diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -9276,6 +9276,51 @@
   return true;
 }
 
+// Emit a remark if there are stores to floats that required a floating point
+// extension. If the vectorized loop was generated with floating point there
+// will be a performance penalty from the conversion overhead and the change in
+// the vector width.
+static void checkMixedPrecision(Loop *L, OptimizationRemarkEmitter *ORE) {
+  SmallVector<Instruction *, 4> Worklist;
+  for (BasicBlock *BB : L->getBlocks()) {
+    for (Instruction &Inst : *BB) {
+      if (auto *S = dyn_cast<StoreInst>(&Inst)) {
+        if (S->getValueOperand()->getType()->isFloatTy())
+          Worklist.push_back(S);
+      }
+    }
+  }
+
+  // Traverse the floating point stores upwards searching, for floating point
+  // conversions.
+  SmallPtrSet<const Instruction *, 4> Visited;
+  SmallPtrSet<const Instruction *, 4> EmittedRemark;
+  while (!Worklist.empty()) {
+    auto *I = Worklist.pop_back_val();
+    if (!L->contains(I))
+      continue;
+    if (!Visited.insert(I).second)
+      continue;
+
+    // Emit a remark if the floating point store required a floating
+    // point conversion.
+    // TODO: More work could be done to identify the root cause such as a
+    // constant or a function return type and point the user to it.
+    if (isa<FPExtInst>(I) && EmittedRemark.insert(I).second)
+      ORE->emit([&]() {
+        return OptimizationRemarkAnalysis(LV_NAME, "VectorMixedPrecision",
+                                          I->getDebugLoc(), L->getHeader())
+               << "floating point conversion changes vector width. "
+               << "Mixed floating point precision requires an up/down "
+               << "cast that will negatively impact performance.";
+      });
+
+    for (Use &Op : I->operands())
+      if (auto *OpI = dyn_cast<Instruction>(Op))
+        Worklist.push_back(OpI);
+  }
+}
+
 LoopVectorizePass::LoopVectorizePass(LoopVectorizeOptions Opts)
     : InterleaveOnlyWhenForced(Opts.InterleaveOnlyWhenForced ||
                                !EnableLoopInterleaving),
@@ -9594,6 +9639,9 @@
              << NV("VectorizationFactor", VF.Width)
              << ", interleaved count: " << NV("InterleaveCount", IC) << ")";
     });
+
+    if (ORE->allowExtraAnalysis(LV_NAME))
+      checkMixedPrecision(L, ORE);
   }
 
   Optional<MDNode *> RemainderLoopID =
diff --git a/llvm/test/Transforms/LoopVectorize/mixed-precision-remarks.ll b/llvm/test/Transforms/LoopVectorize/mixed-precision-remarks.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/LoopVectorize/mixed-precision-remarks.ll
@@ -0,0 +1,64 @@
+; RUN: opt -force-vector-interleave=2 -force-vector-width=4 -loop-vectorize -pass-remarks-analysis=loop-vectorize -disable-output < %s 2>&1 | FileCheck %s
+
+; CHECK: remark: mixed-precision.c:3:26: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
+define void @f(float* noalias nocapture %X, i64 %N) {
+entry:
+  br label %for.body
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  %i = phi i64 [ %inc, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds float, float* %X, i64 %i
+  %0 = load float, float* %arrayidx, align 4
+  %conv = fpext float %0 to double, !dbg !9
+  %mul = fmul double %conv, 0x3FD5555555555555
+  %conv3 = fptrunc double %mul to float
+  store float %conv3, float* %arrayidx, align 4
+  %inc = add nuw i64 %i, 1
+  %exitcond.not = icmp eq i64 %inc, %N
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+}
+
+; CHECK: remark: mixed-precision.c:7:16: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
+; CHECK-NOT: remark: mixed-precision.c:7:16: floating point conversion changes vector width. Mixed floating point precision requires an up/down cast that will negatively impact performance.
+define void @g(float* noalias nocapture %X, float* noalias nocapture %Y, i64 %N) {
+entry:
+  %pi = alloca double
+  store double 0x400921FB54442D18, double* %pi
+  %fac = load double, double* %pi
+  br label %for.body
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  %i = phi i64 [ %inc, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds float, float* %X, i64 %i
+  %0 = load float, float* %arrayidx, align 4
+  %conv = fpext float %0 to double, !dbg !10
+  %mul = fmul double %conv, %fac
+  %conv1 = fptrunc double %mul to float
+  store float %conv1, float* %arrayidx, align 4
+  %arrayidx5 = getelementptr inbounds float, float* %Y, i64 %i
+  %mul2 = fmul float %conv1, %conv1
+  store float %mul2, float* %arrayidx5, align 4
+  %inc = add nuw nsw i64 %i, 1
+  %exitcond.not = icmp eq i64 %inc, %N
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 12.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "mixed-precision.c", directory: "/tmp/mixed-precision.c")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!6 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!7 = distinct !DISubprogram(name: "g", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 3, column: 26, scope: !6)
+!10 = !DILocation(line: 7, column: 16, scope: !7)