diff --git a/llvm/include/llvm/CodeGen/TargetLowering.h b/llvm/include/llvm/CodeGen/TargetLowering.h
--- a/llvm/include/llvm/CodeGen/TargetLowering.h
+++ b/llvm/include/llvm/CodeGen/TargetLowering.h
@@ -3563,6 +3563,14 @@
   bool SimplifyDemandedVectorElts(SDValue Op, const APInt &DemandedElts,
                                   DAGCombinerInfo &DCI) const;
 
+  /// Return true if the target supports simplifying demanded vector elements by
+  /// converting them to undefs.
+  virtual bool
+  shouldSimplifyDemandedVectorElts(SDValue Op,
+                                   const TargetLoweringOpt &TLO) const {
+    return true;
+  }
+
   /// Determine which of the bits specified in Mask are known to be either zero
   /// or one and return them in the KnownZero/KnownOne bitsets. The DemandedElts
   /// argument allows us to only collect the known bits that are shared by the
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -2640,6 +2640,10 @@
 
   KnownUndef = KnownZero = APInt::getZero(NumElts);
 
+  const TargetLowering &TLI = TLO.DAG.getTargetLoweringInfo();
+  if (!TLI.shouldSimplifyDemandedVectorElts(Op, TLO))
+    return false;
+
   // TODO: For now we assume we know nothing about scalable vectors.
   if (VT.isScalableVector())
     return false;
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h
@@ -113,6 +113,10 @@
     report_fatal_error("llvm.clear_cache is not supported on wasm");
   }
 
+  bool
+  shouldSimplifyDemandedVectorElts(SDValue Op,
+                                   const TargetLoweringOpt &TLO) const override;
+
   // Custom lowering hooks.
   SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;
   SDValue LowerFrameIndex(SDValue Op, SelectionDAG &DAG) const;
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -911,6 +911,30 @@
   return TargetLoweringBase::getPreferredVectorAction(VT);
 }
 
+bool WebAssemblyTargetLowering::shouldSimplifyDemandedVectorElts(
+    SDValue Op, const TargetLoweringOpt &TLO) const {
+  // ISel process runs DAGCombiner after legalization; this step is called
+  // SelectionDAG optimization phase. This post-legalization combining process
+  // runs DAGCombiner on each node, and if there was a change to be made,
+  // re-runs legalization again on it and its user nodes to make sure
+  // everythiing is in a legalized state.
+  //
+  // The legalization calls lowering routines, and we do our custom lowering for
+  // build_vectors (LowerBUILD_VECTOR), which converts undef vector elements
+  // into zeros. But there is a set of routines in DAGCombiner that turns unused
+  // (= not demanded) nodes into undef, among which SimplifyDemandedVectorElts
+  // turns unused vector elements into undefs. But this routine does not work
+  // with our custom LowerBUILD_VECTOR, which turns undefs into zeros. This
+  // combination can result in a infinite loop, in which undefs are converted to
+  // zeros in legalization and back to undefs in combining.
+  //
+  // So after DAG is legalized, we prevent SimplifyDemandedVectorElts from
+  // running for build_vectors.
+  if (Op.getOpcode() == ISD::BUILD_VECTOR && TLO.LegalOps && TLO.LegalTys)
+    return false;
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 // WebAssembly Lowering private implementation.
 //===----------------------------------------------------------------------===//
diff --git a/llvm/test/CodeGen/WebAssembly/simd-simplify-demanded-vector-elts.ll b/llvm/test/CodeGen/WebAssembly/simd-simplify-demanded-vector-elts.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/simd-simplify-demanded-vector-elts.ll
@@ -0,0 +1,29 @@
+; RUN: llc < %s -mattr=+simd128 -verify-machineinstrs
+
+target triple = "wasm32-unknown-unknown"
+
+; After DAG legalization, in SelectionDAG optimization phase, ISel runs
+; DAGCombiner on each node, among which SimplifyDemandedVectorElts turns unused
+; vector elements into undefs. And in order to make sure the DAG is in a
+; legalized state, it runs legalization again, which runs our custom
+; LowerBUILD_VECTOR, which converts undefs into zeros, causing an infinite loop.
+; We prevent this from happening by creating a custom hook , which allows us to
+; bail out of SimplifyDemandedVectorElts after legalization.
+
+; This is a reduced test case from a bug reproducer reported. This should not
+; hang.
+define void @test(i8 %0) {
+  %2 = insertelement <4 x i8> <i8 -1, i8 -1, i8 -1, i8 poison>, i8 %0, i64 3
+  %3 = zext <4 x i8> %2 to <4 x i32>
+  %4 = mul nuw nsw <4 x i32> %3, <i32 257, i32 257, i32 257, i32 257>
+  %5 = add nuw nsw <4 x i32> %4, <i32 1, i32 1, i32 1, i32 1>
+  %6 = lshr <4 x i32> %5, <i32 1, i32 1, i32 1, i32 1>
+  %7 = mul nuw nsw <4 x i32> %6, <i32 20000, i32 20000, i32 20000, i32 20000>
+  %8 = add nuw nsw <4 x i32> %7, <i32 32768, i32 32768, i32 32768, i32 32768>
+  %9 = and <4 x i32> %8, <i32 2147418112, i32 2147418112, i32 2147418112, i32 2147418112>
+  %10 = sub nsw <4 x i32> <i32 655360000, i32 655360000, i32 655360000, i32 655360000>, %9
+  %11 = ashr exact <4 x i32> %10, <i32 16, i32 16, i32 16, i32 16>
+  %12 = trunc <4 x i32> %11 to <4 x i16>
+  store <4 x i16> %12, <4 x i16>* undef, align 4
+  ret void
+}