Index: lib/Analysis/DependenceAnalysis.cpp
===================================================================
--- lib/Analysis/DependenceAnalysis.cpp
+++ lib/Analysis/DependenceAnalysis.cpp
@@ -830,6 +830,14 @@
     return isLoopInvariant(Src, LoopNest);
   const SCEV *Start = AddRec->getStart();
   const SCEV *Step = AddRec->getStepRecurrence(*SE);
+  const SCEV *UB = SE->getBackedgeTakenCount(AddRec->getLoop());
+  if (!isa<SCEVCouldNotCompute>(UB)) {
+    if (SE->getTypeSizeInBits(Start->getType()) <
+        SE->getTypeSizeInBits(UB->getType())) {
+      if (!AddRec->getNoWrapFlags())
+        return false;
+    }
+  }
   if (!isLoopInvariant(Step, LoopNest))
     return false;
   Loops.set(mapSrcLoop(AddRec->getLoop()));
@@ -848,6 +856,14 @@
     return isLoopInvariant(Dst, LoopNest);
   const SCEV *Start = AddRec->getStart();
   const SCEV *Step = AddRec->getStepRecurrence(*SE);
+  const SCEV *UB = SE->getBackedgeTakenCount(AddRec->getLoop());
+  if (!isa<SCEVCouldNotCompute>(UB)) {
+    if (SE->getTypeSizeInBits(Start->getType()) <
+        SE->getTypeSizeInBits(UB->getType())) {
+      if (!AddRec->getNoWrapFlags())
+        return false;
+    }
+  }
   if (!isLoopInvariant(Step, LoopNest))
     return false;
   Loops.set(mapDstLoop(AddRec->getLoop()));
@@ -942,13 +958,15 @@
 // All subscripts are all the same type.
 // Loop bound may be smaller (e.g., a char).
 // Should zero extend loop bound, since it's always >= 0.
-// This routine collects upper bound and extends if needed.
+// This routine collects upper bound and extends or truncates if needed.
+// Truncating is safe when subscripts are known not to wrap. Cases without
+// nowrap flags should have been rejected earlier.
 // Return null if no bound available.
 const SCEV *DependenceAnalysis::collectUpperBound(const Loop *L,
                                                   Type *T) const {
   if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
     const SCEV *UB = SE->getBackedgeTakenCount(L);
-    return SE->getNoopOrZeroExtend(UB, T);
+    return SE->getTruncateOrZeroExtend(UB, T);
   }
   return nullptr;
 }
Index: test/Analysis/DependenceAnalysis/PR21585.ll
===================================================================
--- /dev/null
+++ test/Analysis/DependenceAnalysis/PR21585.ll
@@ -0,0 +1,105 @@
+; RUN: opt < %s -analyze -basicaa -globalsmodref-aa -da | FileCheck %s
+define void @i32_subscript(i32* %a) {
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [ 0, %entry ], [ %i.inc, %for.body ]
+  %a.addr = getelementptr i32, i32* %a, i32 %i
+  %a.addr.2 = getelementptr i32, i32* %a, i32 5
+  %0 = load i32, i32* %a.addr, align 4
+  %1 = add i32 %0, 1
+  store i32 %1, i32* %a.addr.2, align 4
+  %i.inc = add nsw i32 %i, 1
+  %i.inc.ext = sext i32 %i to i64
+  %exitcond = icmp ne i64 %i.inc.ext, 100
+  br i1 %exitcond, label %for.body, label %for.end
+
+for.end:
+  ret void
+}
+; CHECK: none
+; CHECK: anti
+; CHECK: output
+
+
+; Test for a bug, which caused an assert in ScalarEvolution because
+; the Dependence Analyzer attempted to zero extend a type to a smaller
+; type.
+
+; void t(unsigned int *a, unsigned int n) {
+;   for (unsigned int i = 0; i != n; i++) {
+;     a[(unsigned short)i] = g;
+;  }}
+
+@g = common global i32 0, align 4
+
+define void @t(i32* noalias %a, i32 %n) nounwind {
+entry:
+  %cmp1 = icmp eq i32 %n, 0
+  br i1 %cmp1, label %for.end, label %for.body
+
+for.body:
+  %i.02 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+  %0 = load i32, i32* @g, align 4
+  %idxprom = and i32 %i.02, 65535
+  %arrayidx = getelementptr inbounds i32, i32* %a, i32 %idxprom
+  store i32 %0, i32* %arrayidx, align 4
+  %inc = add i32 %i.02, 1
+  %cmp = icmp eq i32 %inc, %n
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret void
+}
+; CHECK: input
+; CHECK: none
+; CHECK: output
+
+define void @i16_wrap(i64* %a) {
+entry:
+  br label %for.body
+for.body:
+  %i = phi i64 [0, %entry], [%i.inc, %for.inc]
+  %i.tr = trunc i64 %i to i16
+  %idx = getelementptr i64, i64* %a, i16 %i.tr
+  %0 = load i64, i64* %idx
+  %1 = add i64 %0, 1
+store i64 %1, i64* %idx
+  br label %for.inc
+
+for.inc:
+  %i.inc = add nuw i64 %i, 1
+  %cmp = icmp ult i64 %i.inc, 17179869184
+  br i1 %cmp, label %for.body, label %for.end
+for.end:
+  ret void
+}
+; CHECK: input
+; CHECK: anti
+; CHECK: output
+
+define void @i8_stride_wrap(i32* noalias %a, i32* noalias %b) {
+entry:
+  br label %for.body
+for.body:
+  %i = phi i32 [1,%entry], [%i.inc, %for.inc]
+  %i.tr = trunc i32 %i to i8
+  %idx = getelementptr i32, i32* %a, i8 %i.tr
+  %idx.2 = getelementptr i32, i32* %b, i32 %i
+  %0 = load i32, i32* %idx, align 4
+  %1 = add i32 %0, 1
+  store i32 %1, i32* %idx.2, align 4
+  br label %for.inc
+
+for.inc:
+  %i.inc = add nsw i32 %i, 256
+  %exitcond = icmp ult i32 %i, 65536
+  br i1 %exitcond, label %for.body, label %for.end
+
+for.end:
+  ret void
+}
+; CHECK: input
+; CHECK: none
+; CHECK: none