[LoopUtils, LV] Fix PR26734

The vectorization of first-order recurrences (r261346) caused PR26734. When
detecting these recurrences, we need to ensure that the previous value is
actually defined inside the loop. This patch includes the fix and test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262624 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/LoopUtils.cpp

@@ -543,7 +543,7 @@ bool RecurrenceDescriptor::isFirstOrderRecurrence(PHINode *Phi, Loop *TheLoop,
   // Get the previous value. The previous value comes from the latch edge while
   // the initial value comes form the preheader edge.
   auto *Previous = dyn_cast<Instruction>(Phi->getIncomingValueForBlock(Latch));
-  if (!Previous)
+  if (!Previous || !TheLoop->contains(Previous))
     return false;
 
   // Ensure every user of the phi node is dominated by the previous value. The

test/Transforms/LoopVectorize/AArch64/first-order-recurrence.ll

@@ -207,3 +207,52 @@ for.end.loopexit:
 for.end:
   ret void
 }
+
+; CHECK-LABEL: @PR26734
+;
+; void PR26734(short *a, int *b, int *c, int d, short *e) {
+;   for (; d != 21; d++) {
+;     *b &= *c;
+;     *e = *a - 6;
+;     *c = *e;
+;   }
+; }
+;
+; CHECK-NOT: vector.ph:
+;
+define void @PR26734(i16* %a, i32* %b, i32* %c, i32 %d, i16* %e) {
+entry:
+  %cmp4 = icmp eq i32 %d, 21
+  br i1 %cmp4, label %entry.for.end_crit_edge, label %for.body.lr.ph
+
+entry.for.end_crit_edge:
+  %.pre = load i32, i32* %b, align 4
+  br label %for.end
+
+for.body.lr.ph:
+  %0 = load i16, i16* %a, align 2
+  %sub = add i16 %0, -6
+  %conv2 = sext i16 %sub to i32
+  %c.promoted = load i32, i32* %c, align 4
+  %b.promoted = load i32, i32* %b, align 4
+  br label %for.body
+
+for.body:
+  %inc7 = phi i32 [ %d, %for.body.lr.ph ], [ %inc, %for.body ]
+  %and6 = phi i32 [ %b.promoted, %for.body.lr.ph ], [ %and, %for.body ]
+  %conv25 = phi i32 [ %c.promoted, %for.body.lr.ph ], [ %conv2, %for.body ]
+  %and = and i32 %and6, %conv25
+  %inc = add nsw i32 %inc7, 1
+  %cmp = icmp eq i32 %inc, 21
+  br i1 %cmp, label %for.cond.for.end_crit_edge, label %for.body
+
+for.cond.for.end_crit_edge:
+  %and.lcssa = phi i32 [ %and, %for.body ]
+  store i32 %conv2, i32* %c, align 4
+  store i32 %and.lcssa, i32* %b, align 4
+  store i16 %sub, i16* %e, align 2
+  br label %for.end
+
+for.end:
+  ret void
+}