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
@@ -2046,7 +2046,18 @@
     SmallVector<SDValue, 16> ConstLanes;
     for (const SDValue &Lane : Op->op_values()) {
       if (IsConstant(Lane)) {
-        ConstLanes.push_back(Lane);
+        // Values may need to be fixed so that they will sign extend to be
+        // within the expected range during ISel.
+        auto *Const = dyn_cast<ConstantSDNode>(Lane.getNode());
+        int64_t Val = Const ? Const->getSExtValue() : 0;
+        uint64_t LaneBits = 128 / Lanes;
+        if (Const && LaneT != MVT::i64 &&
+            (Val < -(1 << (LaneBits - 1)) || Val > (1 << (LaneBits - 1)) - 1)) {
+          auto NewVal = ((uint64_t)Val % (1u << LaneBits)) - (1u << LaneBits);
+          ConstLanes.push_back(DAG.getConstant(NewVal, SDLoc(Lane), LaneT));
+        } else {
+          ConstLanes.push_back(Lane);
+        }
       } else if (LaneT.isFloatingPoint()) {
         ConstLanes.push_back(DAG.getConstantFP(0, DL, LaneT));
       } else {
diff --git a/llvm/test/CodeGen/WebAssembly/simd-pr51605.ll b/llvm/test/CodeGen/WebAssembly/simd-pr51605.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/simd-pr51605.ll
@@ -0,0 +1,40 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -verify-machineinstrs -mattr=+simd128 | FileCheck %s
+
+; Regression test for an issue in which DAG combines created a constant i8x16
+; vector with lane values of 255, which was outside the -128 to 127 range
+; expected by our ISel patterns (and similar for the i16 version) and caused an
+; ISel failure. The fix was to adjust out-of-range values manually in
+; BUILD_VECTOR lowering.
+
+target triple = "wasm32-unknown-unknown"
+
+define <4 x i8> @test_i8(<4 x i8> %b) {
+; CHECK-LABEL: test_i8:
+; CHECK:         .functype test_i8 (v128) -> (v128)
+; CHECK-NEXT:  # %bb.0:
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    v128.const -1, -1, -1, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+; CHECK-NEXT:    v128.xor
+; CHECK-NEXT:    v128.const 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+; CHECK-NEXT:    v128.and
+; CHECK-NEXT:    # fallthrough-return
+  %c = and <4 x i8> %b, <i8 1, i8 1, i8 1, i8 1>
+  %d = xor <4 x i8> %c, <i8 1, i8 1, i8 1, i8 1>
+  ret <4 x i8> %d
+}
+
+define <4 x i16> @test_i16(<4 x i16> %b) {
+; CHECK-LABEL: test_i16:
+; CHECK:         .functype test_i16 (v128) -> (v128)
+; CHECK-NEXT:  # %bb.0:
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    v128.const -1, -1, -1, -1, 0, 0, 0, 0
+; CHECK-NEXT:    v128.xor
+; CHECK-NEXT:    v128.const 1, 1, 1, 1, 0, 0, 0, 0
+; CHECK-NEXT:    v128.and
+; CHECK-NEXT:    # fallthrough-return
+  %c = and <4 x i16> %b, <i16 1, i16 1, i16 1, i16 1>
+  %d = xor <4 x i16> %c, <i16 1, i16 1, i16 1, i16 1>
+  ret <4 x i16> %d
+}