignedWrap() ||
-        DU.NarrowUse->hasNoSignedWrap() != WideInc->hasNoSignedWrap();
+    auto CanUseWideInc = [&]() {
+      if (!WideInc)
+        return false;
+      // Reuse the IV increment that SCEVExpander created. Recompute flags,
+      // unless the flags for both increments agree and it is safe to use the
+      // ones from the original inc. In that case, the new use of the wide
+      // increment won't be more poisonous.
+      bool NeedToRecomputeFlags =
+          !SCEVExpander::canReuseFlagsFromOriginalIVInc(
+              OrigPhi, WidePhi, DU.NarrowUse, WideInc) ||
+          DU.NarrowUse->hasNoUnsignedWrap() != WideInc->hasNoUnsignedWrap() ||
+          DU.NarrowUse->hasNoSignedWrap() != WideInc->hasNoSignedWrap();
+      return WideAddRec.first == WideIncExpr &&
+             Rewriter.hoistIVInc(WideInc, DU.NarrowUse, NeedToRecomputeFlags);
+    };
+
     Instruction *WideUse = nullptr;
-    if (WideAddRec.first == WideIncExpr &&
-        Rewriter.hoistIVInc(WideInc, DU.NarrowUse, NeedToRecomputeFlags))
+    if (CanUseWideInc())
       WideUse = WideInc;
     else {
       WideUse = cloneIVUser(DU, WideAddRec.first);

llvm/test/Transforms/IndVarSimplify/pr106239.ll

@@ -0,0 +1,36 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=indvars < %s | FileCheck %s
+
+target datalayout = "n8:16:32:64"
+
+; Make sure it does not crash.
+
+define i32 @m() {
+; CHECK-LABEL: define i32 @m() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[FOR_BODY_I6:.*]]
+; CHECK:       [[FOR_BODY_I6]]:
+; CHECK-NEXT:    br i1 true, label %[[I_EXIT:.*]], label %[[IF_END_I:.*]]
+; CHECK:       [[IF_END_I]]:
+; CHECK-NEXT:    store i64 0, ptr null, align 8
+; CHECK-NEXT:    br label %[[FOR_BODY_I6]]
+; CHECK:       [[I_EXIT]]:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %div.i4 = sdiv i32 1, 0
+  br label %for.body.i6
+
+for.body.i6:                                      ; preds = %if.end.i, %entry
+  %add57.i = phi i32 [ %add.i7, %if.end.i ], [ 0, %entry ]
+  br i1 true, label %i.exit, label %if.end.i
+
+if.end.i:                                         ; preds = %for.body.i6
+  %add.i7 = add i32 %add57.i, %div.i4
+  %conv.i = zext i32 %add57.i to i64
+  store i64 %conv.i, ptr null, align 8
+  br label %for.body.i6
+
+i.exit:                                           ; preds = %for.body.i6
+  ret i32 0
+}